Allied Telesis AT-8550 - Manual (Page 132)

Overview of IP Unicast Routing
8-4
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 8-5 , later in this chapter)
❑ Static routes
❑ Directly attached network interfaces that are not active.
Note
If you define multiple default routes, the route that has the lowest
metric is used. If there are multiple default routes that 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 or with OSPF routes. In OSPF, this capability is referred to as
equal cost multi-path (ECMP) routing. To use IP route sharing, first
enable it. Next, configure static routes and/or OSPF as you would
normally. As many a five ECMP routes can be used for a given
destination.
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.