Routing Information Protocol


Distance-Vector Routing Protocols (RIP)

The distance-vector routing algorithm passes complete routing table contents to neighboring routers, which then combine the received routing table entries with their own routing tables to complete the router’s routing table. This is called routing by rumor, because a router receiving an update from a neighbor router believes the information about remote networks without actually finding out for itself. It’s possible to have a network that has multiple links to the same remote network, and if that’s the case, the administrative distance of each received update is checked first. If the AD is the same, the protocol will have to use other metrics to determine the best path to use to that remote network. RIP uses only hop count to determine the best path to a network. If RIP finds more than one link with the same hop count to the same remote network, it will automatically perform a round-robin load balancing. RIP can perform load balancing for up to six equal-cost links (four by default).

For avoiding routing Loops in (RIP)

  • Maximum Hop Count:- RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable.
  • Split Horizon This reduces incorrect routing information and routing overhead in a distance-vector network by enforcing the rule that routing information cannot be sent back in the direction from which it was received.
  • Route Poisoning When Network goes down, Router initiates route poisoning by advertising Network as 16, or unreachable.
  • Holddowns

    It is the time a router think that a route is up without receiving an update about that root.

    RIP is a true distance-vector routing protocol. RIP sends the complete routing table out to all active interfaces every 30 seconds. RIP only uses hop count to determine the best way to a remote network, but it has a maximum allowable hop count of15 by default, meaning that 16 is deemed unreachable. RIP works well in small networks, but it’s inefficient on large networks with slow WAN links or on networks with a large number of routers installed.

RIP version 1 uses only class-ful routing, which means that all devices in the network must use the same subnet mask. This is because RIP version 1 doesn’t send updates with subnet-mask information in tow. 

RIP version 2 provides something called prefix routing and does send subnet mask information with the route updates. This is called classless routing.

In the following sections, we will discuss the RIP timers and then RIP configuration.

RIP Timers

  • Route update timer

    Sets the interval (typically 30 seconds) between periodic routing updates in which the router sends a complete copy of its routing table out to all neighbors.
  • Route invalid timer

    Determines the length of time that must elapse (180 seconds) before a router determines that a route has become invalid. It will come to this conclusion if it hasn’t heard any updates about a particular route for that period. When that happens, the router will send out updates to all its neighbors letting them know that the route is invalid.
  • Holddown timer

    This sets the amount of time during which routing information is suppressed. Routes will enter into the holddown state when an update packet is received that indicated the route is unreachable. This continues either until an update packet is received with a better metric or until the holddown timer expires. The default is 180 seconds.
  • Route flush timer

    Sets the time between a route becoming invalid and its removal from the routing table (240 seconds). Before it’s removed from the table, the router notifies its neighbors of that route’s impending demise. The value of the route invalid timer must be less than that of the route flush timer. This gives the router enough time to tell its neighbors about the invalid route before the local routing table is updated.

Configuring RIP Routing

  • To configure RIP routing, just turn on the protocol with the router rip command and tell the RIP routing protocol which networks to advertise. That’s it. Let’s configure our five-router internetwork with RIP routing.

    RIP has an administrative distance of 120. Static routes have an administrative distance of 1 by default, and since we currently have static routes configured, the routing tables won’t be propagated with RIP information. We can add the RIP routing protocol by using the router rip command and the network command. The network command tells the routing protocol which class-ful network to advertise.Look at the Corp router configuration and see how easy this is:
     
    Corp # config t
    Corp (config) # router rip
    Corp (config-router) # network 10.0.0.0
    
    R1 (config-router) # do show ip route
    


    R2

    Let’s configure our R2 router with RIP:
     
    R2 # config t
    R2 (config) #router rip
    R2 (config-router) #network 10.0.0.0
    
    R2 (config-router) #do show ip route
    

Verifying the RIP Routing Tables
 

  • R3# sh ip route

    The show ip protocols command shows you the routing protocols that are configured on your router.

    Troubleshooting with the show ip protocols Command

    Router # sh ip protocols
    Router # sh ip interface brief
    The debug ip rip Command

    The debug ip rip command sends routing updates as they are sent and received on the router to the console session. If you are telnetted into the router, you’ll need to use the terminal monitor command to be able to receive the output from the debug commands.
     
    R3#debug ip rip
    RIP protocol debugging is on
    R3#terminal monitor
    R3#undeug all
    

  •  

discusses the differences between RIPv1 and RIPv2.
 

RIPv1 RIPv2
Distance vector Distance vector
Maximum hop count of 15 Maximum hop count of 15
Classful Classless
Broadcast based Uses multicast 224.0.0.9
No support for VLSM Supports VLSM networks
No authentication Allows for MD5 authentication
No support for dis-contiguous Supports dis-contiguous networks networks

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