Configuring RIP (v1)

RIP version 1 overview

-RIP v1 is a classful routing protocol, as a result, auto-summarization is used and subnet masks are not sent in routing updates.  RIP cannot be used in discontinguous networks.

-RIP determines the subnet mask for subnet ID's by looking at the address masks configured on its own interfaces for the classful network in question.  This is why a VLSM IP address scheme cannot be used.  If the router does not have any interfaces assigned to the address received in the routing update, RIP will assume the classful mask.

-Routing updates are sent as a broadcast by default every 30 seconds on UDP port 520

-RIP Timers

This can be a confusing topic, and in general, there is conflicting information on the operation of these timers. By default, RIP uses a 30 second update timer, a 180 second invalid timer, a 240 second flush timer, and a 180 second holddown timer.

Each time a routing advertisement is received for a particular route (30 seconds by default), the invalid and flush timers reset to zero and begin counting upwards.  If the invalid timer reaches 180 seconds, the route is marked as invalid, and the hold-down timer begins.  The flush timer counts alongside the invalid timer, and upon reaching 240 seconds, the route in question will be flushed from the routing table.

There is controversy concerning the operation of the hold-down timer.  The first, and most reoccurring theory  is that during the hold-down timer, the router can accept routes from any source as long as the metric is equal to, or lower then the original metric used for the route. This is what I saw happen in my physical lab.

The second theory is that the router cannot install any routes until the hold-down timer expires, regardless of the metric.

I tested this using physical lab equipment, and it seems that using the default settings and timers, the routers will accept a route from the original source, even if the hold-down timer has not yet expired, perhaps the router behavior is different based upon the IOS version in use.

My topology consisted of :

R1 LAN --->R1 ------ > R2 ------- > R3

When I set R1's serial link to not send RIP traffic (passive-interface s0/0 in router config mode), R2 continued to send updates to R3 every 30 seconds for R1's attached fa0/0 LAN.  After the invalid timer expired, R2 sent a poisoned route for R1's LAN out all interfaces-it increased the metric from 1 to 16, declaring it an invalid route.  When I re-enabled RIP traffic on R1's serial, R2 received the advertisement for R1's LAN, and before the hold-down timer had expired, installed the route and resumed forwarding advertisements with the correct metric to R3.