RIP (Routing Information Protocol)

RIP is an intra-AS (interior gateway) routing protocol based on the distance vector algorithm (Bellman-Ford). It works inside a single autonomous system and is defined in RFC 1058 (RIPv1) and RFC 2453 (RIPv2).

Key characteristics

• Metric = hop count. Every link costs 1. The cost of a route is the number of routers (hops) to the destination.
• Maximum hop count = 15. A metric of 16 means infinity (unreachable), so RIP suits only small networks.
• Transport: RIP messages are carried in UDP, port 520.
• Updates: each router sends its entire routing table to its directly connected neighbours every 30 seconds (periodic advertisement).
• Versions: RIPv1 is classful and broadcasts updates; RIPv2 supports CIDR/subnet masks, multicast (224.0.0.9) and authentication; RIPng supports IPv6.

Routing table entry

Destination network | Next hop | Distance (hops)

Algorithm (Bellman-Ford update)

When router X receives a vector from neighbour N:

d(X, D) = min over neighbours [ 1 + d(N, D) ]

1. Add 1 to the cost of every route advertised by the neighbour.
2. If the destination is new, add it with next hop = N.
3. If it is already known via N, replace the cost with the new value (even if larger, since N is the next hop and its information is authoritative).
4. If it is known via a different next hop, replace it only if the new cost is smaller.

RIP message types

• Request: asks a neighbour for all or some routes (sent at start-up).
• Response: carries routing table entries, either periodic or triggered.

Timers

Timer	Value	Purpose
Periodic	30 s	Send routing updates
Expiration (timeout)	180 s	If no update for a route within this time, mark it invalid (metric 16)
Garbage collection	120 s	Keep the invalid route advertised as 16 before deleting it

Problems and solutions

Count-to-infinity / slow convergence: bad news travels slowly, so routers may keep increasing the cost of a failed route until it reaches 16.

Fixes:

• Split horizon: do not advertise a route back out of the interface from which it was learned.
• Poison reverse: advertise it back with metric 16 (infinity).
• Triggered updates: send an update immediately when a route changes.
• Hold-down timers and a low infinity value (16).

Advantages and disadvantages

Advantages	Disadvantages
Simple and easy to configure	Limited to 15 hops
Low router resource usage	Slow convergence, count-to-infinity
Widely supported	Hop count ignores bandwidth and delay
	Sends the full table every 30 s, wasting bandwidth

OSPF is preferred for larger networks.