AIS AI2524 user manual Static Routing, Routing Decisions

Chapter 9: AI2524 OSI/CLNP Configuration Steps

routing ISO IGRP; it can be configured to be Level 1 only, Level2 only, or both Level 1 and Leve l2 when routing IS-IS.

ESs communicate with ISs using the ES-IS protocol. Lev el1 and

Level 2 ISs communicate with each other using either ISO IS-IS or

Cisco's ISO IGRP protocol.

Static Routing

Static routing is used when it is not possible or desirable to use dy- namic routing. Here are some instances of when you would use static routing:

zIf your network includes WAN links that involve paying for con- nect time or for per-packet charges, use static routing, rather than paying to run a routing protocol and all its routing update packets over that link.

zIf you want routers to advertise connectivity to external networks, but you are not running an interd o mainrouting protocol, you must use static routes.

zIf you must interoperate with another vendor's equipment that does not support any of the dynamic routing protocols that Cisco supports, you must use static routing.

zFor operation over X.25, Frame Relay, or SMDS networks, static routing is generally preferable.

Warning: An interface that is configured for static routing cannot reroute around failed links.

Routing Decisions

A CLNP packet sent to any of the defined NSAP addresses or NETs will be received by the router. The Cisco IOS software uses this algo- rithm to select which NET to use when it sends a packet:

zIf no dynamic routing protocol is running, use the NET defined for the outgoing interface, if it exists; otherwise, use the NET defined for the router.

zIf ISO IGRP is running, use the NET of the ISO IGRP routing pro- cess that is running on the interface.

zIf IS-IS is running, use the NET of the IS-IS routing process that is running on the interface.

August 1997	Page 9-9
2524UM