Extreme Networks Summit 300-48 - Manual (Page 155)

Overview of IP Unicast Routing
Summit 300-48 Switch Software User Guide 155
— 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.
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. 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 58, 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.
IP Route Sharing
IP route sharing allows multiple equal-cost routes to be used concurrently. IP route sharing can be used
with static routes. To use IP route sharing, use the following command:
enable iproute sharing
Next, configure static routes as you would normally. ExtremeWare supports unlimited route sharing
across static routes.
Route sharing is useful only in instances where you are constrained for bandwidth. This is typically not
the case using Extreme switches. Using route sharing makes router troubleshooting more difficult
because of the complexity in predicting the path over which the traffic will travel.