Default Routes "IP Route 0.0.0.0 0.0.0.0 and Load Balancing.

[dhogshead]

I found this document saying you can configure two default routes using the "ip route 0.0.0.0 0.0.0.0" command and the router will load balance between the two routes. Is this correct? Is this something new, and if so what version of the IOS was it implemented in? I thought load balancing need a routing protocal such as OSPF or RIP2 to work.

See:http://www.cisco.com/en/US/customer/tech/tk365/tk554/technologies_tech_note09186a0080094374.shtml

You will find the following Note at end of the document:

Note: If you configure multiple networks as candidate default routes using the ip default-network command, the network that has the lowest administrative distance is chosen as the network for the gateway of last resort. If all the networks have the same administrative distance then the network listed first in the routing table (show ip route lists the routing table) is chosen as the network for the gateway of last resort. If you use both the ip default-network and ip route 0.0.0.0 0.0.0.0 commands to configure candidate default networks, and the network used by the ip default-network command is known statically, the network defined with the ip default-network command takes precedence and is chosen for the gateway of last resort. Otherwise if the network used by the ip default-network command is derived by a routing protocol, the ip route 0.0.0.0 0.0.0.0 command, which will have a lower administrative distance, takes precedence and is chosen for the gateway of last resort. Lastly, if you use multiple ip route 0.0.0.0 0.0.0.0 commands to configure a default route, traffic is load-balanced over the multiple routes.

[omohamed]

Hi,

Load-balancing is a concept that allows a router to take advantage of multiple best paths to a given destination. The paths are derived either statically or with dynamic protocols, such as RIP, EIGRP, OSPF, and IGRP.

So in this scenario if we have multiple of default static routes in the routing and depending on the switching mechanish we per-packet or per-destination based load-balancing. To summarize as long as there are multiple routes in the routing table for the same destination with equal cost the router will automatically do load-balancing.

Hope this helps.

[dhogshead]

Yes thats what I wanted to know. My coworkers and myself were not aware that simple static or default routes would load balance automaticly. I just wanted to be sure it was true, and not an error in the document.

Thanks for the help!

[ruwhite]

This capability was put in someplace before 10.0, I think.... Think of the routing table as a simple database. There are several processes putting information in the routing table--routing protocols, static routes, MPLS, Multicast, and other things--and several processes that use information from the routing table. Load sharing is actually done by two of the processes that use information from the routing table, the process switching path and CEF.

So, it doesn't matter how the information gets into the routing table, as long as there are multiple equal cost paths (and in some cases, unequal cost paths) in the routing table, the switching processes will use the multiple paths to load share.

I wouldn't use the ip default-network command, I would always use the ip route 0.0.0.0 0.0.0.0 command. ip default-network was specifically designed for IGRP, and IGRP is gone from the code, so....

:-)

Russ.W