bgp.ppt

Border Gateway Protocol (BGP4)

Border Gateway Protocol (BGP)
• Routing/Forwarding basics
• Building blocks
• Exercises
• BGP protocol basics
• Exercises
• BGP path attributes
• Best path computation
• Exercises

Border Gateway Protocol (BGP)...
• Typical BGP topologies
• Routing Policy
• Exercises
• Redundancy/Load sharing
• Best current practices

IP route lookup:Longest match
routing
R2
R3
R1 R4
All 10/8 except
10.1/16
10.1/16
Packet: Destination
IP address: 10.1.1.1
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
30/8 -> R6
…..
R2’s IP routing table

IP route lookup: Longest match
routing
R2
R3
R1 R4
All 10/8 except
10.1/16
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
10.1.1.1 & FF.0.0.0
is equal to
10.0.0.0 & FF.0.0.0
Match!
Packet: Destination

routing
R2
R3
R1 R4
All 10/8 except
10.1/16
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
10.1.1.1 & FF.FF.0.0
is equal to
10.1.0.0 & FF.FF.0.0
Match as well!
Packet: Destination

routing
R2
R3
R1 R4
All 10/8 except
10.1/16
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
10.1.1.1 & FF.0.0.0
is equal to
20.0.0.0 & FF.0.0.0
Does not match!
Packet: Destination

routing
R2
R3
R1 R4
All 10/8 except
10.1/16
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
Packet: Destination
Longest match, 16 bit netmask

routing
• default is 0.0.0.0/0
• can handle it using the normal longest
match algorithm
• matches everything. Always the shortest
match.

Forwarding
• Uses the routing table built by routing
protocols
• Performs the lookup to find next-hop and
outgoing interface
• Switches the packet with new encapsulation
as per the outgoing interface

Building Blocks
• Autonomous System (AS)
• Types of Routes
• IGP/EGP
• DMZ
• Policy
• Egress
• Ingress

Autonomous System (AS)
• Collection of networks with same policy
• Single routing protocol
• Usually under single administrative control
• IGP to provide internal connectivity
AS 100

Autonomous System(AS)...
• Identified by ‘AS number’
• Public & Private AS numbers
• Examples:
– Service provider
– Multi-homed customers
– Anyone needing policy discrimination

Routing flow and packet flow
For networks in AS1 and AS2 to communicate:
AS1 must announce routes to AS2
AS2 must accept routes from AS1
AS2 must announce routes to AS1
AS1 must accept routes from AS2
accept
announce
announce
accept
AS 1 AS2
packet
flow
packet flow
Routing flow
egress
ingress

Egress Traffic
• Packets exiting the network
• Based 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
• Packets entering your network
• Ingress traffic depends on:
– What information you send and to who
– Based on your addressing and ASes
– Based on others’ policy (what they accept from
you and what they do with it)

Types of Routes
• Static Routes
– configured manually
• Connected Routes
– created automatically when an interface is ‘up’
• Interior Routes
– Routes within an AS
• Exterior Routes
– Routes exterior to AS

What Is an IGP?
• Interior Gateway Protocol
• Within an Autonomous System
• Carries information about internal prefixes
• Examples—OSPF, ISIS, EIGRP…

What Is an EGP?
• Exterior Gateway Protocol
• Used to convey routing information between
ASes
• De-coupled from the IGP
• Current EGP is BGP4

Why Do We Need an EGP?
• Scaling to large network
– Hierarchy
– Limit scope of failure
• Define administrative boundary
• Policy
– Control reachability to prefixes

• Interior
– Automatic
discovery
– Generally trust
your IGP routers
– Routes go to all
IGP routers
• Exterior
Specifically configured
peers
Connecting with outside
networks
Set administrative
boundaries
Interior vs. Exterior
Routing Protocols

Hierarchy of Routing Protocols
Local NAP
FDDI
Other ISP’s
BGP4 BGP4/Static
BGP4 / OSPF
Customers
BGP4

AS 100 AS 101
AS 102
DMZ
Network
A
B
C
D
E
• Shared network between ASes
Demilitarized Zone (DMZ)

Addressing - ISP
• Need to reserve address space for its
network.
• Need to allocate address blocks to its
customers.
• Need to take “growth” into consideration
• Upstream link address is allocated by
upstream provider

BGP Basics
• Terminology
• Protocol Basics
• Messages
• General Operation
• Peering relationships (EBGP/IBGP)
• Originating routes

Terminology
• Neighbor
– Configured BGP peer
• NLRI/Prefix
– NLRI - network layer reachability information
– Reachability information for a IP address &
mask
• Router-ID
– Highest IP address configured on the router
• Route/Path
– NLRI advertised by a neighbor

Protocol Basics
• Routing protocol used
between ASes
–if you aren’t connected
to multiple ASes, you
don’t need BGP :)
• Runs over TCP
• Path vector protocol
AS 100 AS 101
AS 102
E
B D
A C
Peering

BGP Basics ...
• Each AS originates a set of NLRI
• NLRI is exchanged between BGP peers
• Can have multiple paths for a given prefix
• Picks the best path and installs in the IP
forwarding table
• Policies applied (through attributes)
influences BGP path selection

AS 100 AS 101
AS 102
A C
BGP speakers
are called peers
BGP Peers
eBGP TCP/IP
Peer Connection
Peers in different AS’s
are called External Peers
Note: eBGP Peers normally should be directly connected.
E
B D
220.220.8.0/24 220.220.16.0/24
220.220.32.0/24

AS 100 AS 101
A C
BGP speakers are
called peers
BGP Peers
iBGP TCP/IP
Peer Connection
Peers in the same AS
are called Internal Peers
AS 102
E
B D
Note: iBGP Peers don’t have to be directly connected.
220.220.8.0/24 220.220.16.0/24
220.220.32.0/24

AS 100 AS 101
A C
BGP Peers
AS 102
D
220.220.8.0/24 220.220.16.0/24
220.220.32.0/24
E
B
BGP Peers exchange
Update messages
containing Network
Layer Reachability
Information (NLRI)
BGP Update
Messages

Configuring BGP Peers
interface Serial 0
ip address 222.222.10.2 255.255.255.252
router bgp 100
network 220.220.8.0 mask 255.255.255.0
neighbor 222.222.10.1 remote-as 101
interface Serial 0
ip address 222.222.10.1 255.255.255.252
router bgp 101
network 220.220.16.0 mask 255.255.255.0
eBGP TCP Connection
• BGP Peering sessions are established using the BGP
“neighbor” configuration command
222.222.10.0/30
B C D
A
AS 100 AS 101
.2
220.220.8.0/24 220.220.16.0/24
.2 .1 .2 .1
.1
– External (eBGP) is configured when AS numbers are different

– Internal (iBGP) is configured when AS numbers are same
AS 100 AS 101
222.222.10.0/30
.2
interface Serial 1
ip address 220.220.16.2 255.255.255.252
router bgp 101
network 220.220.16.0 mask 255.255.255.0
B
interface Serial 1
ip address 222.220.16.1 255.255.255.252
router bgp 101
network 220.220.16.0 mask 255.255.255.0
C
iBGP TCP Connection
• BGP Peering sessions are established using the BGP
“neighbor” configuration command
D
220.220.8.0/24 220.220.16.0/24
A .2 .1 .2 .1
.1
– External (eBGP) is configured when AS numbers are different

• Each iBGP speaker must peer with every other
iBGP speaker in the AS
iBGP TCP/IP
Peer Connection
AS 100
A
B
C

• Loopback interface are normally used as
peer connection end-points
AS 100
215.10.7.1
215.10.7.2
215.10.7.3
A
B
C
iBGP TCP/IP
Peer Connection

iBGP TCP/IP
Peer Connection
AS 100
A
215.10.7.1
215.10.7.2
215.10.7.3
C
B
interface loopback 0
ip address 215.10.7.1 255.255.255.255
router bgp 100
network 220.220.1.0
neighbor 215.10.7.2 update-source loopback0
A

AS 100
A
215.10.7.1
215.10.7.2
215.10.7.3
C
A
ip address 215.10.7.2 255.255.255.255
router bgp 100
network 220.220.5.0
B
iBGP TCP/IP
Peer Connection

AS 100
A
215.10.7.1
215.10.7.2
215.10.7.3
A
B
ip address 215.10.7.3 255.255.255.255
router bgp 100
network 220.220.1.0
C
iBGP TCP/IP
Peer Connection

BGP Updates — NLRI
• Network Layer Reachability Information
• Used to advertise feasible routes
• Composed of:
– Network Prefix
– Mask Length

BGP Updates — Attributes
• Used to convey information associated with
NLRI
– AS path
– Next hop
– Local preference
– Multi-Exit Discriminator (MED)
– Community
– Origin
– Aggregator

• Sequence of ASes a route
has traversed
• Loop detection
• Apply policy
AS 100
AS 300
AS 200
AS 500
AS 400
170.10.0.0/16 180.10.0.0/16
150.10.0.0/16
Network Path
180.10.0.0/16 300 200 100
170.10.0.0/16 300 200
150.10.0.0/16 300 400
Network Path
180.10.0.0/16 300 200 100
170.10.0.0/16 300 200
AS-Path Attribute

160.10.0.0/16
150.10.0.0/16
192.10.1.0/30
.2
AS 100
AS 200
Network Next-Hop Path
160.10.0.0/16 192.20.2.1 100
C
Next Hop Attribute
.1
BGP Update
Messages
B
A
.1
.2
AS 300
E
D
• Next hop to reach a network
• Usually a local network is the next
hop in eBGP session
140.10.0.0/16

• Next hop to reach a network
• Usually a local network is the next
hop in eBGP session
160.10.0.0/16
150.10.0.0/16
192.10.1.0/30
.2
AS 100
AS 200
C
Next Hop Attribute
.1
B
A
.1
.2
BGP Update
Messages
E
D
• Next Hop updated between
eBGP Peers
AS 300
140.10.0.0/16
150.10.0.0/16 192.10.1.1 200
160.10.0.0/16 192.10.1.1 200 100

• Next hop not changed
between iBGP peers
160.10.0.0/16
150.10.0.0/16
192.10.1.0/30
.2
AS 100
AS 200
150.10.0.0/16 192.10.1.1 200
160.10.0.0/16 192.10.1.1 200 100
C
Next Hop Attribute
.1
B
A
.1
.2
BGP Update
Messages
D
E
AS 300
140.10.0.0/16

Next Hop Attribute (more)
• IGP should carry route to next hops
• Recursive route look-up
• Unlinks BGP from actual physical topology
• Allows IGP to make intelligent forwarding
decision

BGP Updates —
Withdrawn Routes
• Used to “withdraw” network reachability
• Each Withdrawn Route is composed of:
– Network Prefix
– Mask Length

BGP Updates —
Withdrawn Routes
AS 321
AS 123
192.168.10.0/24
192.192.25.0/24
.1 .2
x
Connectivity lost
BGP Update
Message
Withdraw Routes
192.192.25.0/24
150.10.0.0/16 192.168.10.2 321 200
192.192.25.0/24 192.168.10.2 321

BGP Routing Information Base
BGP RIB
D 10.1.2.0/24
D 160.10.1.0/24
D 160.10.3.0/24
R 153.22.0.0/16
S 192.1.1.0/24
router bgp 100
network 160.10.0.0 255.255.0.0
no auto-summary
Route Table
*>i160.10.1.0/24 192.20.2.2 i
*>i160.10.3.0/24 192.20.2.2 i
BGP ‘network’ commands are normally
used to populate the BGP RIB with
routes from the Route Table

BGP RIB
router bgp 100
network 160.10.0.0 255.255.0.0
aggregate-address 160.10.0.0 255.255.0.0 summary-only
no auto-summary
Route Table
D 10.1.2.0/24
D 160.10.1.0/24
D 160.10.3.0/24
R 153.22.0.0/16
S 192.1.1.0/24
*> 160.10.0.0/16 0.0.0.0 i
* i 192.20.2.2 i
s> 160.10.1.0/24 192.20.2.2 i
s> 160.10.3.0/24 192.20.2.2 i
BGP ‘aggregate-address’ commands
may be used to install summary routes
in the BGP RIB

BGP ‘redistribute’ commands can also
be used to populate the BGP RIB with
routes from the Route Table
BGP RIB
router bgp 100
network 160.10.0.0 255.255.0.0
redistribute static route-map foo
no auto-summary
access-list 1 permit 192.1.0.0 0.0.255.255
route-map foo permit 10
match ip address 1
Route Table
D 10.1.2.0/24
D 160.10.1.0/24
D 160.10.3.0/24
R 153.22.0.0/16
S 192.1.1.0/24
*> 160.10.0.0/16 0.0.0.0 i
* i 192.20.2.2 i
s> 160.10.1.0/24 192.20.2.2 i
s> 160.10.3.0/24 192.20.2.2 i
*> 192.1.1.0/24 192.20.2.2 ?

BGP RIB
IN Process
Update
173.21.0.0/16 192.20.2.1 100
* 173.21.0.0/16 192.20.2.1 100
• BGP “in” process
• receives path information from peers
• results of BGP path selection placed in the BGP table
• “best path” flagged (denoted by “>”)
Update
*>i160.10.1.0/24 192.20.2.2 i
*>i160.10.3.0/24 192.20.2.2 i
OUT Process
>

OUT Process
160.10.1.0/24 192.20.2.2 200
160.10.3.0/24 192.20.2.2 200
173.21.0.0/16 192.20.2.2 200 100
192.20.2.1
BGP RIB
> 173.21.0.0/16 192.20.2.1 100
*>i160.10.1.0/24 192.20.2.2 i
*>i160.10.3.0/24 192.20.2.2 i
*
IN Process
Update Update
• BGP “out” process
• builds update using info from RIB
• may modify update based on config
• Sends update to peers
Next-Hop changed

BGP RIB
D 10.1.2.0/24
D 160.10.1.0/24
D 160.10.3.0/24
R 153.22.0.0/16
S 192.1.1.0/24
*>i160.10.1.0/24 192.20.2.2 i
*>i160.10.3.0/24 192.20.2.2 i
*> 173.21.0.0/16 192.20.2.1 100
• Best paths installed in routing table if:
B 173.21.0.0/16
Route Table
• prefix and prefix length are unique
• lowest “protocol distance”

The ‘Bible’ & other resources
• Route-views.oregon-ix.net
• Internet Routing Architectures
– Bassam Halabi
– pg. 168 BGP Decision Process Summary

Types of BGP Messages
• OPEN
– To negotiate and establish peering
• UPDATE
– To exchange routing information
• KEEPALIVE
– To maintain peering session
• NOTIFICATION
– To report errors (results in session reset)

Internal BGP Peering (IBGP)
• BGP peer within the same AS
• Not required to be directly connected
• Maintain full IBGP mesh or use Route Reflection
AS 100
A
E
B
D

External BGP Peering (EBGP)
AS 100 AS 101
C
B
A
• Between BGP speakers in different AS
• Directly connected or peering address is reachable

An Example…
Learns about 35.0.0.0/8 from F & D
AS3561
B
E
C
D
F
A
AS200
AS101
AS21
AS675
35.0.0.0/8

Basic BGP commands
Configuration commands
router bgp <AS-number>
neighbor <ip address> remote-as <as-number>
Show commands
show ip bgp summary
show ip bgp neighbors

Originating routes...
• Using network command or redistribution
network <ipaddress>
redistribute <protocol name>
• Requires the route to be present in the
routing table

Originating routes/Inserting
prefixes into BGP
• network command
• network 198.10.4.0 mask 255.255.254.0
• ip route 198.10.0.0 255.255.254.0 serial 0
• matching route must exist in the routing
table before network is announced!
• Origin: IGP

Update message
• Withdrawn routes
• Path Attributes
• Advertised routes

Stable IBGP peering
• Unlinks IBGP peering from physical
topology.
• Carry loopback address in IGP
router ospf <ID>
passive-interface loopback0
• Unlink peering from physical topology
router bgp <AS1>
neighbor <x.x.x.x> remote-as <AS1>
neighbor <x.x.x.x> update-source loopback0

BGP Path Attributes: Why ?
• Encoded as Type, Length & Value (TLV)
• Transitive/Non-Transitive attributes
• Some are mandatory
• Used in path selection
• To apply policy for steering traffic

BGP Path Attributes...
• Origin
• AS-path
• Next-hop
• Multi-Exit Discriminator (MED)
• Local preference
• BGP Community
• Others...

AS-PATH
• Updated by the sending router with its AS
number
• Contains the list of AS numbers the update
traverses.
• Used to detect routing loops
– Each time the router receives an update, if it
finds its AS number, it discards the update

• Sequence of ASes a route has
traversed
• Loop detection
AS-Path
AS 100
AS 300
AS 200
AS 500
AS 400
170.10.0.0/16 180.10.0.0/16
150.10.0.0/16
180.10.0.0/16 300 200 100
170.10.0.0/16 300 200
150.10.0.0/16 300 400
180.10.0.0/16
dropped

Next-Hop
• Next hop router to reach a network
• Advertising router/Third party in EBGP
• Unmodified in IBGP
160.10.0.0/16
150.10.0.0/16
150.10.1.1 150.10.1.2
AS 100
AS 300
AS 200
150.10.0.0/16 150.10.1.1
160.10.0.0/16 150.10.1.1
A B
20
Cisco Systems Confidential
0799_04F7_c2

Third Party Next Hop
192.68.1.0/24
150.1.1.3
150.1.1.1
150.1.1.2
192.68.1.0/24 150.1.1.3
AS 201
AS 200
C
A B
• More efficient, but
bad idea!
peering

Next Hop...
• IGP should carry route to next hops
• Recursive route look-up
• Unlinks BGP from actual physical topology
• Allows IGP to make intelligent forwarding
decision

Local Preference
• Not for EBGP, mandatory for IBGP
• Default value is 100 on Ciscos
• Local to an AS
• Used to prefer one exit over another
• Path with highest local preference wins

Local Preference
AS 400
AS 200
160.10.0.0/16
AS 100
AS 300
160.10.0.0/16 500
> 160.10.0.0/16 800
500 800 E
B
C
A
D

Multi-Exit Discriminator
• Non-transitive
• Represented as a numeric value (0-0xffffffff)
• Used to convey the relative preference of entry points
• Comparable if paths are from the same AS
• Path with lower MED wins
• IGP metric can be conveyed as MED

Multi-Exit Discriminator (MED)
AS 201
AS 200
192.68.1.0/24
C
A B
192.68.1.0/24 1000
192.68.1.0/24 2000
preferred

Origin
• Conveys the origin of the prefix
• Three values:
– IGP - Generated using “network” statement
• ex: network 35.0.0.0
– EGP - Redistributed from EGP
– Incomplete - Redistribute IGP
• ex: redistribute ospf
• IGP < EGP < INCOMPLETE

Communities
• Transitive, Non-mandatory
• Represented as a numeric value (0-0xffffffff)
• Used to group destinations
• Each destination could be member of multiple
communities
• Flexibility to scope a set of prefixes within or
across AS for applying policy

Customer AS 201
Service Provider AS 200
192.68.1.0/24
C
A B
Community:201:110 Community:201:120
D
Community Local Preference
201:110 110
201:120 120
Community...

Synchronization
• C not running BGP (non-pervasive BGP)
• A won’t advertise 35/8 to D until the IGP is in sync
• Turn synchronization off!
– Run pervasive BGP
router bgp 1880
no sync
1880
209
690
B
A
C
35/8
D OSPF

BGP Route Selection (bestpath)
Only one path as the bestpath !
• Route has to be synchronized
Prefix in forwarding table
• Next-hop has to be accessible
Next-hop in forwarding table
• Largest weight
Local to the router
• Largest local preference
Spread within AS
• Locally sourced
Via redistribute or network statement

BGP Route Selection ...
• Shortest AS-path length
number of ASes in the AS-path attribute
• Lowest origin
IGP < EGP < INCOMPLETE
• Lowest MED
between paths from same AS
• External over internal
closest exit from a router
• Closest next-hop
Lower IGP metric, closer exit from as AS
• Lowest router-id
• Lowest IP address of neighbor

BGP Route Selection...
AS 400
AS 200
AS 100
AS 300
B
A
D
AS 400’s Policy to reach AS100
AS 200 preferred path
AS 300 backup
Increase AS path attribute
length by at least 1

Stub AS
• Typically no need for BGP
• Point default towards the ISP
• ISP advertises the stub network to
Internet
• Policy confined within ISP policy

Stub AS
AS 100
AS 101
B
A
Provider
Customer

Multi-homed AS
• Only border routers speak BGP
• IBGP only between border routers
• Exterior routes must be redistributed in
a controlled fashion into IGP or use
defaults

Multi-homed AS
AS 100
AS 200
AS 300
D
C
B
A
provider
provider
customer

Service Provider Network
• IBGP used to carry exterior routes
• IGP keeps track of topology
• Full IBGP mesh is required

Common Service Provider
Network
AS 100 AS 200
AS 400
AS 300
F
E
D
G
H
C
B
A
provider

Routing Policy
• Why?
– To steer traffic through preferred paths
– Inbound/Outbound prefix filtering
– To enforce Customer-ISP agreements
• How ?
– AS based route filtering - filter list
– Prefix based route filtering - distribute list
– BGP attribute modification - route maps

Distribute list - using IP access lists
access-list 1 deny 10.0.0.0
access-list 1 permit any
access-list 2 permit 20.0.0.0
… more access-lists as prefixes are added ...
router bgp 100
neighbor 171.69.233.33 distribute-list 1 in
neighbor 171.69.233.33 distribute-list 2 out

Filter list rules
Regular Expressions
• RE is a pattern to match against an input
string
• Used to match against AS-path attribute
• ex: ^3561.*100.*1$
• Flexible enough to generate complex filter
list rules

Filter list - using as-path access list
ip as-path access-list 1 permit 3561
ip as-path access-list 2 deny 35
ip as-path access-list 2 permit .*
router bgp 100
neighbor 171.69.233.33 filter-list 1 in
neighbor 171.69.233.33 filter-list 2 out

Route Maps
router bgp 300
neighbor 2.2.2.2 route-map SETCOMMUNITY out
!
route-map SETCOMMUNITY permit 10
match ip address 1
match community 1
set community 300:100
!
access-list 1 permit 35.0.0.0
ip community-list 1 permit 100:200

Route-map match & set clauses
Match Clauses Set Clauses
• AS-path
• Community
• IP address
• AS-path prepend
• Community
• Local-Preference
• MED
• Origin
• Weight
• Others...

H H
eth H H
eth
H H
eth
H H
eth
C31
C22
C21
C32
ISP3
ISP2
Inbound route-map
to set community
Route-map Configuration Example
neighbor <x.x.x.x> route-map AS100_IN in
!
route-map AS100_IN permit 10
set community 100:200
neighbor <y.y.y.y> route-map AS200_IN in
!
route-map AS200_IN permit 10
match community 1
set local-preference 200
!
ip community-list 1 permit 100:200

Load Sharing & Redundancy
using BGP

Load-sharing - single path
AS100 AS200
Router A:
ip address 20.200.0.1 255.255.255.255
!
router bgp 100
neighbor 10.200.0.2 ebgp-multi-hop 2
!
ip route 10.200.0.2 255.255.255.255 <DMZ-link1, link2>
A
Loopback 0
10.200.0.2
Loopback 0
20.200.0.1

100 200
A
Note:A still only advertises one “best” path to ibgp peers
Router A:
router bgp 100
maximum-paths 2
Load Sharing - Multiple paths
from the same AS

Redundancy - Multi-homing
•Reliable connection to Internet
•3 common cases of multi-homing:
- default from all providers
- customer + default routes from all
- full routes from all

Default from all providers
• Low memory/CPU solution
• Provider sends BGP default
– provider is selected based on IGP metric
• Inbound traffic decided by providers’ policy
– Can influence using outbound policy, example:
AS-path prepend

Default from all providers
AS 400
Provider
AS 200
Provider
AS 300
E
B
C
A
D

Customer + default from all
providers
• Medium memory and CPU solution
• Granular routing for customer routes and
default for the rest
• Inbound traffic decided by providers’ policy
– Can influence using outbound policy

Customer routes from all
providers
AS 400
Provider
AS 200
Customer
AS 100
160.10.0.0/16
Provider
AS 300
E
B
C
A
D
C chooses shortest AS
path

Full routes from all providers
• More memory/CPU
• Full granular routing
• Usually transit ASes take full routes
• Usually pervasive BGP

Full routes from all providers
AS 400
AS 200
AS 100
AS 300
E
B
C
A
D
C chooses shortest AS
path
AS 500

Best Practices
IGP in Backbone
• IGP connects your backbone together, not
your client’s routes
• IGP must converge quickly
• IGP should carry netmask information -
OSPF, IS-IS, EIGRP

Best Practices...
Connecting to a customer
• Static routes
– You control directly
– No route flaps
• Shared routing protocol or leaking
– You must filter your customers info
– Route flaps
• BGP for multi-homed customers

Best Practices...
Connecting to other ISPs
• Use BGP4
• Advertise only what you serve
• Take back as little as you can
• Take the shortest exit

Best Practices...
The Internet Exchange
• Long distance connectivity is expensive
• Connect to several providers at a single
point

bgp.ppt

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