Nokia Network Voyager, Reference Manual

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6
Configuring Routing
258 Voyager Reference Guide
Generation of Exterior Routes
IGRP has three defined types of routes that an update packet can carry:
! Interior
!
System
! Exterior
Note
See the IGRP specification for a detailed explanation of the different route
types.
An exterior route is conceptually the same as a system route, with the added
feature that an exterior route can be used as a default route. Exterior routes are
always propagated as exterior. When it is necessary to locally generate an
exterior default route, the default route should be redistributed into IGRP. The
next-hop network of the default route, determined from the next-hop
interface, is advertised in the appropriate IGRP update messages as exterior.
Note that a direct interface route is advertised only once. Therefore, a direct
interface route that is marked exterior is not also advertised as interior or as
system.
Aliased Interfaces
When an interface has multiple addresses configured, each address is treated
as a distinct interface since it represents a logical subnet. This implies that an
update is sent for each IGRP-configured address. In the configuration syntax,
it is possible to specify a particular address of an interface on which to run
IGRP as opposed to the complete interface (all addresses of the interface).
IGRP Aggregation
Most routing aggregation occurs only if explicitly configured; therefore, it is
worth noting some of the implicit aggregation that occurs in IGRP. By