1 Unicast Routing Protocols

2 Outline  Routing basic  RIP  OSPF  BGP

3 Routing Basic  IP Routing  Autonomous System (AS)  IGP/EGP  Distance-vector(DV)/Link-state(LS)  How routing protocol works?

4 IP Routing  Route entry Destination/netmask Nexthop  Longest-match  Default-route  Equal Cost Multipath Protocol(ECMP)  Static routing/Dynamic routing

5 Autonomous System (AS)  Collection of networks with same policy  Usually under single administrative control  IGP to provide internal connectivity  Identified by a short number Public & Private AS numbers  public:  private: – AS 100

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

7 What Is an EGP?  Exterior Gateway Protocol  Used to convey routing information between ASes  Independent from the IGP  Current EGP is BGP4

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

9 Hierarchy of Routing Protocols Customers Other ISP’s BGP4BGP4/Static BGP4 / IGP Customers BGP4

10 Distance-vector (Bellman-Ford)  Routers only know their local state link metric and neighbor estimates  Examples – RIP, BGP (path-vector)

11 Link-state  Routers have knowledge of the global state topology database global optimization (Shortest Path First - Dijkstra)  Examples – OSPF, ISIS

12 How Routing Protocol works?  Neighbor Discovery  Route Exchange between neighbors learning/flooding/invalidation/refresh  Best route choice and routing table management  Responsibility  Fast convergence and loop-free  Scalability  Robustness  Some control of routing choices

13 Routing Information Protocol (RIP)  RIP basic  General operation  RIP v2 VS RIP v1  Conclusion

14 RIPv2 basic  Distance-vector protocol  Metric – hops Metric is increased when routes are updated to neighbors Network span limited to 15 (16 means unreachable)  Encapsulated as UDP packets, port 520

15 RIPV2 General operation On startup, send request on all interfaces. When a request is received, a response is sent. - Response contains entire routing table. A response is also gratuitously sent every 30s. – Response contains entire routing table. A response is also sent when update detected. - Response only contains changed routes. Route metric is set to 16 when network becomes inaccessible or not refreshed during 6 update periods(180s) Invalid routes are flushed after another 4 update periods(120s)

16 Count of infinity  What happens when a link dies? ABC A: 0 B: 1, B C: 2, B A: 1, A B: 0 C: 1, C A: 2, B B: 1, B C: 0 A: 0 B: 1, B C: 2, B A: 1, A B: 0 C: 3, A A: 2, B B: 1, B C: 0 A: 0 B: 1, B C: 4, B A: 1, A B: 0 C: 3, A A: 2, B B: 1, B C: 0 A: 0 B: 1, B C: 15, B A: 1, A B: 0 C: 16, A A: 2, B B: 1, B C: 0

17 Split horizon To speed up convergence Simple - do not claim reachability for a destination network to the neighbor from which the route was learned. Poison reverse - includes such routes in updates, but sets their metrics to infinity

18 Split horizon - simple ABC A: 0 B: 1, B C: 2, B A: 1, A B: 0 C: 1, C A: 2, B B: 1, B C: 0 A: 0 B: 1, B C: 16, B A: 1, A B: 0 C: 16 A: 2, B B: 1, B C: 0

19 Split horizon – poison reverse ABC A: 0 B: 1, B C: 2, B A: 1, A B: 0 C: 1, C A: 2, B B: 1, B C: 0 A: 0 B: 1, B C: 16, B A: 1, A B: 0 C: 16 A: 2, B B: 1, B C: 0 C: 16

20 RIPv2 vs RIPv1  broadcast  Variable Length Subnet Mask(VLSM) - Classless Inter-Domain Routing (CIDR, no prefix/subnet information, derived from address class)  Authentication - none

21 Conclusion  Simplicity  Slow convergence  Not suited for large and complex networks

22  OSPF Basic  OSPF Neighbors  OSPF Area  SPF and LSA database  OSPF Messages  Conclusion Open Shortest Path First (OSPF)

23 OSPF Basic  Encapsulated as RAW IP packets, protocol ID 89  Uses metrics — path cost(1 – 65,535)

24 OSPF Basic - general operation  Use Hello Protocol to establish neighbors  All routers exchange Link State Advertisement (LSA) to build and maintain a consistent database  Each router runs SPF on LSA database independently and gets optimal routes  Periodic flooding of LSAs every 30 minutes  LSA age 0 when created Incremented as time elapsed. Max age 3600 indicates invalid Remove a LSA by incrementing age to 3600, reflooding and flushing.

25 OSPF Network type  Broadcast  Point-to-Point/Point-to-Multipoint  NBMA(Non-Broadcast Multiple Access)

26 Neighbor discovery  Hello packets Periodically Multicasting , including  RouterId, AreaId, Netmask, hello interval, Priority, DR, BDR, Neighbor list Neighbor state machine Works differently on different network

27 DR/BDR/Others  For broadcast and NBMA networks  Optimize the flooding procedure  Designated Router(DR) Adjacent to all routers Describe all routers on the network Send updates to all routers on the network  Backup Designated Router(BDR) Adjacent to all routers Act as new DR when previous DR fails  Others Only adjacent to DR/BDR, only send updates to DR/BDR

28 OSPF Area  Why divide the network into different areas? Limit the scope of updates and computational overhead independent SPF computing in each area inject aggregated information on routes into other areas  32 bit number  Backbone area – area 0 or All areas must connect to backbone area. Backbone area must be continuous Virtual link when the above fails  Area Border Routers(ABR)

29 Virtual Link Area 1 Area 0 Area 2 Area 3 Virtual link ABR

30 Shortest Path First AB CD

31 Candidat e Root cost SPF treeDescription A, A, 0Root tree A, B, 3 A, C, 1 A, D, A, A, 0Add adjacent links to A into Candidate and calculate cost to A. A, B, 3 A, D, 10 C, D, A, A, 0 A, C, 1 Choose the lowest cost link (A, C, 1), add it into SPF tree and remote it from Candidate. Add adjacent links to C into Candidate and calculate cost to A. Because the new lowest cost to D is 8, is remoted. C, D, 7 B, D, A, A, 0 A, C, 1 A, B, 3 Choose the lowest cost link(A, B, 3), Add it into SPF tree and remote it from Candidate. Add adjacent links to B into Candidate and calculate cost to A. because the new lowest cost to D is 7, is removed. A, A, 0 A, C, 1 A, B, 3 B, D, 4 Choose the lowest cost link(B, D, 4), Add it into SPF tree and remote it from Candidate. Because the Candidate is empty, the process is over.

32 OSPF SPF process  SPF calculation is performed independently for each area  Router LSA Each router creates a router LSA for each area Describe links to an area  DR/BDR(broadcast)  Neighboring router(point-to-point)  Prefix/mask(stub network)  metric  Network LSA Only DR creates a network LSA for a network Describe all routers on the network

33 Inter-area routes  Network Summary LSA Created by ABR Advertise optimal routes in one area into another area  Prefix/mask  Metric Flood only in one area

34 Inter-AS routes  Autonomous System Border Router(ASBR)  Autonomous System External LSA Created by ASBR Describe routes redistributed from other AS  Prefix/mask  Metric Flood across area in an AS(except stub area)  ASBR summary LSA Created by ABR Describe ASBR routers in one area  ASBR router id  metric

35 Stub area  AS External LSA are forbidden in stub area  Why stub area? When many networks are connected only via one router All external networks aggregated into default route Reduce routing table sizes

36 OSPF Messages  Hello Used to establish neighbor relationship  Database description Used to describe brief information of LSA  Link-state request Used to request LSAs  Link-state update Used to update LSAs  Link-state acknowledgment Used to assure LSA flooding reliable by including brief description of received LSA

37 Conclusion  2-level hierarchical model  Faster convergence  Relatively low, steady state bandwidth requirements

38 Border Gateway Protocol (BGP)  BGP Basic  BGP Peers  BGP Updates – NLRI and Path Attributes  Synchronization with IGP  Route Reflector and AS Confederation  Routing policy  BGP Messages  Conclusion

39 BGP Basic  Based on TCP connection, port 179  BGP peer is configured manually  BGP Peers exchange Update messages containing Network Layer Reachability Information (NLRI)  Path attributes are with NLRI to avoid loop and facilitate policy control  No routes refresh

40 AS 100 AS 101 AS 102 AC BGP Peers - eBGP eBGP TCP/IP Peer Connection Peers in different AS’s are called External Peers Note: eBGP Peers normally should be directly connected. E BD / / /24 eBGP

41 AS 100 AS 101 AC BGP Peers - iBGP iBGP TCP/IP Peer Connection Peers in the same AS are called Internal Peers AS 102 E BD Note: iBGP Peers don’t have to be directly connected. Loopback interface are normally used as peer connection end-points. In this case, recursive route look-up is needed / / /24 iBGP

42 Full mesh  Each iBGP speaker must peer with every other iBGP speaker in the AS (full mesh)  IBgp speaker never floods routes received from another iBGP peer to any other iBGP peer. AS 100 A B CD

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

44 BGP Updates — Path Attributes  Used to convey information associated with NLRI Origin- mandatory AS path - mandatory Next hop - mandatory Local preference Multi-Exit Discriminator (MED) Community Origin Aggregator  Rich policy control

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

46  Sequence of ASes a route has traversed  Loop detection  Apply policy AS 100 AS 300 AS 200 AS 500 AS / / /16 Network Path / / / Network Path / / AS-Path Attribute

47  Sequence of ASes a route has traversed  Loop detection AS-Path Loop detection AS 100 AS 300 AS 200 AS 500 AS / / / / / / /16 dropped

/ / /30.2 AS 100 AS 200 Network Next-Hop Path / 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  Next Hop updated between eBGP Peers  Next hop not changed between iBGP peers / /30 Network Next-Hop Path / / Network Next-Hop Path / /

49 Local Preference AS 400 AS /16 AS 100 AS / > / E B C A D 500 Multi-homed AS Only for iBGP Local to an AS Path with highest local preference wins

50 Multi-Exit Discriminator (MED) AS 201 AS /24 C AB / / preferred 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

51 Customer AS 201 Service Provider AS /24 C AB Community:201:110Community:201:120 D 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 Communities

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

53 Synchronization with IGP  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 B A C 35/8 DOSPF

54 Alternative to Full Mesh – Router-reflection AS 100 RR ClientNon-client Client  Non-client peers are full-mesh connected  RR reflects routes from non-client peers to all client peers  RR reflects routes from client peers to all non-client peers and other client peers  Route Reflector (RR)  Client peers  Non-client peers

55 Alternative to Full Mesh – Confederation AS Confederation 100 Member-AS Member - AS Divided into member AS, marked by private AS number Full-mesh in member AS Peers between member AS are most similar with eBGP, except that inserted AS path is confederation AS path When routes get out of AS confederation, remove confederation AS path

56 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

57 BGP Messages  OPEN To negotiate and establish peering  UPDATE To exchange routing information(NLRI, Path attributes, Withdrawn routes)  KEEPALIVE To maintain peering session  NOTIFICATION To report errors (results in session reset)

58 Conclusion  The single extant protocol for interdomain routing  Fundamentally simple algorithms but can provide complex and flexible policy control  More future applications, such as BGP/MPLS VPN networks