RIPv2: Route Exchange

RIPv2 exchanges routes by sending RIPv2 updates on each RIPv2 enabled interface based on the update timer.  A RIPv2 router will advertise its connected routes as well as other learned routes in the routing table.

RIPv2 does not form neighborships with other RIPv2 enabled routers, nor do they use any sort of Hello protocol to track an adjacencies connectivity. All they do is simply sends updates to a destination multicast address 224.0.0.9.

There are two types of RIPv2 Messages:  Requests and Responses

The message format is identical between the two types as shown below:

ripv2message.PNG

The message contains a 4Byte header containing the following fields:

  • Command field – 1 for Request 2 for Response
  • Version Field – 2 for RIPv2
  • The remainder of the message consists of routing entries with each entry containing 20 octets in total.

At most 25 routing entries can be placed into a single RIP message.  Each routing entry contains the address family identifier identifying the format of the address information in the routing entry.

A Request message is used to ask RIPv2 devices to send a partial or full RIP update immediately,  instead of waiting on the update timer to resend the RIP messages.

A Full RIP update is requested when the request message contains exactly one route entry with the address family id set to 0 and metric set to 16.  Otherwise if the request message lists one or more networks on the the update on these networks is requested.

Full updates are requested initially when a router comes online or when routes are cleared using the clear ip route command

RIPv2 uses the hop count metric, counting the number of routers that need to be traversed to get to the destination network.  15 hops is the largest distance RIP will consider to be a valid path.  16 is considered to be infinity, instead of adding its own metric to the route it first adds 1 to each metric when building the routing update.

When RIPv2 learns multiple routes to the same subnet, the lowest metric route is chosen if equal metric routes exist to the same network, the router will install up to 4 such routes into the routing table.

ripv2.PNG

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