Wrou01

Routing Workshop I

Contact: training@apnic.net

WROU01_v1.0

Overview
Routing Workshop (3 Days)
– 
– 
– 
– 
– 
– 
– 
– 
– 
– 
– 

Introduction to IP Routing
Routing Protocol Basic
IPv6 Address Structure
Routing Lab Topology Overview
Operation of OSPF Routing Protocol
Lab Exercise on Basic Router and OSPF Dynamic Routing Configuration
Basic BGP Operation
BGP Attributes and Path Selection Process
BGP Scaling Techniques
Lab Exercise on iBGP, eBGP, RR, Peer group etc
Internet Exchange [IX] Policy Overview and Configuration requirement

Overview

–  Introduction to IP Routing
– 
– 
– 
– 
– 
– 
– 
– 
– 
– 

Basic BGP Operation

IPv4
•  Internet uses IPv4
–  Addresses are 32 bits long
–  Range from 1.0.0.0 to 223.255.255.255
–  0.0.0.0 to 0.255.255.255 and 224.0.0.0 to 255.255.255.255 have
“special” uses

•  IPv4 address has a network portion and a host portion

IPv4 address format
•  Address and subnet mask
–  written as
– 
12.34.56.78 255.255.255.0 or
– 
12.34.56.78/24
–  mask represents the number of network bits in the 32 bit address
–  the remaining bits are the host bits

What does a router do?
•  ?

A day in a life of a router
•  find path
•  forward packet, forward packet, forward packet, forward
packet...
•  find alternate path
•  forward packet, forward packet, forward packet, forward
packet…
•  repeat until powered off

Routing versus Forwarding
•  Routing = building maps
and giving directions
•  Forwarding = moving
packets between interfaces
according to the
“directions”

IP Routing – finding the path
•  Path derived from information received from a routing
protocol
•  Several alternative paths may exist
–  best path stored in forwarding table

•  Decisions are updated periodically or as topology changes
(event driven)
•  Decisions are based on:
–  topology, policies and metrics (hop count, filtering, delay, bandwidth,
etc.)

IP route lookup
•  Based on destination IP address
•  “longest match” routing
–  More specific prefix preferred over less specific prefix
–  Example: packet with destination of 10.1.1.1/32 is sent to the router
announcing 10.1/16 rather than the router announcing 10/8.

IP route lookup
Packet: Destination
IP address: 10.1.1.1
R1

10/8 announced
from here

R3

R2

10/8 → R3
10.1/16 → R4
20/8 → R5
30/8 → R6
…..
R2’s IP routing table

R4

10.1/16 announced
from here

IP route lookup:
Longest match routing
Packet: Destination
R1

10/8 announced
from here

R3

R2

10.1.1.1 && FF.0.0.0
10/8 → R3
vs.
Match!
10.1/16 → R4
10.0.0.0 && FF.0.0.0
20/8 → R5
30/8 → R6
…..

R4

10.1/16 announced
from here

IP route lookup:
Packet: Destination
R1

10/8 announced
from here

R3

R2
R4

10/8 → R3
10.1/16 → R4
20/8 → R5
30/8 → R6
…..

10.1/16 announced
10.1.1.1 && FF.FF.0.0
from here
Match as well!
vs.
10.1.0.0 && FF.FF.0.0


IP route lookup:
Packet: Destination
R1

10/8 announced
from here

R3

R2
R4

10/8 → R3
10.1/16 → R4
20/8 → R5
30/8 → R6
…..

10.1/16 announced
from here

10.1.1.1 && FF.0.0.0
Does not match!
vs.
20.0.0.0 && FF.0.0.0


IP route lookup:
Packet: Destination
R1

10/8 announced
from here

R3

R2
R4

10/8 → R3
10.1/16 announced
10.1/16 → R4
from here
20/8 → R5
10.1.1.1 && FF.0.0.0
Does not match!
30/8 → R6
vs.
…..
30.0.0.0 && FF.0.0.0

IP route lookup:
Packet: Destination
R1

10/8 announced
from here

R3

R2
R4

10/8 → R3
10.1/16 → R4
20/8 → R5
30/8 → R6
…..

Longest match, 16 bit netmask

10.1/16 announced
from here

IP Forwarding
•  Router decides which interface a packet is sent to
•  Forwarding table populated by routing process
•  Forwarding decisions:
–  destination address
–  class of service (fair queuing, precedence, others)
–  local requirements (packet filtering)

•  Forwarding is usually aided by special hardware

Routing Information Base (RIB)

Forwarding Information Base (FIB)

Routing Tables Feed the Forwarding
Table
BGP 4 Routing Table
OSPF – Link State Database
Connected Routes
Static Routes

RIBs and FIBs
•  FIB is the Forwarding Table
–  It contains destinations and the interfaces to get to those destinations
–  Used by the router to figure out where to send the packet
–  Careful! Some people still call this a route!

•  RIB is the Routing Table
–  It contains a list of all the destinations and the various next hops used
to get to those destinations – and lots of other information too!
–  One destination can have lots of possible next-hops – only the best
next-hop goes into the FIB

Explicit versus Default Routing
•  Default:
–  simple, cheap (cycles, memory, bandwidth)
–  low granularity (metric games)

•  Explicit (default free zone)
–  high overhead, complex, high cost, high granularity

•  Hybrid
–  minimise overhead
–  provide useful granularity
–  requires some filtering knowledge

Egress Traffic
•  How packets leave your network
•  Egress traffic depends on:
– 
– 
– 
– 

route availability (what others send you)
route acceptance (what you accept from others)
policy and tuning (what you do with routes from others)
Peering and transit agreements

Ingress Traffic
•  How packets get to your network and your customers’
networks
•  Ingress traffic depends on:
–  what information you send and to whom
–  based on your addressing and AS’s
–  based on others’ policy (what they accept from you and what they do
with it)

Autonomous System (AS)
•  Collection of networks with same routing policy
•  Single routing protocol
•  Usually under single ownership, trust and administrative
control

AS 100

Definition of terms
•  Neighbours
–  AS’s which directly exchange routing information
–  Routers which exchange routing information

•  Announce
–  send routing information to a neighbour

•  Accept
–  receive and use routing information sent by a neighbour

•  Originate
–  insert routing information into external announcements (usually as a
result of the IGP)

•  Peers
–  routers in neighbouring AS’s or within one AS which exchange routing
and policy information

Routing flow and packet flow
packet flow

AS 1

accept
announce

routing flow

announce
accept

packet flow

For networks in AS1 and AS2 to communicate:
AS1 must announce to AS2
AS2 must accept from AS1
AS2 must announce to AS1
AS1 must accept from AS2

AS 2

Routing flow and Traffic flow
•  Traffic flow is always in the opposite direction of the flow of
Routing information
–  Filtering outgoing routing information inhibits traffic flow inbound
–  Filtering inbound routing information inhibits traffic flow outbound

Routing Flow/Packet Flow:
With multiple ASes
AS 1

AS 34

N1

AS16
AS 8

N16

•  For net N1 in AS1 to send traffic to net N16 in AS16:
– 
– 
– 
– 

AS16 must originate and announce N16 to AS8.
AS8 must accept N16 from AS16.
AS8 must forward announcement of N16 to AS1 or AS34.
AS1 must accept N16 from AS8 or AS34.

•  For two-way packet flow, similar policies must exist for N1

Routing Flow/Packet Flow:
With multiple ASes
AS 1

AS 34

N1

AS16
AS 8

N16

•  As multiple paths between sites are implemented it is easy
to see how policies can become quite complex.

Routing Policy
•  Used to control traffic flow in and out of an ISP network
•  ISP makes decisions on what routing information to accept
and discard from its neighbours
– 
– 
– 
– 

Individual routes
Routes originated by specific ASes
Routes traversing specific ASes
Routes belonging to other groupings
•  Groupings which you define as you see fit

Routing Policy Limitations
red

red

Internet
green

AS99
green
packet flow

•  AS99 uses red link for traffic to the red AS and the green
link for remaining traffic
•  To implement this policy, AS99 has to:
–  Accept routes originating from the red AS on the red link
–  Accept all other routes on the green link

Routing Policy Limitations

red

red

Internet

AS99

AS22
green

green
packet flow

•  AS99 would like packets coming from the green AS to use
the green link.
•  But unless AS22 cooperates in pushing traffic from the
green AS down the green link, there is very little that AS99
can do to achieve this aim

Routing Policy Issues
•  End May 2012:
–  400000+ prefixes
•  Not realistic to set policy on all of them individually

–  40000 origin AS’s
•  Too many to try and create individual policies for

•  Routes tied to a specific AS or path may be unstable
regardless of connectivity
•  Solution: Groups of AS’s are a natural abstraction for
filtering purposes

Overview

–  Routing Protocol Basic
– 
– 
– 
– 
– 
– 
– 
– 
– 

Basic BGP Operation

1: How Does Routing Work?
•  Internet is made up of the ISPs who connect to each
other’s networks
•  How does an ISP in Kenya tell an ISP in Japan what
customers they have?
•  And how does that ISP send data packets to the customers
of the ISP in Japan, and get responses back
–  After all, as on a local ethernet, two way packet flow is needed for
communication between two devices

•  ISP in Kenya could buy a direct connection to the ISP in
Japan
–  But this doesn’t scale – thousands of ISPs, would need thousands of
connections, and cost would be astronomical

•  Instead, ISP in Kenya tells his neighbouring ISPs what
customers he has
–  And the neighbouring ISPs pass this information on to their
neighbours, and so on
–  This process repeats until the information reaches the ISP in Japan

•  This process is called “Routing”
•  The mechanisms used are called “Routing Protocols”
•  Routing and Routing Protocols ensures that the Internet
can scale, that thousands of ISPs can provide connectivity
to each other, giving us the Internet we see today

•  ISP in Kenya doesn’t actually tell his neighbouring ISPs the
names of the customers
–  (network equipment does not understand names)

•  Instead, he has received an IP address block as a member
of the Regional Internet Registry serving Kenya
–  His customers have received address space from this address block
as part of their “Internet service”
–  And he announces this address block to his neighbouring ISPs – this
is called announcing a “route”

Routing Protocols
•  Routers use “routing protocols” to exchange routing
information with each other
–  IGP is used to refer to the process running on routers inside an ISP’s
network
–  EGP is used to refer to the process running between routers
bordering directly connected ISP networks

What Is an IGP?
•  Interior Gateway Protocol
•  Within an Autonomous System
•  Carries information about internal infrastructure prefixes
•  Two widely used IGPs in service provider network:
–  OSPF
–  ISIS

Why Do We Need an IGP?
•  ISP backbone scaling
–  Hierarchy
–  Limiting scope of failure
–  Only used for ISP’s infrastructure addresses, not customers or
anything else
–  Design goal is to minimise number of prefixes in IGP to aid scalability
and rapid convergence

What Is an EGP?
•  Exterior Gateway Protocol
•  Used to convey routing information between Autonomous
Systems
•  De-coupled from the IGP
•  Current EGP is BGP

Why Do We Need an EGP?
•  Scaling to large network
–  Hierarchy
–  Limit scope of failure

•  Define Administrative Boundary
•  Policy
–  Control reachability of prefixes
–  Merge separate organisations
–  Connect multiple IGPs

Interior versus Exterior
Routing Protocols
•  Interior
– 
– 
– 
– 
– 

Automatic neighbour discovery
Generally trust your IGP routers
Prefixes go to all IGP routers
Binds routers in one AS together
Carries ISP infrastructure
addresses only
–  ISPs aim to keep the IGP small for
efficiency and scalability

•  Exterior
– 
– 
– 
– 
– 
– 
– 

Specifically configured peers
Connecting with outside networks
Set administrative boundaries
Binds AS’s together
Carries customer prefixes
Carries Internet prefixes
EGPs are independent of ISP
network topology

Hierarchy of Routing Protocols
Other ISPs
BGP4

BGP4
and OSPF/ISIS

BGP4
IXP

Static/BGP4
Customers

FYI: Cisco IOS Default Administrative
Distances
Route Source

Default Distance

Connected Interface
Static Route
Enhanced IGRP Summary Route
External BGP
Internal Enhanced IGRP
IGRP
OSPF
IS-IS
RIP
EGP
External Enhanced IGRP
Internal BGP
Unknown

0
1
5
20
90
100
110
115
120
140
170
200
255

Overview

–  IPv6 Address Structure
– 
– 
– 
– 
– 
– 
– 
– 

Basic BGP Operation

IPv6 Addressing
•  An IPv6 address is 128 bits long
•  So the number of addresses are 2^128

=340282366920938463463374607431768211455
(39 decimal digits)
=0xffffffffffffffffffffffffffffffff (32 hexadecimal digits)

•  In hex 4 bit (nibble) is represented by a hex digit
•  So 128 bit is reduced down to 32 hex digit

IPv6 Address Representation
•  Hexadecimal values of eight 16 bit fields
-  X:X:X:X:X:X:X:X (X=16 bit number, ex: A2FE)
-  16 bit number is converted to a 4 digit hexadecimal number

•  Example:
-  FE38:DCE3:124C:C1A2:BA03:6735:EF1C:683D

–  Abbreviated form of address
- 
- 
- 
- 

4EED:0023:0000:0000:0000:036E:1250:2B00
→4EED:23:0:0:0:36E:1250:2B00
→4EED:23::36E:1250:2B00
(Null value can be used only once)

IPv6 addressing structure
128 bits
64 65

1

Network Prefix

16

32

Interface ID

16

128

64

ISP
/32

Customer
Site /48
End Site Subnet /64

Device 128 Bit Address

IPv6 addressing model
•  IPv6 Address type
–  Unicast

RFC
4291

•  An identifier for a single
interface

–  Anycast

•  An identifier for a set of
interfaces
–  Multicast

•  An identifier for a group of
nodes

Addresses Without a Network Prefix
•  Localhost

::1/128

•  Unspecified Address

::/128

•  IPv4-mapped IPv6 address ::ffff/96 [a.b.c.d]
•  IPv4-compatible IPv6 address ::/96 [a.b.c.d]

53

Local Addresses With Network Prefix
•  Link Local Address
–  A special address used to communicate within the local link of an
interface
–  i.e. anyone on the link as host or router
–  This address in packet destination that packet would never pass
through a router
–  fe80::/10

54

Local Addresses With Network Prefix
•  Unique Local IPv6 Unicast Address
–  Addresses similar to the RFC 1918 / private address like in IPv4 but
will ensure uniqueness
–  A part of the prefix (40 bits) are generated using a pseudo-random
algorithm and it's improbable that two generated ones are equal
–  fc00::/7
–  Example webtools to generate ULA prefix
http://www.sixxs.net/tools/grh/ula/
http://www.goebel-consult.de/ipv6/createLULA

55

Global Addresses With Network Prefix
•  IPV6 Global Unicast Address
–  Global Unicast Range:

0010

2000::/3

0011
3000::/3
–  All five RIRs are given a /12 from the /3 to further distribute within the
RIR region
• 
• 
• 
• 
• 

APNIC
ARIN
AfriNIC
LACNIC
Ripe NCC

2400:0000::/12
2600:0000::/12
2C00:0000::/12
2800:0000::/12
2A00:0000::/12

56

Examples and Documentation Prefix
•  Two address ranges are reserved for examples and
documentation purpose by RFC 3849
–  For example 3fff:ffff::/32
–  For documentation 2001:0DB8::/32

57

Interface ID
•  The lowest-order 64-bit field addresses may be
assigned in several different ways:
–  auto-configured from a 48-bit MAC address expanded into a 64-bit
EUI-64
–  assigned via DHCP
–  manually configured
–  auto-generated pseudo-random number
–  possibly other methods in the future

EUI-64
Mac Address

EUI-64 Address

0

0

0

0

0

00

00

0

0

1

26

B0

0

0

26

B0

E5

83

3C

E5

83

3C

E5

83

3C

0

FF

FE

FF

FE

U/L bit
0

0

0

0

0

Interface Identifier

02

26

B0

Overview
–  IPv6 Address Structure

–  Routing Lab Topology Overview
– 
– 
– 
– 
– 
– 
– 

Basic BGP Operation

Training ISP Network Topology
•  Scenario:
– 
– 
– 
– 

Training ISP has 4 main operating area or region
Each region has 2 small POP
Each region will have one datacenter to host content
Regional network are inter-connected with multiple link


Training ISP Topology Diagram

•  Regional Network:
– 
– 
– 
– 

Each regional network will have 3 routers
1 Core & 2 Edge Routers
2 Point of Presence (POP) for every region
POP will use a router to terminate customer network i.e
Edge Router
–  Each POP is an aggregation point of ISP customer

•  Access Network:
– 
– 
– 
– 

Connection between customer network & Edge router
Usually 10 to 100 MBPS link
Separate routing policy from most of ISP
Training ISP will connect them on edge router with
separate customer IP prefix

•  Transport Link:
– 
– 
– 
– 

Inter-connection between regional core router
Higher data transmission capacity then access link
Training ISP has 2 transport link for link redundancy
2 Transport link i.e Purple link & Green link are connected
to two career grade switch


Training ISP Core IP Backbone

•  Design Consideration:
–  Each regional network should have address summarization
capability for customer block and CS link WAN.
–  Prefix planning should have scalability option for next
couple of years for both customer block and infrastructure
–  No Summarization require for infrastructure WAN and
loopback address

–  All WAN link should be ICMP reachable for link monitoring
purpose (At least from designated host)
–  Conservation will get high preference for IPv4 address
planning and aggregation will get high preference for IPv6
address planning.

–  OSPF is running in ISP network to carry infrastructure IP
prefix
–  Each region is a separate OSPF area
–  Transport core is in OSPF area 0
–  Customer will connect on either static or eBGP (Not OSPF)
–  iBGP will carry external prefix within ISP core IP network

Training ISP IPV6 Addressing Plan
•  IPv6 address plan consideration:
–  Big IPv6 address space can cause very very large routing
table size
–  Most transit service provider apply IPv6 aggregation prefix
filter (i.e. anything other then /48 & <=/32 prefix size
–  Prefix announcement need to send to Internet should be
either /32 or /48 bit boundary

•  IPv6 address plan consideration (RFC3177):
–  WAN link can be used on /64 bit boundary
–  End site/Customer sub allocation can be made between /
48~/64 bit boundary
–  APNIC Utilization/HD ratio will be calculated based on /56
end site assignment/sub-allocation

Addressing Plans – ISP Infrastructure
•  What about LANs?
–  /64 per LAN

•  What about Point-to-Point links?
–  Protocol design expectation is that /64 is used
–  /127 now recommended/standardised
•  http://www.rfc-editor.org/rfc/rfc6164.txt
•  (reserve /64 for the link, but address it as a /127)

–  Other options:
•  /126s are being used (mirrors IPv4 /30)
•  /112s are being used

–  Leaves final 16 bits free for node IDs
•  Some discussion about /80s, /96s and /120s too

Addressing Plans – ISP Infrastructure
•  ISPs should receive /32 from their RIR
•  Address block for router loop-back interfaces
–  Generally number all loopbacks out of one /48
–  /128 per loopback

•  Address block for infrastructure
– 
– 
– 
– 

/48 allows 65k subnets
/48 per region (for the largest international networks)
/48 for whole backbone (for the majority of networks)
Summarise between sites if it makes sense

Addressing Plans – Customer
•  Customers get one /48
–  Unless they have more than 65k subnets in which case they get a
second /48 (and so on)

•  In typical deployments today:
–  Several ISPs give small customers a /56 or single LAN end-sites a /
64, e.g.:
–  /64 if end-site will only ever be a LAN
–  /56 for medium end-sites (e.g. small business)
–  /48 for large end-sites
–  (This is another very active discussion area)

Addressing Plans – Advice
•  Customer address assignments should not be reserved or
assigned on a per PoP basis
– 
– 
– 
– 

Same principle as for IPv4
ISP iBGP carries customer nets
Aggregation within the iBGP not required and usually not desirable
Aggregation in eBGP is very necessary

•  Backbone infrastructure assignments:
–  Number out of a single /48
•  Operational simplicity and security

–  Aggregate to minimise size of the IGP

Addressing Plans
Planning
•  Registries will usually allocate the next block to be
contiguous with the first allocation
–  Minimum allocation is /32
–  Very likely that subsequent allocation will make this up to a /31
–  So plan accordingly

Example Address Plan
•  IPv6 Allocation Form Registry is
–  2406:6400::/32

•  IPv4 Allocation From Registry is
–  172.16.0.0/19


•


Table 4: Datacenter prefix summarization options
Block#
12
13
14
15

!

Prefix
2406:6400:0800:0000::/39
2406:6400:0a00:0000::/39
2406:6400:0c00:0000::/39
2406:6400:0e00:0000::/39

Region
Region
Region
Region

1
2
3
4

Description
DC Summary [R2]
DC Summary [R5]
DC Summary [R8]
DC Summary [R11]

Reverse Domain

172.16.2.0/23

R1

R4

1

1
e1/0

e1/3

e1/1
e1/1

fa0/1

e1/1
e1/0

e1/1

49

1

fa0/0

53

e1/3

e1/3

e1/1

172.16.10.76/30

fa0/1

fa0/5
fa0/8
4

fa0/0

e1/0

fa0/1

77

R12

lo 0
172.16.15.7/32

3

4

e1/1

172.16.30.0/23

1

R8

57

lo 0
172.16.15.11/32

lo 0
172.16.15.8/32

lo 0
172.16.15.9/32

58

R9
fa0/0

e1/0

e1/0
78

54

172.16.6.0/23

172.16.4.0/23

Training ISP IPv4 Address Plan

1

172.16.26.0/23

fa0/0

R11

73

fa0/0
50

172.16.10.56/30

82

1

1

lo 0
172.16.15.12/32

R7
e1/0

172.16.10.52/30

172.16.10.80/30

lo 0
172.16.15.10/32

fa0/8

3

172.16.10.72/30

e1/1

172.16.28.0/23

81

fa0/5

SW2

172.16.10.48/30

74

fa0/0
1

30

fa0/11

e0/0

R6

172.16.24.0/23

R10
1
fa0/0

e1/0

e1/3

e1/0

1

fa0/0

e1/1

172.16.10.4/30

1

SW1

34
e1/0

e1/1

172.16.10.0/30

e1/1
e1/1

172.16.18.0/23

fa0/2
fa0/11

lo 0
172.16.15.6/32

172.16.22.0/23

6

1

172.16.10.28/30

172.16.12.0/24

e1/0

fa0/0

29
2

fa0/2

R3

lo 0
172.16.15.4/32

25

fa0/0

1

33

172.16.10.32/30

fa0/1

5

10

R5

172.16.13.0/24

2

lo 0
172.16.15.3/32

R2

1

1

172.16.10.8/30

lo 0
172.16.15.1/32

lo 0
172.16.15.5/32

lo 0
172.16.15.2/32

26

172.16.10.24/30

2
9

fa0/0

e1/0

e1/1

fa0/0

172.16.20.0/23

172.16.16.0/23

172.16.0.0/23

Overview
– 
– 
– 
– 


–  Operation of OSPF Routing Protocol
– 
– 
– 
– 
– 
– 

Basic BGP Operation

OSPF
•  Open Shortest Path First
•  Link state or SPF technology
•  Developed by OSPF working
group of IETF (RFC 1247)
•  OSPFv2 (IPv4) standard
described in RFC2328
•  OSPFv3 (IPv6) standard
described in RFC2740

•  Designed for:
– 
– 
– 
– 
– 
– 

TCP/IP environment
Fast convergence
Variable-length subnet masks
Discontiguous subnets
Incremental updates
Route authentication

•  Runs on IP, Protocol 89

Link State Routing Protocol
Z’s Link State
Q’s Link State
Z

Q

Y

X
X’s Link State

A
B
C

Q
Z
X

2
13
13

Topology Information is kept
in a Database separate from
the Routing Table

What is Link State Routing
•  Do not send full routing table on periodic interval
•  Maintain three tables to collect routing information
–  Neighbor table
–  Topology Table
–  Routing table

•  Use Shortest Path First (SPF) algorithm to select
best path from topology table
•  Send very small periodic (Hello) message to
maintain link condition
•  Send triggered update instantly when network
change occur

Link State Data Structure
•  Neighbor Table
–  List of all recognized neighboring router to whom routing
information will be interchanged

•  Topology Table
–  Also called LSDB which maintain list of routers and their link
information i.e network destination, prefix length, link cost etc

•  Routing table
–  Also called forwarding table contain only the best path to forward
data traffic

Shortest Path First (SPF) Tree

•  Every router in an OSPF network maintain an identical
topology database
•  Router place itself at the root of SPF tree when calculate
the best path

Low Bandwidth Utilisation
LSA

X

R1
LSA

•  Only changes propagated
•  Uses multicast on multi-access broadcast networks

Fast Convergence
•  Detection Plus LSA/SPF
–  Known as the Dijkstra Algorithm

Alternate Path

N1

R1

R2

X

Primary Path

R3

N2

Fast Convergence
•  Finding a new route
– 
– 
– 
– 

LSA flooded throughout area
Acknowledgement based
Topology database synchronised
Each router derives routing table to
destination network

LSA
N1
R1

X

Basic OSPF Operation
•  Neighbor discovery
–  Send L3 multicast message (hello) to discover neighbors

•  Exchanging topology table (LSDB)
–  Send L3 multicast message (DBD packets)

•  Use SPF algorithm to select best path
–  Each router independently calculates best path from an identical
topology database of an OSPF network or area

•  Building up routing table
–  All the SPF selected best paths are installed in routing table for the
traffic to be forwarded

OSPF Neighbor Discovery Process

•  Use IP packet to send hello message. At start routers are at
OSPF Down State
•  Use multicast address 224.0.0.5/FF02::5 to make sure
single IP packet will be forwarded to every router within
OSPF network. Router now at OSPF Init State

OSPF Neighbor Discovery Process

•  All neighboring router with OSPF enabled receive the hello
packet
•  Checks contents of the hello message and if certain
information match it reply (Unicast) to that hello with
sending its router ID in the neighbor list.
•  This is OSPF Two-way State

Contents Of A Hello Packet
•  Required information to build up adjacency:
– 
– 
– 
– 
– 
– 
– 
– 

Router ID of sending router
Hello and dead interval time *
List of neighbors
Network mask
Router priority
Area ID *
DR & BDR IP
Authentication information (If any) *

* Need to match to create neighbor relationship

Discovering Network Information
•  After creating 2-way neighbor relationship neighboring
routers will start exchanging network related information
•  At this stage they will decide who will send network
information first. Router with the highest router ID will start
sending first. This stage is called OSPF Exstart Stage
•  Then they will start exchanging link state database. This
stage is Exchange Stage

Adding Network Information
•  When router receive the LSDB it perform following action:
–  Acknowledge the receipt of DBD by sending Ack packet (LSAck)
–  Compare the information it received with the existing DB (if any)
–  If the new DB is more up to date the router send link state request
(LSR) for detail information of that link. This is Loading Stage

•  When all LSR have been satisfied and all routers has an
identical LSDB this stage is OSPF Full Stage

Maintaining Routing Information
•  Send periodic updates (Hello) to all neighbors to make sure
link with the neighbor is active. I.e 10 sec for LAN
•  Send triggered (Instant) update if any network information
changed
•  Maintain link state sequence number to make sure all
information are up-to-date
•  Sequence number is 4-byte number that begins with
0x80000001 to 0x7fffffff

OSPF Packet Types
•  OSPF use following five packet types to flow routing
information between routers:
-  1: hello [every 10 sec]
•  Hello Builds adjacencies between neighbors

-  2: DBD [Database Descriptor Packet]
•  DBD for database synchronization between routers

-  3: LSR [Link State Request Packet]
•  Requests specific link-state records from router to router

-  4: LSU [Link State Update Packet]
•  Sends specifically requested link-state records

-  5: LSAck [Link State Ack Packet]
•  Acknowledges the above packet types

Format of OSPF Packet

•  All five OSPF packets encapsulated in IP payload (Not
TCP)
•  To ensure reliable deliver using IP packet OSPF use its own
Ack packet (Type 5)

Format of OSPF Packet Header Field
•  Version number
– 

Either OSPF version 2 (IPv4) or version 3 (IPv6)

•  Packet type
–  Differentiates the five OSPF packet types [Type 1 to Type 5]

•  Packet length
–  Length of OSPF packet in bytes

•  Router ID
–  Defines which router is the source of the packet [Not always source address of IP header]

•  Area ID
–  Defines the area where the packet originated

•  Checksum
–  Used for packet-header error-detection to ensure that the OSPF packet was not corrupted during
transmission

•  Authentication type
–  An option in OSPF that describes either clear-text passwords or encrypted Message Digest 5
(MD5) formats for router authentication

Content of OSPF Packet Data
•  Data (for hello packet):
–  Contains a list of known neighbors

•  Data (for DBD packet):
–  Contains a summary of the LSDB, which includes all known router IDs
and their last sequence number, among a number of other fields

•  Data (for LSR packet):
–  Contains the type of LSU needed and the router ID of the needed LSU

•  Data (for LSU packet):
–  Contains the full LSA entry. Multiple LSA entries can fit in one OSPF
update packet

•  Data (for LSAck packet):
–  Is empty

Difference is OSPFv3 for IPv6

•  OSPFv3 still use 32 bit number as router ID
•  So OSPFv3 operation and packet types are same as OSPFv2
•  Change will be in IP header where source address will be interface
address and destination will be FF02::5 which is 128 bit address.
•  Change will be in DBD [t2] and LSU packet [t4] to carry 128 bit prefix

OSPF Network Topology
•  OSPF network can made up of different types of network
links
•  Neighbor relationship behavior will also be different for
each network type
•  It is important for OSPF to be configured correctly based on
its network types to be functioned properly
•  Some network type create neighbor relationship
automatically some need to create it manually

OSPF Network Topology
Broadcast
Multi-access

Point-to-Point

Non Broadcast
Multi-access (NBMA)

X.25/
Frame Relay/
ATM

Broadcast Multi-access Network

•  Generally LAN type of technologies like Ethernet or Token
Ring
•  Neighbor relationship are created automatically
•  DB/BDR election is required
•  Default OSPF hello is 10 sec dead interval is 40 sec

Broadcast Multi-access Network
•  Broadcast network use flooding process to send routing
update
•  Broadcast network use DR/BDR concept to reduce routing
update traffic in the LAN
•  Packet sent to DR/BDR use 224.0.0.6/FF02::6 multicast
address
•  Packets from DR to all other routers use 224.0.0.5/FF02::5
multicast address
•  All neighbor routers form full adjacencies relation with the
DR and BDR only

DB/BDR Election Process
•  Router with the highest priority value is the DR, Second highest
is BDR
•  In the event of tie router with the highest IP address on an
interface become DR and second highest is BDR
•  DR/BDR election can be manipulated by using router-ID
command.
•  In practice loopback IP address is used as router ID and the
highest IP address will become DR, Second highest is BDR
•  The DR/BDR election is non-preemptive
•  Generates network link advertisements
•  Assists in database synchronization

Point-to-Point Network

•  Usually a serial interface running either PPP or HDLC
•  Neighbor relationship are created automatically
•  No DR or BDR election required
•  Default OSPF hello is 10 sec and dead interval is 40 sec

Non Broadcast Multi-access Network
X.25/
Frame Relay/
ATM

•  A single interface interconnects multiple sites like Frame
Relay/ATM/X.25
•  NBMA topologies support multiple routers, but without
broadcasting capabilities
•  OSPF neighbor relation need to create manually, DR/BDR
will be elected
•  Default OSPF hello is 30 sec and dead interval is 120 sec

OSPF Areas
•  Area is a group of
contiguous hosts and
networks
–  Reduces routing traffic

R1

R2

Area 2

Rc

•  Per area topology database
–  Invisible outside the area

•  Backbone area MUST be
contiguous
–  All other areas must be
connected to the backbone

Area 0
Backbone Area

Rd

Rb

Ra

R5
R8

R4

R7

Area 4
R6

Area 3

Area 1
R3

Virtual Links between OSPF Areas
•  Virtual Link is used when it
is not possible to physically
connect the area to the
backbone
•  ISPs avoid designs which
require virtual links
–  Increases complexity
–  Decreases reliability and
scalability

Rc

Area 0
Backbone Area

Rd

Rb

Ra

Area 4
R5
R8

R4

R7

Area 1
R6

R3

Classification of Routers
IR

R1

R2

Area 2

Area 3
Rc

Rb

ABR/BR

•  Internal Router (IR)

Area 0
Rd

Ra

ASBR
To other AS

IR

IR/BR
R5

R4

Area 1
R3

•  Area Border Router (ABR)
•  Backbone Router (BR)
•  Autonomous System
Border Router (ASBR)

OSPF Route Types
IR

R1

Area 2

Area 3
Rc

Rb

ABR/BR

•  Intra-area Route

Area 0
Rd

Ra

–  all routes inside an area

•  Inter-area Route

ASBR
To other AS

IR

R2

R5

R4

Area 1
R3

–  routes advertised from one
area to another by an Area
Border Router

•  External Route
–  routes imported into OSPF
from other protocol or static
routes

External Routes
•  Prefixes which are redistributed into OSPF from other
protocols
•  Flooded unaltered throughout the AS
–  Recommendation: Avoid redistribution!!

•  OSPF supports two types of external metrics
–  Type 1 external metrics
–  Type 2 external metrics (Cisco IOS default)

OSPF

R2
Redistribute

RIP
EIGRP
BGP
Static
Connected
etc.

External Routes
•  Type 1 external metric: metrics are added to the
summarised internal link cost
Cost = 10

R2

to N1
External Cost = 1

R1

Cost = 8
Network
N1
N1

Type 1
11
10

Next Hop
R2
R3

R3

to N1
External Cost = 2

Selected Route

External Routes
•  Type 2 external metric: metrics are compared without
adding to the internal link cost
Cost = 10

R2

to N1
External Cost = 1

R1

Cost = 8
Network
N1
N1

Type 1
1
2

Next Hop
R2
R3

R3

to N1
External Cost = 2

Selected Route

Topology/Link State Database
•  A router has a separate LS database for each area to which
it belongs
•  All routers belonging to the same area have identical
database
•  SPF calculation is performed separately for each area
•  LSA flooding is bounded by area
•  Recommendation:
–  Limit the number of areas a router participates in!!
–  1 to 3 is fine (typical ISP design)
–  >3 can overload the CPU depending on the area topology complexity

Different Types of LSAs
•  Six distinct type of LSAs
– 
– 
– 
– 
– 
– 

Type 1 :
Type 2 :
Type 3 & 4:
Type 5 & 7:
Type 6:
Type 9, 10 & 11:

Router LSA
Network LSA
Summary LSA
External LSA (Type 7 is for NSSA)
Group membership LSA
Opaque LSA (9: Link-Local, 10: Area)

Router LSA (Type 1)
•  Describes the state and cost of the router’s links to the
area
•  All of the router’s links in an area must be described in a
single LSA
•  Flooded throughout the particular area and no more
•  Router indicates whether it is an ASBR, ABR, or end point
of virtual link

Network LSA (Type 2)
•  Generated for every transit broadcast and NBMA network
•  Describes all the routers attached to the network
•  Only the designated router originates this LSA
•  Flooded throughout the area and no more

Summary LSA (Type 3 and 4)
•  Describes the destination outside the area but still in the AS
•  Flooded throughout a single area
•  Originated by an ABR
•  Only inter-area routes are advertised into the backbone
•  Type 4 is the information about the ASBR

External LSA (Type 5 and 7)
•  Defines routes to destination external to the AS
•  Default route is also sent as external
•  Two types of external LSA:
–  E1: Consider the total cost up to the external destination
–  E2: Considers only the cost of the outgoing interface to the external
destination

•  (Type 7 LSAs used to describe external LSA for one
specific OSPF area type)

Types of Areas
•  Regular
•  Stub
•  Totally Stubby
•  Not-So-Stubby
•  Only “regular” areas are useful for ISPs
–  Other area types handle redistribution of other routing protocols into
OSPF – ISPs don’t redistribute anything into OSPF

•  The next slides describing the different area types are
provided for information only

Regular Area (Not a Stub)
•  From Area 1’s point of view, summary networks from other
areas are injected, as are external networks such as X.1
ASBR
X.1
2
3

X.1 External
networks
1
2 X.1

Area 0

X.1
1
3

X.1

1.A

1.C

1.B

1.D

X.1
X.1

2.A

2.C

3.A

2.B
3.C

2.D

3.B

3.D

Normal Stub Area
•  Summary networks, default route injected
•  Command is area x stub
Default
2
3

ASBR

X.1 External
networks
1
2 Default

Area 0

Default
1
3

X.1

1.A

1.C

1.B

1.D

X.1
X.1

2.A

2.C

3.A

2.B
3.C

2.D

3.B

3.D

Totally Stubby Area
•  Only a default route injected
–  Default path to closest area border router

•  Command is area x stub no-summary

ASBR

Totally
Stubby Area

X.1

Default

X.1 External
networks
1
2 Default

Area 0

Default
1
3

1.A

1.C

1.B

1.D

X.1
X.1

2.A

2.C

3.A

2.B
3.C

2.D

3.B

3.D

ISP Use of Areas
•  ISP networks use:
–  Backbone area
–  Regular area

•  Regular area
–  Summarisation of point to point link addresses used within areas
–  Loopback addresses allowed out of regular areas without
summarisation

Overview
– 
– 
– 
– 
– 


–  Lab Exercise on Basic Router and OSPF Dynamic
Routing Configuration
– 
– 
– 
– 
– 

Basic BGP Operation

Configuration of OSPF as IGP
Scenario:
•  Training ISP need to configure OSPF as IGP for both IPv4 and IPv6
•  Dual stack mechanism will be used to ensure both IPv4 and IPv6
operation
•  OSPFv3 supports IPv6 routed protocol
•  IGP is used to carry next hop only for BGP

Minimum Router OS require for OSPF3:
•  Cisco IOS
-  12.2(15)T or later (For OSPFv3)
-  12.2(2)T or later (For IPv6 support)
•  Jun OS
-  JUNOS 8.4 or later

•  Before enabling OSPF3 on an Interface following steps
must be done on a Router:
–  Enable IPv6 unicast routing
–  Enable IPv6 CEF (Optional)
config t!
ipv6 unicast-routing!
ipv6 cef (distributed cef)!

Configure interface for both IPv4 and IPv6:
!
interface e1/0!
description WAN R1-R2!
no ip redirects!
no ip directed-broadcast!
no ip unreachables!
ip address 172.16.10.2 255.255.255.252!
no shutdown!
interface e1/0!
ipv6 address 2406:6400:000F:0000::2/64 !
ipv6 enable!

Verify Interface configuration:
sh ip interface e0/0!
ping 172.16.10.1!

sh ipv6 interface e0/0!
ping 2406:6400:000F:0000::2!

IPv4 Interface configuration for Router1:
interface loopback 0!
description Router1 Loopback!
no ip redirects!
no ip unreachables!
ip address 172.16.15.1 255.255.255.255!
no shutdown!
interface e1/0!
no ip redirects!
no ip unreachables!
ip address 172.16.10.2 255.255.255.252!
no shutdown!

interface e1/1!
no ip redirects!
no ip unreachables!
ip address 172.16.10.9 255.255.255.252!
no shutdown!
interface fa0/0!
description Router1 customer network!
no ip redirects!
no ip unreachables!
no cdp enable!
ip address 172.16.16.1 255.255.255.0!
no shutdown!

ipv6 address 2406:6400:0000:0000::1/128 !
ipv6 enable!
interface e1/0!
ipv6 address 2406:6400:000F:0000::2/64 !
ipv6 enable!
interface e1/1!
ipv6 address 2406:6400:000F:0002::1/64 !
ipv6 enable!
interface fa0/0!
ipv6 address 2406:6400:0100:0000::1/48 !
ipv6 enable!

•  OSPF Configuration for IPv4:
–  OSPF for IPv4 can be configured from global configuration
mode
–  Interface mode configuration will also activate OSPF
process on your running config

•  OSPF Configuration for IPv6:
–  OSPF for IPv6 need to configure from Interface
configuration mode
–  Interface mode configuration will automatically activate
OSPF process on your running config

OSPF for IPv6 Configuration Command:
router ospf 17821!
log-adjacency-changes!
passive-interface default!
network 172.16.15.1 0.0.0.0 area 1!
no passive-interface e1/0!
network 172.16.10.0 0.0.0.3 area 1!
network 172.16.10.8 0.0.0.3 area 1!

OSPF for IPv6 Configuration Command:
ipv6 ospf 17821 area 1!
interface e1/0!
interface e1/1!

Verify OSPF configuration:
sh run!
!!
interface Ethernet1/0!
ip address 172.16.10.2 255.255.255.252!
no ip redirects!
no ip unreachables!
half-duplex!
ipv6 address 2406:6400:F::2/64!
ipv6 enable!

Example OSPF configuration for Router1:
!
router ospf 17821!
log-adjacency-changes!
passive-interface default!
network 172.16.15.1 0.0.0.0 area 1!
network 172.16.10.0 0.0.0.3 area 1!
network 172.16.10.8 0.0.0.3 area 1!

Example OSPF configuration for Router1:
interface e1/0!
interface e1/1!

OSPF Packet Type
Five OSPF Packet Type:
t: Specifies the OSPF packet type:
1: hello
[every 10 sec]
2: DBD
[Database Descriptor Packet]
3: LSR
[Link State Request Packet]
4: LSU
[Link State Update Packet]
5: LSAck
[Link State Ack Packet]
debug ip ospf packet
debug ipv6 ospf packet

Overview
– 
– 
– 
– 
– 
– 


–  Basic BGP Operation
– 
– 
– 
– 


What is Border Gateway Protocol?
•  BGP:
–  A path vector routing protocol to exchange routing information
between different Autonomous System (AS)
–  ASes are the building block of BGP operational unites
–  AS is a collection of routers with a common routing policy
–  Specification is defined in RFC4271

What is an Autonomous System (AS)
•  An AS is a collection of networks with same routing policy
•  Usually under a single administrative control unit
•  A public AS is identified by a unique number called AS
number
•  Around 32000 ASes are visible on the Internet now

BGP features
•  Path Vector Routing Protocol
•  Send incremental updates to peers
•  Runs over TCP –Port 179
•  Select path based on routing policy/ organization’s
business requirement
•  Support Classless Inter Domain Routing (CIDR) concept
•  Widely used in today’s Internet Backbone
•  Current BGP version is MP-BGP

What is Path Vector Routing Protocol
•  A path vector routing protocol is used to span different
autonomous systems
•  It defines a route as a collection of a number of AS that it
passes through from source AS to destination AS
•  This list of ASes are called AS path and used to avoid
routing loop
•  AS path is also used to select path to destination

What is AS path?
•  An AS path example:

BGP Traffic Arrangement Definition
•  Transit
–  Forwarding traffic through the network usually for a fee
–  I.e Internet service from upstream ISP

•  Peering
–  Exchanging traffic without any fee
–  I.e Connection in an IXP

•  Default
–  Where to send traffic if there no explicit route match in the routing
table

What is Default Free Zone?
•  Default free zone is made up of Tire One ISP routers which
have explicit routing information about every part of the
Global Internet
•  So there is no need of default route
•  If there is no destination network match, then that prefix is
still not announced/ used by any ISP yet

ISP Hireracial Connection
•  Connectivity Diagram:

BGP General Operation
•  BGP maintain 3 database i.e Neighbor Table, BGP Table
and Forwarding Table
•  Learns multiple paths via internal and external BGP
speakers
•  Picks the best path and installs them on the forwarding
tables
•  Best path is sent to external BGP neighbors
•  Policies are applied by influencing the best path selection

Constructing the Forwarding Table
•  BGP “In” process
–  Receives path information from peers
–  Results of BGP path selection placed in the BGP table “best path”
flagged

•  BGP “Out” process
–  Announce “best path” information to peers

•  Best path installed in forwarding table if:
–  Prefix and prefix length are equal
–  Lowest protocol distance

Constructing the Forwarding Table
•  Flowchart:
•

BGP Terminology
•  Neighbor
–  Any two routers that have formed a TCP connection to exchange
BGP routing information are called peers or neighbors

•  iBGP
–  iBGP refers to the BGP neighbor relationship within the same AS.
–  • The neighbors do not have to be directly connected.

•  eBGP
–  When BGP neighbor relationship are formed between two peers
belongs to different AS are called eBGP.
–  • EBGP neighbors by default need to be directly connected.

Building Neighbor Relationship
•  After adding BGP neighbor:
–  Both router establish a TCP connection and send open message
–  If open message is accepted then both send keepalive message to
each other to confirm open message
–  After both confirm open message by sending keepalive message
they establish BGP neighbor relationship and exchange routing
information

BGP message type
•  Open Message
–  To establish BGP neighbor relationship

•  Keepalive message
–  Only contain message header to maintain neighbor relationship. Sent
every periodic interval

•  Update message
–  Contain path information. One update message contain one path
information. Multiple path need multiple update message to be sent

•  Notification message
–  Sent when an error condition occur and BGP connection closed
immediately

BGP Open message
•  Open message contain:
–  BGP Version number
–  AS number of the local router
–  BGP holdtime in second to elapse between the successive keepalive
message
–  BGP router ID which is a 32 bit number. Usually an IPv4 address is
used as router ID
–  Optional parameters i.e types, length and value encoded. An
example optional parameter is session authentication info

BGP Keepalive Message
•  Send between BGP peers after every periodic interval (60
Sec)
•  It refresh hold timer from expiration (180sec)
•  A keepalive message contain only the message header

BGP Update Message
•  An update message contain:
–  Withdrawn routes: a list contain address prefix that are withdrawn
from service
–  Path attributes: includes AS path, origin code, local pref etc
–  Network-layer reachablity information: includes a list of address prefix
reachable by this path

BGP Notification message
•  Only sent when an error condition occur and detected in a
network and BGP connection is closed immediately
•  Notification message contain an error code, an error
subcode, and data that are related to that error

BGP Neighbor Relationship States
•  BGP neighbor goes through following steps:
–  Idle: Router is searching its routing table to reach the neighbor
–  Connect: Router found route and completed TCP three-way
handshake
–  Open Sent: Open message sent with the parameter for BGP session
–  Open Confirm: Router receive agreement on the parameter to
establish BGP session
–  Established: Peering is established and routing information exchange
began

Troubleshoot BGP Neighbor Relation
•  Idle:
–  The router can not find address of the neighbor in its routing table

•  Active:
–  Router found address of the neighbor in its routing table sent open
message and waiting for the response from the neighbor

•  Cycle between Active/Idle
–  Neighbor might peer with wrong address
–  Does not have neighbor statement on the other side
–  BGP open message source IP address does not match with remote
side neighbor statement or no route to source IP address

iBGP Peering
•  BGP peer within the same AS
•  Not required to be directly connected
•  iBGP peering require full mesh peering
– 
– 
– 
– 

Within an AS all iBGP speaker must peer with other iBGP speaker
They originate connected network
Pass on prefixes learned from outside AS
They do not forward prefixes learned from other iBGP peer

Training ISP IPV6 Addressing Pan

Training ISP IPv6 Address Plan

iBGP Peering with Loopback Interface

•  If iBGP speakers has multiple connection then it is
advisable to peer with loopback
•  Connected network can go down which might loose iBGP
peering
•  Loopback interface will never go down

iBGP Neighbor Update Source
•  This command allows the BGP process to use the IP
address of a specified interface as the source IP address of
all BGP updates to that neighbor
•  A loopback interface is usually used as it will never goes
down as long as the router is operational
•  All BGP message will use the referenced interface as
source of the messages

eBGP Peering

•  Peering with BGP speaker in different AS
•  Peers should be directly connected and share same WAN
link
•  eBGP neighbors are usually routed through connected
network

BGP Next Hop Behavior
•  BGP is an AS-by-AS routing
protocol not a router-by router
routing protocol.
•  In BGP, the next hop does not
mean the next router it means
the IP address to reach the
next AS
–  I.e Router A advertise
150.10.0.0/16 and 160.10.0.0/16
to router B in eBGP with next hop
150.10.1.1
–  Router B will update Router C in
iBGP keeping the next hop
unchanged

iBGP Next Hop

•  Next hop is iBGP router loopback address
•  Recursive route look-up
•  Loopback address need to announce through IGP (OSPF)

BGP Synchronous Rule
•  BGP do not use or advertise any route to an external
neighbor learned by iBGP until a matching route has been
learned from an IGP i.e OSPF or static
•  It ensure consistency of information throughout the AS
•  Avoid black hole route within an AS
•  It is safe to turn off if all routers with in the AS run full-mesh
iBGP
•  Advisable to disable this feature (BCP)

Overview
– 
– 
– 
– 
– 
– 
– 

Basic BGP Operation

–  BGP Attributes and Path Selection Process
–  BGP Scaling Techniques
–  Lab Exercise on iBGP, eBGP, RR, Peer group etc
–  Internet Exchange [IX] Policy Overview and Configuration requirement

BGP Attributes
BGP metrics are called path attributes. Here
is the classifications BGP attributes:
Well-known mandatory
–  AS-Path
–  Next-hop
–  Origin

Optional transitive
-  Community
-  Aggregator

Well-known discretionary Optional non-transitive
−  Local preference
−  Atomic aggregate

- Multi-exit-discriminator
(MED)

Well-Known Attributes
•  Must be recognized by all compliant BGP implementations
•  Are propagated to other neighbors

Well-Known Mandatory Attributes
- Must be present in all update
messages
- AS Path
- Next-hop
- Origin

Well-Known Discretionary Attributes
- May be present in update messages
- Local preference
- Atomic aggregate

Optional Attributes
•  Recognized by some implementations (could be private)
expected not to be recognized by everyone
•  Recognized optional attributes are propagated to other
neighbors based on their meaning

Optional Transitive Attributes
- If not recognized, are marked as
partial and propagated to other
neighbors
- Community
- Aggregator

Optional Non Transitive attributes
- Discarded if not recognized
- Multi Exit Discriminator (MED)

AS Path Attribute

•  Sequence of ASes a route has traversed
•  Used for
–  Loop detection
–  Path metrics where the length of the AS Path is used as in path
selection

AS Path Loop Detection

•  180.10.0.0/16 is not accepted by AS100 as the prefix has
AS100 in its AS-PATH
•  This is loop detection in action

AS Path Attribute (2 byte and 4 byte)

•  Internet with 16-bit and 32-bit ASNs
–  32-bit ASNs are 65536 and above
–  AS-PATH length maintained

AS Path and AS4 Path Example
Router5:
Network

Next Hop

Metric LocPrf Weight

*> 2001::/32

Path

2406:6400:F:41::1
0 23456 38610 6939 I

*i

2406:6400:D::5 0

*> 2001:200::/32

100 0 45192 4608 4826 6939 i

2406:6400:F:41::1
0 23456 38610 6939 2500 i

*i

2406:6400:D::5 0 100 0 45192 4608 4826 6939 2500 i

eBGP Next Hop

•  The IP address to reach the next AS
–  Router A advertise 150.10.0.0/16 and 160.10.0.0/16 to router
– 
B in eBGP with next hop 150.10.1.1 (Change it to own IP)
–  Router B will update Router C in iBGP keeping the next hop
unchanged

•  Well known mandatory attribute

iBGP Next Hop

•  Next hop is iBGP router loopback address
•  Recursive route look-up
•  Loopback address need to announce through IGP (OSPF)
•  iBGP send update next-hop unchanged

Next Hop Best Practice
•  IOS default is for external next-hop to be propagated
unchanged to iBGP peers
–  This means that IGP has to carry external next-hops
–  Forgetting means external network is invisible
–  With many eBGP peers, it is unnecessary extra load on IGP

•  ISP Best Practice is to change external next-hop to be that
of the local router
–  neighbor x.x.x.x next-hop-self

Next Hop Self Configuration
•  Next hop default behavior can be changed by using nexthop-self command
•  Forces all updates for this neighbor to be advertised with
this router as the next hop
•  The IP address used for next-hop-self will be the same as
the source IP address of the BGP packet

BGP Origin Attribute
•  The origin attribute informs all autonomous systems how
the prefix introduced into BGP
•  Well known mandatory attribute
•  Three values: IGP, EGP, incomplete
–  IGP generated by BGP network statement
–  EGP generated by EGP
–  Incomplete redistributed from another routing protocol

BGP Origin Attribute Example
Status codes: s suppressed, d damped, h history, * valid, >
best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? – incomplete
Network

Next Hop

Metric LocPrf Weight

Path

*> 2001::/32 2406:6400:F:41::1 0 23456 38610 6939 i
*i

2406:6400:D::5 0 100 0 45192 4608 4826 6939 i

BGP Local Preference Attribute
•  Local preference is used to advertise to IBGP neighbors
only about how to leave their AS (Outbound Traffic).
•  Paths with highest preference value are most desirable
•  Local preference attribute is well-known and discretionary
and is passed only within the AS
•  Cisco Default Local Pref is 100

BGP Local Preference Attribute

•  For destination 160.10.0.0/16 Router A advertise local pref
500 and Router B advertise local pref 800 in iBGP
•  800 will win best path (Router B)

BGP Local Pref Attribute Example
Network

Next Hop Metric LocPrf Weight

Path

*> 2001::/32 2406:6400:F:41::1
0 23456 38610 6939 i
*i
6939 i

2406:6400:D::5 0

100 0 45192 4608 4826

*> 2001:200::/32 2406:6400:F:41::1
0 23456 38610 6939 2500 i
*i
2406:6400:D::5
6939 2500 i

0 100 0 45192 4608 4826

BGP MED Attribute
•  MED is used to advertise to EBGP neighbors about how to
exit their AS to reach networks owned by this AS (Incoming
traffic).
•  • MED is sent to EBGP neighbors only.
•  The paths with the lowest MED value are the most
desirable
•  The MED attribute is optional and non transitive

BGP MED Attribute

•  For prefix 120.68.1.0/24 Router B send MED 1000 and
router A send MED 2000 to eBGP neighbor
•  Incoming traffic from AS200 will choose Router B since
lowest MED will win

BGP MED Example
Network Next Hop Metric LocPrf Weight Path
*> 2001::/32 2406:6400:F:41::1
0 23456 38610 6939 i
* i 2406:6400:D::5 0

100 0 45192 4608 4826 6939 i

*> 2001:200::/32 2406:6400:F:41::1
0 23456 38610 6939 2500 i
* i 2406:6400:D::5

0 100 0 45192 4608 4826 6939 2500 i

BGP Community Attribute
•  Community is a tagging technique to mark a set of routes
•  Upstream service provider routers can then use these flags
to apply specific routing polices (i.e local preference etc)
within their network
•  Represented as two 16 bit integers (RFC1998)
•  Common format is <local-ASN>:xx
•  I.e 0:0 to 0:65535 and 65535:0 to 65535:65535 are
reserved
•  Very useful in applying policies within and between ASes
•  Optional & transitive attribute

BGP Route Selection Process
•  Step 1: Prefer highest weight (local to router)
•  Step 2: Prefer highest local preference (global within AS)
•  Step 3: Prefer route originated by the local router
•  Step 4: Prefer shortest AS path
•  Step 5: Prefer lowest origin code (IGP < EGP < incomplete)
•  Step 6: Prefer lowest MED (from other AS)
•  Step 7: Prefer EBGP path over IBGP path
•  Step 8: Prefer the path through the closest IGP neighbor
•  Step 9: Prefer oldest route for EBGP paths
•  Step 10: Prefer the path with the lowest neighbor BGP router ID

Overview
– 
– 
– 
– 
– 
– 
– 
– 

Basic BGP Operation

–  BGP Scaling Techniques

BGP Peer Group
•  Defines a template with parameters set for a group of
neighbors instead of individually
•  Useful when many neighbors have the same outbound
policies
•  Members can have a different inbound policy
•  Updates generated once per peer group
•  Simplifies configuration

BGP Peer Group
•  Problem – how to scale iBGP
–  Large iBGP mesh slow to build
–  iBGP neighbors receive the same update
–  Router CPU wasted on repeat calculations

•  Solution – peer-groups
–  Group peers with the same outbound policy
–  Updates are generated once per group

BGP Peer Group -Advantages
•  Makes configuration easier
•  Makes configuration less prone to error
•  Makes configuration more readable
•  Lower router CPU load
•  iBGP mesh builds more quickly
•  Members can have different inbound policy
•  Can be used for eBGP neighbors too!

BGP Peer Group -BCP
•  Always configure peer-groups for iBGP
–  Even if there are only a few iBGP peers
–  Easier to scale network in the future

•  Consider using peer-groups for eBGP
–  Especially useful for multiple BGP customers using same AS
(RFC2270)
–  Also useful at Exchange Points where ISP policy is generally the
same to each peer

BGP Route Reflector
•  In a transit AS all router in the core need to know the
complete routing table coming from Internet
•  Global routing table size is above 300k prefix
•  Practically impossible to redistribute these route in IGP i.e
OSPF
•  Solution is to forward these large routing information by
iBGP

BGP Route Reflector
•  iBGP use TCP to make reliable delivery of these large
routing information across all core router in a transit ISP
•  TCP can not broadcast so need to make individual delivery
to all iBGP speaker
•  To protect routing loop iBGP use split horizon rule so can
not send routing information to neighbor learn via iBGP
•  So iBGP need full mesh peering with other core router in a
transit ISP

BGP Route Reflector

•  Avoid 1⁄2n(n-1) iBGP mesh
•  n=1000 ⇒ nearly half a million iBGP sessions!
•  Solution -Route reflector

BGP Route Reflector

•  There will be Reflector, Client and Non-Client
•  Reflector receives path from clients and non-clients
•  Select the best path then If best path is from client, reflect to other
clients and non-clients
•  If best path is from non-client, reflect to clients only
•  Described in RFC4456

Overview
– 
– 
– 
– 
– 
– 
– 
– 
– 

Basic BGP Operation


Case study- Deployment IPv6 in EGP
•  Scenario:
–  BGP4 is used in Training ISP network
–  iBGP is used between internal routers in Training ISP to
carry external prefixes (i.e Customer & Global Internet
Prefixes)
–  Route Reflector is used to resolve iBGP full mesh
scalability issue.

•  Scenario:
–  Transit service with upstream ASes is configured with
eBGP
–  Customer network from downstream can also be
configured with eBGP or static
–  Training ISP is having one native IPv6 transit and one
tunnel IPv6 transit with AS45192 & AS131107 (2.35 as
dot)

•  Basic BGP Configuration:
router bgp 17821!
address-family ipv6!
no synchronization!

Adding iBGP Neighbor:
!
router bgp 17821!
!
neighbor 2406:6400:0000:0000::2 remote-as 17821!
neighbor 2406:6400:0000:0000::2 update-source loopback 0!
neighbor 2406:6400:0000:0000::2 activate!
!

iBGP neighbor is always recommended with loopback interface

Announcing IPv6 Prefix:
!
router bgp 17821!
!

!!
network 2406:6400:0100:0000::/48!

Add Pull-up route if needed:!
router bgp 17821!

!

!!
network 2406:6400:0100:0000::/48!
exit!
exit!
ipv6 route 2406:6400:0100:0000::/48 null 0!

IPv4 iBGP Conf POP Router
Router1
config t!
router bgp 17821!
no auto-summary!
no synchronization!
neighbor 172.16.15.2 remote-as 17821!
neighbor 172.16.15.2 update-source loopback 0!
neighbor 172.16.15.2 activate!
network 172.16.16.0 mask 255.255.254.0!
exit!
exit!
ip route 172.16.16.0 255.255.254.0 null 0 permanent!
exit!
wr!

IPv4 iBGP Configuration Verification
POP Router
sh bgp ipv4 unicast summary!
sh bgp ipv4 unicast!
sh ip route bgp!
sh bgp ipv4 unicast neighbors [router 1……router12
loopback] advertised-routes!
loopback] received-routes!
sh ip route [R2, R5, R8, R11 datacenter prefix]!

IPv6 iBGP Conf POP Router
Router1!
config t!
router bgp 17821!
no synchronization!
network 2406:6400:0100:0000::/45 !
exit!
exit!
ipv6 route 2406:6400:0100:0000::/45 null 0!
exit!

!

!

wr!

POP Router
sh ipv6 route bgp!
sh ipv6 route [R2, R5, R8, R11 datacenter prefix]!

IPv4 iBGP Conf Core Router
Router2 Configuration
"config t!

router bgp 17821!
no auto-summary!
no synchronization!
network 172.16.0.0 mask 255.255.254.0!
exit!
exit!
ip route 172.16.0.0 255.255.254.0 null 0 permanent!
exit!
Wr!

Core Router
sh ip route bgp!
sh ip route [R2, R5, R8, R11 datacenter prefix]!

IPv6 iBGP Conf Core Router
Router2 Configuration
"config t!

!

!

router bgp 17821!
no synchronization!
network 2406:6400:0001:0000::/48!
exit!
exit!
ipv6 route 2406:6400:0001:0000::/48 null 0!
exit!
wr!

•  Core Router
sh ipv6 route bgp!
sh ipv6 route [R2, R5, R8, R11 datacenter prefix]!

iBGP Full Mesh Issue
Route reflector configuration:
!
router bgp 17821!
!

!!
neighbor 2406:6400:0000:0000::1 route-reflector-client!

iBGP Peering For Region 2

268

IPv6 IOS Command For eBGP
Adding eBGP Neighbor:
!
router bgp 17821!
!
neighbor 2406:6400:000D:0000::5 remote-as 45192!
neighbor 2406:6400:000D:0000::5 activate!
!
!

eBGP neighbor is always recommended with directly connected interface

IPv6 Native Transit Configuration
•  Router5!
config t!
router bgp 17821!
neighbor 2406:6400:000D:0000::5
neighbor 2406:6400:000D:0000::5
neighbor 2406:6400:000E:0000::5
neighbor 2406:6400:000E:0000::5
exit!
exit!
exit!
Wr!

remote-as 45192!
activate!
remote-as 45192!
activate!

IPv6 Tunnel Transit Configuration

6 to 4 Tunnel Configuration
IOS Command for Tunnel Interface:!
Router2
config t!
interface Tunnel0!
tunnel source 172.16.15.2!
tunnel destination 192.168.1.1!
tunnel mode ipv6ip!
ipv6 address 2406:6400:F:40::2/64!
ipv6 enable!

6 to 4 Tunnel Configuration
IOS Command for Tunnel Peering:
!
!

router bgp 17821!
neighbor 2406:6400:F:40::1 remote-as 23456!
!

neighbor 2406:6400:F:40::1 activate!

Overview
– 
– 
– 
– 
– 
– 
– 
– 
– 
– 

Basic BGP Operation

–  Internet Exchange [IX] Policy Overview and
Configuration requirement

Case Study- IXP Configuration
•  Two type of traffic exchange between ISPs
•  Transit
–  Where ISP will pay to send/receive traffic
–  Downstream ISP will pay upstream ISP for transit service

•  Peering
–  ISPs will not pay each other to interchange traffic
–  Works well if win win for both
–  Reduce cost on expensive transit link

IX Peering Model
•  BLPA (Bi-Lateral Peering Agreement)
–  IX will only provide layer two connection/switch port to ISPs
–  Every ISPs will arrange necessary peering arrangement with others
by their mutual business understanding.

•  MLPA (Multi-Lateral Peering Agreement)
–  IX will provide layer two connection/switch port to ISPs
–  Each ISP will peer with a route server on the IX.
–  Route server will collect and distribute directly connected routes to
every peers.

IXP Peering Policy
•  BLPA is applicable where different categories of ISPs are
connected in an IX
–  Large ISPs can choose to peer with large ISPs (base on their traffic
volume)
–  Small ISPs will arrange peering with small ISPs

•  Would be preferable for large ISPs
–  They will peer with selected large ISPs (Equal traffic interchange)
–  Will not loose business by peering with small ISP

IXP Peering Policy
•  MLPA model works well to widen the IX scope of operation
(i.e national IX).
•  Easy to manage peering
–  Peer with the route server and get all available local routes.
–  Do not need to arrange peering with every ISPs connected to the IX.

•  Unequal traffic condition can create not intersected situation
to peer with route server

IXP Peering Policy
•  Both peering model can be available in an IX.
•  Member will select peering model i.e either BLPA or MLPA
(Route Server Peering)
•  IX will provide switch port
•  Mandatory MLPA model some time not preferred by large
ISP (Business Interest)
–  Can create not interested situation to connect to an IX

IXP Operating Cost
•  Access link
•  Link maintenance
•  Utility
•  Administration

IXP Cost Model
•  Not for profit
•  Cost sharing
•  Membership based
•  Commercial IX

•  Required global & interface commands to enable IPv6
Router(Config)#ipv6 unicast-routing 
Router(Config)#ipv6 cef (optional)"

•  Configure IPv6 address on interface
Router(Config-if)#ipv6 address 2001:0df0:00aa::1/64 
Router(Config-if)#ipv6 enable"

•  Verify IPv6 configuration
Router#sh ipv6 interface fa0/0"

•  Verify connectivity
Router#ping

2001:0df0:00aa::1"

•  Required BGP commands to enable IPv6 routing
Router(config)# router bgp 1 
Router2(config-router)#bgp router-id 10.0.0.1
address on any interface)

(if no 32 bit

Router(config-router)# address-family ipv6 
Router(config-router-af)# no synchronization 
Router(config-router-af)#neighbor 2001:0df0:00aa::1
remote-as 2 (EBGP) 
Router(config-router-af)#neighbor 2001:0df0:00aa::1
activate 
Router(config-router-af)#network 2001:0df0:00aa::/48
•  Verify BGP IPv6 configuration
Router#sh bgp ipv6 unicast summary (summarized
neighbor list) 
Router#sh bgp ipv6 unicast (BGP database)  
Router#sh ipv6 route bgp (BGP routing table)"

Required command to add IX prefix filter
•  Create prefix filter in global mode
Router(config)#ipv6 prefix-list AS1 seq 2 permit 2001:0df0:aa::/48"

•  Apply prefix filter in BGP router configuration mode
Router(config-router)# address-family ipv6"
Router(config-router-af)#neighbor 2001:0df0:aa::1
prefix-list AS1 in"
Router(config-router-af)#neighbor 2001:0df0:aa::1
prefix-list AS1 out"

•  Controlling routing update traffic (Not data traffic)
•  Incoming routing update (Will control outgoing data traffic)
•  Outgoing routing update (Will control incoming data traffic)

Wrou01

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Wrou01 (20)

Recently uploaded (20)

Wrou01