Monday 16 May 2011

RIP intro


Welcome to the wonderful world of dynamic routing protocols. Routing Information Protocol; aka: RIP, will be the first of 3 dynamic routing protocols you must know as a CCNA network engineer. RIP was widely deployed in 1982 due to the inclusion of its code in the BSD 4.2 Unix OS then a few years later, June of 1988 became an IETF Standard. RIP Replaced GGP (Gateway to Gateway Protocol) which is a protocol you’ll never hear about again as its long gone and turned into ash.
Before you jump onto the router lets first discuss a little bit of the technology relating to RIP;
RIP is a distance vector routing protocol which has an administrative distance of 120 and uses the Bellman-Ford Algorithm. RIP uses hop counts as a metric to determine the best path to a network. e.g; If 10.70.10.0/24 is 2 routers away then its effectively 2 hop counts away thus having a metric of 2. Keep in mind that if you have a PC trying to get to a server on a network and one path has 2 hop counts which uses T1′s links and the other path has 3 hop and uses 3 DS3′s then the lower metric will be chosen.
RIP operated at UDP Port 520 by default. Version 1 uses a broadcast delivery mechanism for updates whereas version 2 uses multicast. This will be discussed further in detail in the next lab.
By standard; RIP utilizes three different timers to function properly however if you’re running RIP on a Cisco router a 4th timer will be used called the Hold Down timer. Timer functions given below;
Update Timer: This interval at which “hello packets” are sent to neighboring routers. The Cisco Default is every 30 seconds +/- 5 Seconds to prevent synchronization
Invalid Timer: The invalid timer is the interval at which the router will mark the route(s) invalid by using a metric of 16 and advertise them with an unreachable metric (16). When a route is marked invalid on a Cisco router it is enters hold down. The Cisco default invalid timer is 180 Seconds.
Hold Down Timer: The hold down timer is the interval at which a specific route that has been previously marked invalid will be suppressed updates with an equal or greater metric. This timer is intended to prevent inaccurate routing updates (routing by rumor) till the topology has had the time to converge or a route with a better metric has been received.. The hold down timer is a Cisco extension of the RIP protocol and is not included int he IETF standard.
Flushed Timer: The interval at which after the invalid timer the router waits after a route has been marked invalid before it flushes it (removes it) from the routing table.

No comments:

Post a Comment