Foundry Networks Switch and Router Installation And Configuration Manual page 492

Foundry Switch and Router Installation and Configuration Guide
When the software uses the default network route, it also uses the default network route's next hop gateway as
the gateway of last resort.
This feature is especially useful in environments where network topology changes can make the next hop
gateway unreachable. This feature allows the Layer 3 Switch to perform default routing even if the default
network route's default gateway changes.
The feature thus differs from standard default routes. When you configure a standard default route, you also
specify the next hop gateway. If a topology change makes the gateway unreachable, the default route becomes
unusable.
For example, if you configure 10.10.10.0/24 as a candidate default network route, if the IP route table does not
contain an explicit default route (0.0.0.0/0), the software uses the default network route and automatically uses
that route's next hop gateway as the default gateway. If a topology change occurs and as a result the default
network route's next hop gateway changes, the software can still use the default network route. To configure a
default network route, use the following CLI method.
If you configure more than one default network route, the Layer 3 Switch uses the following algorithm to select one
of the routes:
1. Use the route with the lowest administrative distance.
2. If the administrative distances are equal:
• Are the routes from different routing protocols (RIP, OSPF, or BGP4)? If so, use the route with the lowest
IP address.
• If the routes are from the same routing protocol, use the route with the best metric. The meaning of
"best" metric depends on the routing protocol:
• RIP – The metric is the number of hops (additional routers) to the destination. The best route is the route
with the fewest hops.
• OSPF – The metric is the path cost associated with the route. The path cost does not indicate the
number of hops but is instead a numeric value associated with each route. The best route is the route
with the lowest path cost.
• BGP4 – The metric is the Multi-exit Discriminator (MED) associated with the route. The MED applies to
routes that have multiple paths through the same AS. The best route is the route with the lowest MED.
Configuring a Default Network Route
To configure a default network route, use one of the following methods. You can configure up to four default
network routes.
USING THE CLI
To configure a default network route, enter commands such as the following:
BigIron(config)# ip default-network 209.157.22.0
BigIron(config)# write memory
Syntax: ip default-network <ip-addr>
The <ip-addr> parameter specifies the network address.
To verify that the route is in the route table, enter the following command at any level of the CLI:
BigIron(config)# show ip route
Total number of IP routes: 2
Start index: 1
Destination
1 209.157.20.0
2 209.157.22.0
This example shows two routes. Both of the routes are directly attached, as indicated in the Type column.
However, one of the routes is shown as type "*D", with an asterisk (*). The asterisk indicates that this route is a
candidate default network route.
15 - 50
B:BGP D:Connected R:RIP
NetMask
255.255.255.0
255.255.255.0
S:Static O:OSPF *:Candidate default
Gateway Port
0.0.0.0 lb1
0.0.0.0 4/11
Cost Type
1 D
1 *D
December 2000