AIS Router AI2524 - Manual (Page 166)

AI2524 Router Card User’s Manual
Page 9-8 August 1997
2524UM
Understand
Routing Processes The basic function of a router is to forward packets: receive a packet
in one interface and send it out another (or the same) interface to the
proper destination. All routers do this by looking up the destination ad-
dress in a table. The tables can be built either dynamically or statically.
If you are configuring all the entries in the table yourself, you are using
static routing. If you use a routing process to build the tables, you ar
using dynamic routing. It is possible, and sometimes necessary, to use
both static and dynamic routing simultaneously.
When you configure only ISO CLNS and not routing protocols, the
Cisco IOS software only makes forwarding decisions. It does not per-
form other routing-related functions. In such a configuration, the soft-
ware compiles a table of adjacency data, but does not advertise this
information. The only information that is inserted into the routing table
is the NSAP and NET addresses of this router, static routes, and adja-
cency information.
You can route ISO CLNS on some interfaces and transparently bridge
it on other interfaces simultaneously. To do this, you must enable con-
current routing and bridging by using th
bridge crb command.
Dynamic Routing
Cisco supports these two dynamic routing protocols for ISO CLNP
networks:
z ISO IGRP
z IS-IS
When dynamically routing, you can choose either ISO IGRP or IS-IS,
or you can enable both routing protocols at the same time. Both routing
protocols support the concept of areas. Within an area, all routers
know how to reach all the system IDs. Between areas, routers know
how to reach the proper area.
ISO IGRP supports three levels of routing: system routing, area rout-
ing, and interdomainrout ing. Routing across domains
(interdomai nrouting) can be static or dynamic with ISO IGRP. IS-IS
supports two levels of routing: station routing (within an area) and are
routing (between areas).
Intermediate Systems (IS) and End Systems (ES)
Some ISs keep track of how to communicate with all the ESs in their
areas and thereby function as Level 1 routers (also referred to as local
routers). Other ISs keep track of how to communicate with other areas
in the domain, functioning as Le vel2 routers (sometimes referred to as
area routers). The AI2524 router is always Le vel1 and L evel2 when