RIP

This is how a new RIP enabled router knows about routes from its connected routers. RIP operates on UDP 520. A new RIP enabled router broadcasts a Request message out of each RIP enabled interface. The neighbor respond to the Request mesage with updates in a Response message. The router then extracts the prefix & next-hop information to build a routing table. There after every 30 seconds the router updates its entire routing table to the neighbors in form of Response message.

RIPv1

• Classfull Routing,doesnt carry mask with updates
• Updates are Broadcast every 30 sec.
• No authentication of routes.

RIPv2

• Classless routing, carry mask with updates
• Authentication of routing updates
• Carries next-hop with each route entry
• External route-tags to identify redistributed routes
• Multicast routing updates on 224.0.0.9

Each RIP message can send updates for up to 25 routes.

RIP Update Timers

If a router doesnt hear update for a prefix in the 30 sec i.e. usual update time, it starts the invalid timer which lasts for upto 180 sec. At the end of this 180 sec, holdown timer starts till the timer for route reaches 240 sec. After 240 sec, the router flushes the rote from its routing table.

If during this period, an update is recieved from a router for the same route with a higher metric then the router waits for either the holdown timer to expire or flush timer whichever is earlist.

Any router interface can be configured to send & receive either v1 or v2 updates with following comands: "ip rip send version #", "ip rip recieve version #".

I'm yet to find out the significance of "next-hop" in the v2 updates. I thought next-hop should be there with every update. However, Jeff Doyle book mentioned it that way. I'll update this once I find answer to my query.

I got it from Wendell Odom & JDoyle books. Next-hop feature in v2 allow RIP to advertise a different next-hop router than the advertising router which may be a better option to reach destination than advertising router. If the field is set to 0.0.0.0 in the update, it means that the advertising router is the best router for destination.

Offset-lists

Help vary the metric of routes for prefixes being send out or recvd from neighbor. ACL is used to match the prefixes, then specific offset is added to the matches in acl & advertised based on the direction mentioned on interface "in / out". For routes not matching the acl, metric remains unchanged. "offset-list acl# in/out offset# interface#"