User guide

Overview of IP Unicast Routing
Summit24e3 Switch Installation and User Guide 169
— Locally, by way of interface addresses assigned to the system
— By other static routes, as configured by the administrator
NOTE
If you define a default route, and subsequently delete the VLAN on the subnet associated with the
default route, the invalid default route entry remains. You must manually delete the configured default
route.
Dynamic Routes
Dynamic routes are typically learned by way of RIP or OSPF. Routers that use RIP or OSPF exchange
information in their routing tables in the form of advertisements. Using dynamic routes, the routing
table contains only networks that are reachable.
Dynamic routes are aged out of the table when an update for the network is not received for a period of
time, as determined by the routing protocol.
Static Routes
Static routes are manually entered into the routing table. Static routes are used to reach networks not
advertised by routers.
Static routes can also be used for security reasons, to control which routes you want advertised by the
router. You can decide if you want all static routes to be advertised, using one of the following
commands:
[enable | disable] rip export static
[enable | disable] ospf export static
The default setting is disabled. Static routes are never aged out of the routing table.
A static route must be associated with a valid IP subnet. An IP subnet is associated with a single VLAN
by its IP address and subnet mask. If the VLAN is subsequently deleted, the static route entries using
that subnet must be deleted manually.
Multiple Routes
When there are multiple, conflicting choices of a route to a particular destination, the router picks the
route with the longest matching network mask. If these are still equal, the router picks the route using
the following criteria (in the order specified):
• Directly attached network interfaces
• ICMP redirects (refer to Table 50, later in this chapter)
• Static routes
• Directly attached network interfaces that are not active.
If you define multiple default routes, the route that has the lowest metric is used. If multiple
default routes have the same lowest metric, the system picks one of the routes.
You can also configure blackhole routes — traffic to these destinations is silently dropped.