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E rou01 routing_basics
- 1. APNIC eLearning: Routing Basics Contact: [email protected] eROU01_v1.0
- 2. Overview • What does a router do? • Routing vs. Forwarding • IP Route Lookup • RIB and FIB • Explicit and Default Routing • Autonomous Systems • Routing Policy • Routing Protocols • IGP and EGP
- 3. What does a router do? • ?
- 4. 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
- 5. Routing versus Forwarding • Routing = building maps and giving directions • Forwarding = moving packets between interfaces according to the “directions”
- 6. 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.)
- 7. 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.
- 8. IP route lookup • Based on destination IP address 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
- 9. IP route lookup: Longest match routing • Based on destination IP address Packet: Destination IP address: 10.1.1.1 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 ….. R2’s IP routing table R4 10.1/16 announced from here
- 10. IP route lookup: Longest match routing • Based on destination IP address Packet: Destination IP address: 10.1.1.1 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 R2’s IP routing table
- 11. IP route lookup: Longest match routing • Based on destination IP address Packet: Destination IP address: 10.1.1.1 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 R2’s IP routing table
- 12. IP route lookup: Longest match routing • Based on destination IP address Packet: Destination IP address: 10.1.1.1 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 R2’s IP routing table
- 13. IP route lookup: Longest match routing • Based on destination IP address Packet: Destination IP address: 10.1.1.1 R1 10/8 announced from here R3 R2 R4 10/8 → R3 10.1/16 → R4 20/8 → R5 30/8 → R6 ….. R2’s IP routing table Longest match, 16 bit netmask 10.1/16 announced from here
- 14. 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
- 15. 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
- 16. 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
- 17. 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)
- 18. Autonomous System (AS) • Collection of networks with same routing policy • Single routing protocol • Usually under single ownership, trust and administrative control AS 100
- 19. 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
- 20. 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
- 21. 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
- 22. 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
- 23. 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.
- 24. 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
- 25. 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
- 26. 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
- 27. 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
- 28. 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
- 29. 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
- 30. What Is an EGP? • Exterior Gateway Protocol • Used to convey routing information between Autonomous Systems • De-coupled from the IGP • Current EGP is BGP
- 31. 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
- 32. 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
- 33. Hierarchy of Routing Protocols Other ISPs BGP4 BGP4 and OSPF/ISIS BGP4 IXP Static/BGP4 Customers
- 34. 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
- 35. Questions • Please remember to fill out the feedback form – <survey-link> • Slide handouts will be available after completing the survey
- 37. Thank You! End of Session