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RIP Stability Features, Dynamic Routing

Like other routing protocols, RIP uses certain timers to regulate its performance. The biggest drawback to a RIP router is the broadcast it makes. In order to keep the routing tables up to date, each router periodically transmits it entire routing table to all the other routers on the network.

The update timer is generally set to 30 seconds, ensuring that each router will send a complete copy of its routing table to all neighbors every 30 seconds. While this alone is not a major detriment to network traffic, the routers also transmit a route response packet. This is controlled by the route invalid timer, which determines how much time must expire without a router having heard about a particular route before that route is considered invalid.

When a route is marked invalid, neighbors are notified of this fact. This notification must occur prior to expiration of the route flush timer. When the route flush timer expires, the route is removed from the routing table. Typical initial values for these timers are 90 seconds for the route invalid timer and 270 seconds for the route flush timer.

Stability Features

RIP makes its operation more stable using a variety of features designed to accommodate rapid network topology changes. Some of these features includes a hop-count limit, hold-downs, split horizons, and poison reverse updates.

Hop-Count Limit

RIP permits a maximum hop count of 15. Any destination greater than 15 hops away is tagged as unreachable. RIP's maximum hop count greatly restricts its use in large internetworks, but prevents a problem called count to infinity from causing endless network routing loops.

An example would if Router 2's link to Network A is via Router 1's link.

If Router 1's link to network A fails, Router 1 check its information and sees that Router 2 has a one-hop link (which is actually via Router 1) to Network A and it would began advertising it has a two-hop link to Network A and then route all traffic to Network A through Router 2.

This would create a routing loop, since when Router 2 sees that Router 1 get to Network A in two hops, it alters it own routing table entry to show it has a three-hop path to Network A.

Routing Information Protocol, Routing Table Format
Routing Table Format, Stability Features - Hop-Count Limit
Stability Features - Hop Count Limits, Hold-Downs, Split Horizons
Stability Features - Poison Reverse Updates, RIP Version 2 (RIPv2)