6 Configuring Routing258 Voyager Reference GuideGeneration of Exterior RoutesIGRP has three defined types of routes that an update packet can carry:! Interior! System! ExteriorNoteSee the IGRP specification for a detailed explanation of the different routetypes.An exterior route is conceptually the same as a system route, with the addedfeature that an exterior route can be used as a default route. Exterior routes arealways propagated as exterior. When it is necessary to locally generate anexterior default route, the default route should be redistributed into IGRP. Thenext-hop network of the default route, determined from the next-hopinterface, is advertised in the appropriate IGRP update messages as exterior.Note that a direct interface route is advertised only once. Therefore, a directinterface route that is marked exterior is not also advertised as interior or assystem.Aliased InterfacesWhen an interface has multiple addresses configured, each address is treatedas a distinct interface since it represents a logical subnet. This implies that anupdate 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 runIGRP as opposed to the complete interface (all addresses of the interface).IGRP AggregationMost routing aggregation occurs only if explicitly configured; therefore, it isworth noting some of the implicit aggregation that occurs in IGRP. By
