Network Notes: RIP

Overview

Routing information protocol (RIP) is a interior distance vector routing protocol originally defined in RFC1058. RIP has had a number of improvements over the years with version 2 being defined RFC2453 and RIPng adding IPv6 support in RFC2080.

Transport

RIPv1 and RIPv2 utilize UDP/520, RIPng uses UDP/521 to transport RIP messages.

Metric

RIP uses the number of hops to a destination as its metric. RIP has a maximum hop count of 15 with 16 considered to be infinite and unreachable.

Neighborship

RIP does not form a neighborship with adjacent RIP routers. RIP processes routing updates received via either broadcast or mulitcast depending on the RIP version in use.

Routing Updates

	RIP	RIPv2	RIPng
Update Type	Broadcast	Multicast	Multicast
Update Interval	30 seconds	30 seconds	30 seconds
Update Address	255.255.255.255	224.0.0.9	FF02::9
Update Contents	Full update at each interval period.
Triggered Update	Immediate partial update when routing information changes. Only contains changed routes.
Invalid After Timer	If a route stops being received, it will be declared invalid after 180 seconds.
Holddown Timer	Once the <span class="px-1 py-[0.15rem] font-mono font-semibold break-words rounded text-latte-mauve dark:text-mocha-mauve bg-latte-crust dark:bg-zinc-900 dark:text-mocha-mauve">invalid after timer</span> expires the route will be advertised as unreachable for 180 seconds.
Flushed After Timer	Starts at the same time as the <span class="px-1 py-[0.15rem] font-mono font-semibold break-words rounded text-latte-mauve dark:text-mocha-mauve bg-latte-crust dark:bg-zinc-900 dark:text-mocha-mauve">invalid after timer</span>. If no updates are received for the route in 240 seconds the route is flushed from the routing table.

Message Format

RIPv1

RIPv2

RIPng

Summarization

RIPv1 is a classful routing protocol and summarization is not supported. RIPv2 and RIPng added support for classless routing and subnets can be summarized at any arbitrary block.

Loop Prevention

Mechanism	Description
Count to infinity	If a routes metric exceeds the infinity counter cease using that routes next hop.
Split horizon	Routes are not advertised out the same interface they where learned.
Split horizon with poison reverse	Routes learned from an interface are re-advertised out the same interface with an infinite metric.
Route Poisoning	When a route fails, set the routes metric to infinity in the next routing update.

Authentication

RIPv1 does not support authentication. RIPv2 support both plain-text and MD5 authentication. RIPng does not implement authentication and relies on the authentication and encryption capabilities of the underlying IPv6 protocol for routing update integrity.

Packet Captures

Various RIP PCAPs can be found here.

Bibliography

1.Kocharians, N. and Paluch, P. (2014) CCIE Routing and Switching v5.0 Official Cert Guide, Volume 1, 5th Edition - Chapter 7: RIPv2 and RIPng. Indianapolis: Cisco Press