22-56
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 22 Configuring IP Unicast Routing
Configuring Protocol-Independent Features
The switch retains static routes until you remove them (by using the no ip route global configuration
command). However, you can override static routes with dynamic routing information by assigning
administrative distance values. Each dynamic routing protocol has a default administrative distance, as
listed in
Table 22-11
. If you want a static route to be overridden by information from a dynamic routing
protocol, set the administrative distance of the static route higher than that of the dynamic protocol.
Static routes that point to an interface are advertised through RIP, IGRP, and other dynamic routing
protocols, whether or not static redistribute router configuration commands were specified for those
routing protocols. These static routes are advertised because static routes that point to an interface are
considered in the routing table to be connected and hence lose their static nature. However, if you define
a static route to an interface that is not one of the networks defined in a network command, no dynamic
routing protocols advertise the route unless a redistribute static command is specified for these
protocols.
When an interface goes down, all static routes through that interface are removed from the IP routing
table. When the software can no longer find a valid next hop for the address specified as the forwarding
router's address in a static route, the static route is also removed from the IP routing table.
Specifying Default Routes
A router might not be able to determine the routes to all other networks. To provide complete routing
capability, you can use some routers as smart routers and give the remaining routers default routes to the
smart router. (Smart routers have routing table information for the entire internetwork.) These default
routes can be dynamically learned or can be configured in the individual routers. Most dynamic interior
routing protocols include a mechanism for causing a smart router to generate dynamic default
information that is then forwarded to other routers.
Specifying a Default Network
If a router has a directly connected interface to the specified default network, the dynamic routing
protocols running on that device generate or source a default route. In the case of RIP, it advertises the
pseudonetwork 0.0.0.0. In the case of IGRP, the network itself is advertised and flagged as an exterior
route.
A router that is generating the default for a network also might need a default of its own. One way a
router can generate its own default is to specify a static route to the network 0.0.0.0 through the
appropriate device.
Table 22-11 Dynamic Routing Protocol Default Administrative Distances
Route Source
Default Distance
Connected interface
0
Static route
1
Enhanced IRGP summary route
5
Internal Enhanced IGRP
90
IGRP
100
OSPF
110
RIP
120
Unknown
225
