RIP: Routing Information Protocol in IOS

RIP: Routing Information Protocol in IOS

  • Distance vector protocol. Routing by rumour.
  •  Make sure “ip routing” is enabled.
  • RIP uses UDP SRC/DES port number: 520
  • v1packets are broadcast.
  • v2 packets are multicast: 224.0.0.9 and supports authentication/classless option.
    • ‘version 2’ and ‘no auto-summary’ CLI are necessary
  • A directly connected network has a metric of zero; an unreachable network has a metric of 16.
  • Timers:
    • Advertise timer(update): 30s
    • Mark routes as unusable (invalid/holddown): 180s (6x)
    • Remove from RIP database (flush): 240s (8x)
    • “timers basic {update invalid holddown flush}” on router rip configuration mode.
    • If timers are not configured for a specific address-family, system default timer values are used. Values configured on router-rip mode are not used.
    • “ip rip advertise <sec>” on interface mode to override the global timer.
  • Hop count is incremented by 1 for each hop and the sender is indicated as next-hop.
  • Triggered updates are sent independently of regularly scheduled updates.
  • By default, IOS receives both RIP Version 1 and Version 2 packets, but sends only Version 1 packets. “version” command on router rip config mode or “ip rip {send | receive} version {1 | 2}” on interface mode.
  • An offset list is the mechanism for *increasing* incoming and outgoing metrics to routes learned via RIP. “offset-list [acl] {in | out} [offset] [interface ID]” configured under router rip process.
    • To prefer traffic to flow via one interface, configure offset and advertise with higher metric via all other interfaces.
  • Automatic summary is enabled by default. Disabled using “no auto-summary”
  • Metric of the summarized route: lowest metric of the all child routes. Metrics are not changed after summary advertisement.
  • Manual summarization: On interface mode: “ ip summary-address rip <nw> <mask>”
  • Authentication: Clear text and MD5.
    • Using “ ip rip authentication key-chain” and “ ip rip authentication mode {text | md5}”.
    • For clear-text authentication, configure key-chain and then configure “ip rip authentication key-chain <name>” and “ip rip authentication mode text”
    • Space is a valid character in the key-string. Confirm the correct password using “show key chain” command.
  • Split Horizon enabled by default on Ethernet link.
    • To disable on frame-relay network configure “no ip split-horizon” on interface mode.
    • Split horizon is disabled by default for frame-relay encapsulation but enabled on Multi-point frame relay sub interfaces.
  • Inter-packet delay:  By default there is no delay between multiple packet RIP update.
    • Configured using “output-delay <ms>” on router config mode.
  • By default, IOS validates the source IP address of incoming RIP routing updates.
    • “no validate-update-source” on rip config mode disables the SRC IP check. Router can install routes from SRC which may be in different subnet than its receiving interface.
  • IP-RIP delay start: to delay the initiation of RIPv2 neighbor sessions until the network connectivity between the neighbor routers is fully operational
    • Some Non-Cisco routers will not allow an MD5-authenticated RIPv2 neighbor session to start when the sequence number of the first MD5 packet received from the other router is greater than 0.
    • Delay start feature will be useful in this case.
    • “ip rip initial-delay” on the interface mode.
  • RIP optimization on WAN: Only triggered RIP updates are sent. Periodic updates are suppressed.
    • “ip rip triggered” on interface mode.
  • Show commands:
    • Show ip rip database
    • Show ip protocols
    • Show ip route rip
  • How RIP version-1 works: If a link is configured for RIPv1, the router will advertise;
    • All classful networks
    • All contiguous subnets which share the same major network and subnet mask as the link network on which it has to advertise.
    • All other networks are not advertised.
  • Passive interface in RIP: The router can learn updates from passive interface but will not send any updates on passive interface. Configured on the router mode using “passive-interface <>”
  • “ip prefix-list PERMIT_ALL permit 0.0.0.0/0 le 32”   <<< To permit all prefix.
  • “distribute-list prefix PERMIT_ALL  gateway EXCEPT_FROM_R1 in”  <<< To permit all prefix except advertised from R1 router.
  • RIP does not generate NULL route when generating summary address. (because of this, summary routes may generate routing loop when used with distribute-list)
  • “access-list 1 permit 0.0.1.0 255.255.254.255” : to permit  routes which have odd numbers in the third octet.
  • Extended Access-list behaviour:
    • In Redistribution and BGP: Source represent “network ID” and destination represent “network mask”
    • In distribute list of IGP: Source represent “source of the update” and destination represent “network address”.
  • “distance  <#> [src address  <mask>] [access-list for which AD has to change]”
  • RIP cannot advertise a route which is not available in its routing table. (except summary address and a default route generated using “default-information originate”)
  • While working on distribute-list make sure there is no route feedback problem.
  • To advertise default route to only R1 and not to other neighbors:
    • route-map DEFAULT_TO_R1_ONLY permit 10
    • set interface  <interface connecting to R1>
    • default-information originate route-map DEFAULT_TO_R1_ONLY
  • For conditional default route advertisement, match the condition-check route in a route-map.
  • “neighbor x.x.x.x” command will make router to send updates via unicast. But it will not suppress the multicast updates on that interface. To suppress those updates, use passive-interface command.
  • “ip rip v2-broadcast” interface command  makes RIP updates sent to BC instead of MC.
  • “ip rip triggered” interface command suppress  MC/BC updates.
    • Only triggered updates are sent.
    • useful on on-Demand circuit. Works only when both the ends are configured.
    •  Routes are tagged as “permanent”  in “show ip rip database”.
    • Route tables are exchanged initially & updates to the routing table are sent henceforth.
    • Trigger state goes through Down, Init, Loading  to Full. Updates are ACK-ed.
  • Once the router is enabled with RIP, first it sends a ‘request’ packet with 0.0.0.0/0 net and metric 16.
  • Sends the net ID of all active interfaces in ‘response’ packet with metric=1
  • If we shutdown an interface, it sends ‘response’ packet with net ID= shutdown interface and metric=16 to all other interface.
  • RIPng:
    • Port number: 521, MC address: FF02::9
    • Request and response same as in RIP.
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