Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 73)

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 Autonomous System.
The best route is the route with the lowest MED.
Configuring a default network route
You can configure up to four default network routes.
To configure a default network route, enter commands such as the following.
device(config)# ip default-network 10.157.22.0
device(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.
device# show ip route
Total number of IP routes: 2
Start index: 1 B:BGP D:Connected R:RIP S:Static O:OSPF *:Candidate default
Destination NetMask Gateway Port Cost Type
1 10.157.20.0 255.255.255.0 0.0.0.0 lb1 1 D
2 10.157.22.0 255.255.255.0 0.0.0.0 4/11 1 *D
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.
Configuring a default network route
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