The document discusses IP routing protocols RIP, RIP version 2, EIGRP, and OSPF. It provides details on configuration and features of each protocol, including route summarization, route filtering, default routing, and stub routing. It also covers troubleshooting routing loops caused by interface summaries in RIP and using leak maps in EIGRP.

1. IP Routing Kristof De Brouwer

2. Agenda RIPv2 EIGRP OSPF Redistribution

3. IGP RIP

4. RIP Widely available Hop count metric Periodic update Easy to implement Usually free RFC 1058 Simple = limited Slow convergence No VLSM No discontiguous subnets Routing loops Count to infinity

5. RIP V2 RFC 1723 Cisco IOS ™ 11.1 support Variable length subnet masks Route summarization Classless InterDomain Routing (CIDR) Multicast routing updates (TTL 2) Authenticated updates using MD5

6. RIP Default classful  no auto-summary Force unicast  neighbor statement Passive-interface  still receive updates and advertise network but stops sending updates out that interface Route Filtering – Inbound Distribute-list  ACL or Prefix-list ACL: Prefix: Offset-list  Metric 16, poison the route Distance  255, poisoning the route access-list 100 permit ip host source host route Distribute-list prefix route gateway source  /32 prefix

7. RIP Route Filtering – Outbound Distribute-list  ACL or Prefix-list Passive-interface Advertise 204.12.1.0/24, but do not send or receive RIP updates on this interafce Router rip redistribute connected route-map connected->rip metric 1 Route-map connected->rip permit match interface interface

8. RIP Default-information originate  can be filtered by interface using route-map matching on interface Router rip default-information orignate route-map Default Route-map Default permit match interface interface

9. RIP Configure R4 to load balance traffic destined for the OSPF domain between both R5 and SW2 Router rip offset-list 0 in 1 Serial0/1

10. RIP interface summaries cause routing loops ip summary-address rip network mask No summary route pointing to Null 0 installed in Routing table

11. RIP interface summaries cause routing loops hostname A1 ! interface Loopback0 ip address 10.0.1.1 255.255.255.255 ! interface FastEthernet0/0 description LAN 0 (A1-A2) ip address 10.2.1.1 255.255.255.0 ! interface Serial1/0 description Link to R1(ROUTER) S1/0 ip address 10.0.7.9 255.255.255.252 ip summary-address rip 10.0.1.0 255.255.255.0 ! router rip version 2 network 0.0.0.0

12. RIP interface summaries cause routing loops A2#show ip route rip 10.0.0.0/8 is variably subnetted, 9 subnets, 3 masks R 10.0.7.8/30 [120/1] via 10.2.1.1, 00:00:01, FastEthernet0/0 R 10.0.1.1/32 [120/1] via 10.2.1.1, 00:00:01, FastEthernet0/0 R 10.0.1.0/24 [120/3] via 10.0.7.14, 00:00:04, Serial1/0 R 10.0.1.5/32 [120/1] via 10.0.7.14, 00:00:04, Serial1/0 R 10.0.1.4/32 [120/2] via 10.2.1.1, 00:00:01, FastEthernet0/0 [120/2] via 10.0.7.14, 00:00:04, Serial1/0 R 10.0.7.20/30 [120/1] via 10.0.7.14, 00:00:04, Serial1/0 A1#show ip route rip 10.0.0.0/8 is variably subnetted, 11 subnets, 3 masks R 10.0.1.0/24 [120/4] via 10.2.1.2, 00:00:02, FastEthernet0/0 R 10.0.1.2/32 [120/1] via 10.2.1.2, 00:00:02, FastEthernet0/0 R 10.0.1.4/32 [120/1] via 10.0.7.10, 00:00:04, Serial1/0 R 10.0.1.5/32 [120/2] via 10.2.1.2, 00:00:02, FastEthernet0/0 [120/2] via 10.0.7.10, 00:00:04, Serial1/0 R 10.0.7.12/30 [120/1] via 10.2.1.2, 00:00:02, FastEthernet0/0 R 10.0.7.20/30 [120/1] via 10.0.7.10, 00:00:04, Serial1/0

13. RIP interface summaries cause routing loops A1#trace 10.0.1.18 Type escape sequence to abort. Tracing the route to 10.0.1.18 1 10.2.1.2 4 msec 4 msec 4 msec 2 10.0.7.14 16 msec 4 msec 4 msec 3 10.0.7.22 4 msec 8 msec 8 msec 4 10.0.7.9 16 msec 12 msec 8 msec 5 10.2.1.2 20 msec 12 msec 12 msec 6 10.0.7.14 20 msec 24 msec 20 msec 7 10.0.7.22 28 msec 32 msec 28 msec 8 10.0.7.9 24 msec 24 msec 16 msec 9 10.2.1.2 28 msec 40 msec 20 msec 10 10.0.7.14 28 msec 24 msec 32 msec 11 10.0.7.22 28 msec 32 msec 36 msec 12 10.0.7.9 36 msec 60 msec 32 msec 13 10.2.1.2 40 msec 36 msec 36 msec 14 10.0.7.14 40 msec 40 msec 36 msec

14. RIP interface summaries cause routing loops A2#show ip route rip 10.0.0.0/8 is variably subnetted, 8 subnets, 3 masks R 10.0.7.8/30 [120/1] via 10.2.1.1, 00:00:05, FastEthernet0/0 R 10.0.1.1/32 [120/1] via 10.2.1.1, 00:00:05, FastEthernet0/0 R 10.0.1.5/32 [120/1] via 10.0.7.14, 00:00:07, Serial1/0 R 10.0.1.4/32 [120/2] via 10.2.1.1, 00:00:05, FastEthernet0/0 [120/2] via 10.0.7.14, 00:00:07, Serial1/0 R 10.0.7.20/30 [120/1] via 10.0.7.14, 00:00:07, Serial1/0

15. IGP EIGRP

16. EIGRP Extremely fast convergence VLSM support Discontiguous subnets Route summarization Easy to configure Best of DV and LS Low overhead Guaranteed loop-free Reliable, incremental update based Multiprotocol: IP, IPX ® , AppleTalk

17. EIGRP Protocol 88 Multicast 224.0.0.10 (0010.5e00.000a) Blocking EIGRP multicast on L2 : Neighbor statement  Force Unicast no ip igmp snooping vlan vlan ! interface interface switchport ip access-group DENY_EIGRP in  only allowed inbound ! ip access-list extended DENY_EIGRP deny eigrp any any permit ip any any ! mac-address-table static 0100.5e00.000a vlan 1363 interface FastEthernet0/1 FastEthernet0/3 FastEthernet0/6

18. EIGRP EIGRP Advertising Routes Router eigrp 109 network route wildcard redistribute connected route-map connected->eigrp Route-map connected->eigrp permit 10 match interface interface

19. EIGRP Stub When a router running EIGRP loses its connection to a network, it first searches for alternate loop free paths If it finds none, it then sends queries to each of its neighbors, looking for an alternate path B A 10.1.1.0/24 router-a#sho ip eigrp topo IP-EIGRP Topology Table .... P 10.1.1.0/24, 1 successors, FD is 281600 via Connected, Ethernet1/2 router-a#show ip eigrp events Event information for AS 100: .... 12 Active net/peers: 10.1.1.0/24 1 14 FC not sat Dmin/met: 4294967295 128256 15 Find FS: 10.1.1.0/24 128256 .... 18 Conn rt down: 10.1.1.0/24 Ethernet 3/1

20. EIGRP Stub If the neighbor has no path to this destination, it replies The router then removes all references to this route from its local tables In large hub and spoke networks, the hub routers have to build queries and process replies from each of the spokes This limits scaling! B A 10.1.1.0/24 router-a#show ip eigrp events Event information for AS 100: 1 NDB delete: 10.1.1.0/24 1 .... 12 Active net/peers: 10.1.1.0/24 1 14 FC not sat Dmin/met: 4294967295 128256 15 Find FS: 10.1.1.0/24 128256 .... 18 Conn rt down: 10.1.1.0/24 Ethernet 3/1

21. EIGRP Stub If these spokes are remotes sites, they have two connections for resiliency, not so they can transit traffic between A and B A should never use the spokes as a path to anything, so there’s no reason to learn about, or query for, routes through these spokes B A 10.1.1.0/24 don’t use these paths

22. EIGRP Stub To signal A and B that the paths through the spokes should not be used, the spoke routers can be configured as stubs B A 10.1.1.0/24 router#config t# router(config)#router eigrp 100 router(config-router)#EIGRP stub connected

23. EIGRP Stub Marking the spokes as stubs allows them to signal A and B that they are not valid transit paths A will not query stubs, reducing the total number of queries in this example to 1 Marking the remotes as stubs also reduces the complexity of this topology; B now believes it only has 1 path to 10.1.1.0/24, rather than 5 marked as stubs B A 10.1.1.0/24

24. EIGRP Stub If stub connected is configured B will advertise 10.1.2.0/24 to A B will not advertise 10.1.2.0/23 or 10.1.4.0/24 If stub summary is configured B will advertise 10.1.2.0/23 to A B will not advertise 10.1.2.0/24 or 10.1.4.0/24 ip route 10.1.4.0 255.255.255.0 10.1.1.10 ! interface serial 0 ip summary-address eigrp 10.1.2.0 255.255.254.0 ! router eigrp 100 redistribute static 1000 1 255 1 1500 network 10.2.2.2 0.0.0.3 network 10.1.2.0 0.0.0.255 eigrp stub connected eigrp stub summary 10.1.2.0/24 A B 10.2.2.2/30

25. EIGRP Stub If stub static is configured B will advertise 10.1.4.0/24 to A B will not advertise 10.1.2.0/24 or 10.1.2.0/23 If stub receive-only is configured B won’t advertise anything to A, so A needs to have a static route to the networks behind B to reach them ip route 10.1.4.0 255.255.255.0 10.1.1.10 ! interface serial 0 ip summary-address eigrp 10.1.2.0 255.255.254.0 ! router eigrp 100 redistribute static 1000 1 255 1 1500 network 10.2.2.2 0.0.0.1 network 10.1.2.0 0.0.0.255 eigrp stub static eigrp stub receive-only A B 10.2.2.2/30 10.1.2.0/24

26. EIGRP Stub Any combination of the route types can be specified on the eigrp stub statement, except receive-only , which cannot be used with any other option For example: eigrp stub connected summary redistributed If eigrp stub is specified without any options, it will send connected and summary routes

27. EIGRP Stub At A, you can tell B is a stub using show ip eigrp neighbor detail. 10.1.2.0/24 A B 10.2.2.2/30 router-a#show ip eigrp neighbor detail IP-EIGRP neighbors for process 100 H Address Interface Hold Uptime SRTT RTO Q Seq Type (sec) (ms) Cnt Num 0 10.2.2.3 Et1/2 10 00:00:50 320 1920 0 7 Version 12.2/1.2, Retrans: 0, Retries: 0 Stub Peer Advertising ( CONNECTED ) Routes

28. EIGRP Stub leak maps

29. EIGRP Stub leak maps R1: router eigrp 10 network 150.1.12.1 0.0.0.0 network 150.1.13.1 0.0.0.0 network 150.1.14.1 0.0.0.0 eigrp stub connected no auto-summary ! R2: router eigrp 10 network 150.1.12.2 0.0.0.0 no auto-summary R3: router eigrp 10 network 150.1.13.3 0.0.0.0 auto-summary R4: router eigrp 10 network 150.1.14.4 0.0.0.0 no auto-summary redistribute rip met 1 1 1 1 1 router rip redistribute eigrp 10 met 1 R2#sh ip route eigrp 150.1.0.0/24 is subnetted, 3 subnets D 150.1.14.0 [90/2195456] via 150.1.12.1, 00:00:23, Serial1/0 D 150.1.13.0 [90/2195456] via 150.1.12.1, 00:00:23, Serial1/0 R3#sh ip route eigrp 150.1.0.0/24 is subnetted, 3 subnets D 150.1.14.0 [90/307200] via 150.1.13.1, 00:01:15, Ethernet0/0 D 150.1.12.0 [90/2195456] via 150.1.13.1, 00:01:15, Ethernet0/0

30. EIGRP Stub leak maps access-list 1 permit 5.5.0.0 0.0.0.255 access-list 1 permit 5.5.1.0 0.0.0.255 route-map EIGRP_LEAK match ip address 1 router eigrp 10 eigrp stub connected leak-map EIGRP_LEAK R2#sh ip route eigrp 5.0.0.0/24 is subnetted, 2 subnets D EX 5.5.0.0 [170/2560537856] via 150.1.12.1, 00:00:28, Serial1/0 D EX 5.5.1.0 [170/2560537856] via 150.1.12.1, 00:00:28, Serial1/0 150.1.0.0/24 is subnetted, 3 subnets D 150.1.14.0 [90/2195456] via 150.1.12.1, 00:00:28, Serial1/0 D 150.1.13.0 [90/2195456] via 150.1.12.1, 00:00:28, Serial1/0 R3#sh ip route eigrp 5.0.0.0/24 is subnetted, 2 subnets D EX 5.5.0.0 [170/2560051456] via 150.1.13.1, 00:00:20, Ethernet0/0 D EX 5.5.1.0 [170/2560051456] via 150.1.13.1, 00:00:20, Ethernet0/0 150.1.0.0/24 is subnetted, 3 subnets D 150.1.14.0 [90/307200] via 150.1.13.1, 00:00:20, Ethernet0/0 D 150.1.12.0 [90/2195456] via 150.1.13.1, 00:00:20, Ethernet0/0

31. EIGRP Stub leak maps

32. EIGRP Stub leak maps If “match interface” options is not used, routes are leaked on all interfaces. If “match interface” option is used, routes are ONLY leaked on the interface matched. access-list 1 permit 5.5.0.0 0.0.0.255 access-list 1 permit 5.5.1.0 0.0.0.255 route-map EIGRP_LEAK match ip address 1 match interface e0/0 router eigrp 10 eigrp stub connected leak-map EIGRP_LEAK

33. EIGRP Metric Calculate EIGRP load variance (delay value) Feasible Distance: ((10 7 /BW min ) + (sum of delays))*256 Sucessor Metric * Configured Variance Value = Target Metric Target Metric - (10 7 / BW min ) = delay/256 set this value as interface delay subtracting the existing interface delay

34. IGP OSPF

35. OSPF Dynamic routing protocol Link state or SPF technology Developed by OSPF working group of IETF (RFC 1253) Intra-autonomous system (IGP) Designed expressly for TCP/IP Internet environment Fast convergence Variable-length subnet masks Discontiguous subnets No periodic updates Route authentication Delivered two years after IGRP

36. OSPF Metric Derived from bandwidth 10 8 /bandwith in bps Configured via Interface sub-command: bandwidth Interface sub-command: ip ospf cost Router sub-command: ospf auto-cost reference-bandwidth neighbor x.x.x.x cost

37. OSPF Areas Rules Backbone area must be present All other areas must have connection to backbone Backbone must be contiguous Always pass through backbone

38. Classification of OSPF Routers Four overlapping categories of routers Backbone Routers Internal Routers Area Border Router Autonomous System Boundary Router

39. OSPF Algorithm Network changes generate link-state advertisements (LSA) All routers exchange LSAs to build and maintain a consistent database The protocol remains relatively quiet during steady-state conditions Periodic refresh of LSAs every 30 minutes Otherwise, updates only sent when there are changes

40. OSPF Hello packets Multicast 224.0.0.5, 224.0.0.6 on all router interfaces Hello interval 10 sec. LAN, 30 sec. NBMA Used to form adjacencies between routers Describes the optional capabilities

41. OSPF LSA Type 1 - Represents a router Type 2 - Represents the pseudonode (designated router) for a multiaccess link Type 3 - A network link summary (internal route) Type 4 - Represents an ASBR Type 5 - A route external to the OSPF domain Type 7 - Used in stub areas in place of a type 5 LSA

42. OSPF Areas - Standard

43. OSPF Areas - Stub

44. OSPF Areas – Totally Stuby Area

45. OSPF Areas – Not So Stuby Area

46. OSPF Media Options Point-to-Point Non-Broadcast Multi-Access (NBMA) Multi-access media

47. OSPF Point-to-Point Default on HDLC, PPP and Frame-relay Point-to-Point Hello Interval 10 seconds Dead Interval 40 seconds No DR/BDR Election Multicast updates to 224.0.0.5 Next hop address is that of the advertising router

48. OSPF Non-Broadcast Multi-Access Default on Multipoint interface like Frame-relay Hello interval 30 seconds Dead Interval 120 seconds DR/BDR election Updates are sent as unicast Neighbor command required on hub router Next hop is not changed and remains the ip address of the originating router

49. OSPF Multi-access media Default on Ethernet Hello interval 10 seconds Dead Interval 40 seconds DR/BDR election Updates are sent as multicast Next hop is not changed and remains the ip address of the originating router

50. OSPF MTU Mismatch Issue This problem is common when running OSPF between a switch (i.e 3550 or 3560) and a router. The error message that is generated when this problem occurs is: The resolution is to either adjust the system MTU on the switch or have OSPF ignore the MTU. Change the system MTU on the switch ( system mtu routing 1500 ) Have the router and/or switch ignore the MTU ( ip ospf mtu-ignore ) Change the interface MTU on the router %OSPF-5-ADJCHG: Process 1, Nbr 150.8.5.5 on Vlan258 from DOWN to DOWN, Neighbor Down: Dead timer expired %OSPF-5-ADJCHG: Process 1, Nbr 150.8.2.2 on Vlan258 from EXSTART to DOWN, Neighbor Down: Too many DBD retransmitions

51. External default route in NSSA area Conditional ABR non-OSPF default route router ospf 1 log-adjacency-changes area 22 nssa default-information-originate metric 100 metric-type 1 network 0.0.0.0 255.255.255.255 area 22 ! ip route 0.0.0.0 0.0.0.0 Serial0/0

52. External default route in NSSA area ABR-A#show ip ospf database nssa-external | exclude LS|Check OSPF Router with ID (10.0.1.9) (Process ID 1) Type-7 AS External Link States (Area 22) Options: (No TOS-capability, Type 7/5 translation, DC) Link State ID: 0.0.0.0 (External Network Number ) Advertising Router: GW Length: 36 Network Mask: /0 Metric Type: 1 (Comparable directly to link state metric) Metric: 100 Forward Address: 10.0.1.8 External Route Tag: 0 B1#show ip ospf database external | exclude LS|Check OSPF Router with ID (172.16.1.1) (Process ID 1) Type-5 AS External Link States Options: (No TOS-capability, DC) Link State ID: 0.0.0.0 (External Network Number ) Advertising Router: ABR-A Length: 36 Network Mask: /0 Metric Type: 1 (Comparable directly to link state metric) Metric: 100 Forward Address: 10.0.1.8 External Route Tag: 0

53. OSPF Filtering area area-id range ip-address mask not-advertise Interface interface ip ospf database-filter all out 150.1.0.0/16

54. OSPF Filtering based on next hop R1#sh ip route 2.2.2.0 Routing entry for 2.2.2.0/24 Known via "ospf 1", distance 110, metric 74, type intra area Last update from 192.168.1.1 on Serial1/0, 00:00:46 ago Routing Descriptor Blocks: 192.168.1.2 , from 192.168.1.2 , 00:00:46 ago, via Serial1/0 Route metric is 74, traffic share count is 1 * 192.168.1.3 , from 192.168.1.3 , 00:00:46 ago, via Serial1/0 Route metric is 74, traffic share count is 1

55. OSPF Filtering based on next hop router ospf 1 distribute-list route-map ROUTE_FROM_R2 in ! access-list 2 permit 192.168.1.2 ! access-list 12 permit 2.2.2.0 0.0.0.255 ! route-map ROUTE_FROM_R2 deny 10 match ip address 12 match ip next-hop 2 ! route-map ROUTE_FROM_R2 permit 20 R3#sh ip route 2.2.2.0 Routing entry for 2.2.2.0/24 Known via "ospf 1", distance 110, metric 74, type intra area Last update from 192.168.1.1 on Serial1/0, 00:00:46 ago Routing Descriptor Blocks: 192.168.1.3 , from 192.168.1.3 , 00:00:46 ago, via Serial1/0 Route metric is 74, traffic share count is 1

56. OSPF Type 3 LSA Filtering Router(config)# router ospf 1 log-adjacency-changes area 1 filter-list prefix AREA_1_OUT out area 3 filter-list prefix AREA_3_IN in network 10.0.0.0 0.255.255.255 area 3 network 172.16.1.0 0.0.0.255 area 0 network 192.168.0.0 0.255.255.255 area 1 ! ip prefix-list AREA_1_OUT seq 10 permit 10.25.0.0/8 ge 16 ip prefix-list AREA_1_OUT seq 20 permit 172.20.20.0/24 ! ip prefix-list AREA_3_IN seq 10 permit 172.31.0.0/16

57. Challenge: Establish OSPF adjacency on a LAN R1 and R2 should establish OSPF adjacency, but you cannot change or remove any of the existing configuration commands. hostname R1 ! interface FastEthernet 0/0 ip address 192.168.1.17 255.255.255.0 ip ospf 1 area 1 ! router ospf 1 hostname R2 ! interface FastEthernet 0/0 ip address 192.168.1.18 255.255.255.252 ip ospf 1 area 1 ! router ospf 1 hostname R1 ! interface FastEthernet 0/0 ip address 192.168.1.17 255.255.255.0 ip ospf 1 area 1 ip ospf network point-to-point ! router ospf 1 hostname R2 ! interface FastEthernet 0/0 ip address 192.168.1.18 255.255.255.252 ip ospf 1 area 1 ip ospf network point-to-point ! router ospf 1

58. Challenge: Establish OSPF adjacency on a LAN Subnet mask checked only on multi-access interfaces Subnet mask ignored on point-to-point links. RFC 2328 The generic input processing of OSPF packets will have checked the validity of the IP header and the OSPF packet header. Next, the values of the Network Mask, HelloInterval, and RouterDeadInterval fields in the received Hello packet must be checked against the values configured for the receiving interface . Any mismatch causes processing to stop and the packet to be dropped. In other words, the above fields are really describing the attached network's configuration. However, there is one exception to the above rule: on point-to-point networks and on virtual links, the Network Mask in the received Hello Packet should be ignored .

59. Redistribution

60. Redistribution Fundamentals Routes can be redistributed from one routing protocol to another Routes really aren’t redistributed between protocols Routes are taken from the RIB, not another protocol! The redistributing protocol knows which routes to take from the RIB based on the “known via” information EIGRP OSPF Topology Database RIB router#show ip route 10.0.0.0 .... Routing entry for 10.0.0.0/8 Known via “eigrp 100", distance 90, metric 3072256, type internal Redistributing via eigrp 100

61. Redistribution Considerations Routing feedback Suboptimal path selection Routing loops Incompatible routing information Inconsistent convergence time 172.16 OSPF 172.16 RIP 172.16 OSPF 172.16 RIP Area 300 OSPF RIP 172.16.0.0 ASBR ASBR

62. Filter to Avoid Redistribution Feedback Impose split horizon when redistributing EIGRP Process RIP Process Filter 172.16.1.0 Allow 172.16.2.0 Filter 172.16.2.0 Allow 172.16.1.0 172.16.1.0 172.16.2.0

63. Redistribution Filters Filters can control what information is injected into a routing protocol through redistribution Filters can also be used to stop routing loops when mutual redistribution between two routing protocols is configured What information about installed routes does the RIB have (what information can we filter on)?

64. Redistribution Filters Route maps can filter on these fields in the RIB: Match metric Filtering is possible between all protocols based on metric It’s very tricky, as the metric must be exact to match Match tag Some protocols carry route tags: EIGRP, OSPF externals This normally works well, but is sometimes tricky, if the protocol can carry more than one tag Match ip address Matches the prefix, possibly the network mask, depending on the access list type used, etc. More on this in the section on route filtering, later in this presentation

65. Redistribution Filters Route maps can filter on these fields in the RIB: Match ip next-hop Matches on the next hop listed in the routing table Match route-type Internal: EIGRP or OSPF internal routes External: EIGRP or OSPF external routes, type-1 and type-2 options for OSPF Match interface The interface through which the router is forwarding traffic to the destination, as listed in the RIB

66. Redistribution Filters A route is injected into EIGRP as an external; this route is redistributed through B into OSPF The route is transmitted to A through OSPF, and redistributed into EIGRP The metric is set manually in redistribution at A to something lower than the original external injected into EIGRP B prefers this route, building a routing loop A EIGRP OSPF B metric 10 metric 2816000 10.1.1.0/24 metric 2560256 metric 2688000 metric 25 metric 2560256

67. Redistribution Filters To filter based on prefixes, configure access lists which match the address ranges used by each section of the network. Use these access lists to filter routes redistributed between protocols. access-list 10 permit 10.1.0.0 0.0.255.255 access-list 20 permit 10.2.0.0 0.0.255.255 .... router ospf 100 redistribute eigrp 100 metric 10 distribute-list 10 .... router eigrp 100 redistribute ospf 100 metric 1000 1 255 1 1500 distribute-list 20 .... EIGRP 10.1.0.0/16 OSPF 10.2.0.0/16 A B

68. Redistribution Filters A route is injected into EIGRP as an external; this route is redistributed through B into OSPF The route is transmitted to A through OSPF, and redistributed into EIGRP The route is now blocked by distribute list 10, which breaks the routing loop A EIGRP OSPF B metric 10 metric 2816000 10.1.1.0/24 metric 2560256 metric 25

69. Redistribution Filters EIGRP & OSPF can set tags on their external routes Set the tag when redistributing between the protocols. Deny tagged routes at the redistribution point route-map ospf->eigrp deny 10 match tag 90 route-map ospf->eigrp permit 20 set tag 110 route-map eigrp->ospf deny 10 match tag 110 route-map eigrp->ospf permit 20 set tag 90 .... router ospf 100 redistribute eigrp 100 metric 10 route-map eigrp->ospf .... router eigrp 100 redistribute ospf 100 metric 1000 1 255 1 1500 route-map ospf->eigrp .... EIGRP 10.1.0.0/16 OSPF 10.2.0.0/16 A B

70. Redistribution Filters 10.1.1.0/24 route is injected into EIGRP as an external; it is redistributed through B into OSPF, and tagged 10.1.1.0/24 is transmitted to A through OSPF; the route is blocked from being redistributed into EIGRP because of the route tag A EIGRP OSPF B metric 10 metric 2816000 10.1.1.0/24 metric 2560256 metric 25

71. Redistribution Filters If live routing data is only needed in one direction (normally, this is true), redistribute a static in one direction, and between protocols in the other direction ip route 10.2.0.0 255.255.0.0 serial 0/0 .... router ospf 100 redistribute eigrp 100 metric 10 .... router eigrp 100 redistribute static 100 metric 1000 1 255 1 1500 .... EIGRP 10.1.0.0/16 OSPF 10.2.0.0/16 A B

72. Redistribution Filters A route is injected into EIGRP as an external; this route is redistributed through B into OSPF The route is transmitted to A through OSPF; the route is not redistributed back into EIGRP, since redistribution between OSPF and EIGRP is not configured A EIGRP OSPF B metric 10 metric 2816000 10.1.1.0/24 metric 2560256 metric 25

73. Route Loops When crossing a redistribution boundary, information is lost A physical or logical loop causes a route to be advertised back to the redistributing router that first advertised it How does the router know which route to accept? Answer: it can’t know Humans have to re-insert the lost information Distribute-lists are the mechanism

74. Ground Rules Under normal operation, there should be exactly one interior routing protocol on any network segment Use “passive-interface” as necessary to ensure this The number of redistribution boundaries should be kept to a minimum Run as few routing protocols as possible

75. Questions? 161