User's Manual

System Configuration
74
5
Dynamic Routing
The Gateway supports RIP (also referred to as RIP-1) and RIP-2 dynamic routing
protocols. Routing Information Protocol (RIP) is the most widely used method for
dynamically maintaining routing tables. RIP uses a distance vector-based approach
to routing. Routes are chosen to minimize the distance vector, or hop count, which
serves as a rough estimate of transmission cost. Each router broadcasts its
advertisement every 30 seconds, together with any updates to its routing table. This
allows all routers on the network to build consistent tables of next hop links which
lead to relevant subnets.
RIP can utilize any of the following methods to prevent loops from occurring:
• Split horizon – Never propagate routes back to an interface port from which they
have been acquired.
• Poison reverse – Propagate routes back to an interface port from which they have
been acquired, but set the distance-vector metrics to infinity. (This provides faster
convergence.)
•
Triggered updates – Whenever a route gets changed, broadcast an update message
after waiting for a short random delay, but without waiting for the periodic cycle.
RIP-2 is a compatible upgrade to RIP-1. RIP-2 adds useful capabilities for plain text
authentication, multiple independent RIP domains, variable length subnet masks,
and multicast transmissions for route advertising (RFC 1723).
There are several serious problems with RIP that should be considered. First of all,
RIP-1 has no knowledge of subnets, both RIP versions can take a long time to
converge on a new route after the failure of a link or router during which time routing
loops may occur, and its small hop count limitation of 15 restricts its use to smaller
networks. Moreover, RIP-1 wastes valuable network bandwidth by propagating
routing information through broadcasts; it also considers too few network variables
to make the best routing decision.
If the local network connected to the LAN interface does not include any routers,
then it is not necessary to configure either static or dynamic routing for this interface.
Also, if the path from the Gateway back to the router (that is, the remote gateway) at
the ISP’s central office does not pass through any other routers, then this gateway
will always be the first hop, and again it will not be necessary to configure either
static or dynamic routing.