NETGEAR RT210, RT211

Reference Guide for the Model RT210/RT211 ISDN Routers

Introduction 1-5

To work effectively, routers gather information about the networks connected to them and update

the other routers on the network with this information. In this way they build up a logical picture of

the network, known as a routing table, and work out the quickest route to other networks.

Routing Information Protocol

One of the protocols used to build and maintain this picture of the network is the Routing

Information Protocol (RIP). The RIP is a distance vector protocol, which means that all the

decisions about which path to use are based upon distance between source and destination. A

router using RIP would ﬁrst look at directly attached networks. These networks would be

considered to be one hop

away. Other networks that are not directly attached would be multiple

hops away. When there are multiple paths to a network, the path with the least number of hops is

chosen and is regarded as the best path. All other information about how to get to that network is

discarded.

Using RIP, the routers update each other every 30 seconds and check to see whether there is any

new information that should be added to the routing table. The entries in a routing table are

allowed to remain for 180 seconds. If no information about a remote network is received in this

time, the destination network is deemed to be unreachable and removed from the routing table.

An important consideration is how long it takes for a change to the routing topology, which is

typically a link or node failure, to be propagated throughout the entire RIP environment. This

process is called convergence. It is important that it occur as fast as possible so that all routers have

correct information about the changed network in their tables. With RIP, con ver gence takes place 1

hop at a time throughout the entire network. In using the 30 second routing information timer, the

routers pass information from one hop to another. To prevent the information that is being passed

from being excessively long, RIP is limited to 15 hops maximum.

The NETGEAR Model RT210 and Model RT211 ISDN routers support both RIP and RIP2. RIP2

has a number of enhancements, including the ability to transmit subaddress information with the

routing table, allowing networks with variable length subnetwork numbers to be supported.

Routing Updates

As can be seen from the previous routing protocol descriptions, routing updates are broadcast on a

regular basis. This process works well with leased circuit or permanent connections, where the

circuit is either functioning by passing data and routing updates, or not functioning, in which case

routing updates are not received. In this case, a time-out mechanism removes the relevant routes

from the routing table after no updates have been received for a deﬁned period.

NGRRT210.BK Page 5 Friday, October 17, 1997 2:37 PM