eigrp default network command

Unanswered Question
Feb 20th, 2008

Hi all, when adding the default network command say in eigrp, what does this do, as its not the same as a default route, or is it ?

I have this problem too.
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royalblues Wed, 02/20/2008 - 05:29

A router configured with this command considers the network listed in the command as the last-resort gateway that it will announce to other routers

The network specified by this command must be reachable by the router that uses this command before it announces it as a candidate default route to other EIGRP routers. The network specified by this command must also be passed to other EIGRP routers so that those routers can use this network as their default network and set the gateway of last resort to this default network.

This requirement means that the network must either be an EIGRP-derived network in the routing table or be generated using a static route, which has been redistributed into EIGRP.



carl_townshend Wed, 02/20/2008 - 06:36

is this the same as a default route, I cannot understand what this is achieving ? as surely it will have a route to that default network anyway, so why does it need to advertise it again ?

ruwhite Wed, 02/20/2008 - 06:55

The default network command either marks an existing major network or creates and redistributes a major network into EIGRP as the "default network." This is a pseudo default network, which means that any traffic which doesn't have a matching destination in the routing table will be shipped to the router advertising the default network (just like a default route).

In newer CEF code, the default network is actually converted to a 0/0 route, to reclude special handling in the forwarding plane. If there is a default route (0/0) and a default network, this can lead to some unexpected results.

Historically, the default network command was put into the code to support IGRP--the way IGRP encodes metworks, there is no way to encode an empty network (0/0), thus IGRP couldn't support the default route, and a workaround had to be improvised. EIGRP supports this to provide backward compatibility with IGRP. No other routing protocol supports the default network--IE, a default network will cause a "gateway of last resort" to be added to the local routing table, but no protocol other than EIGRP (now that IGRP has been removed from the code) will advertise the route as a default network--it won't replace a default route in OSPF, IS-IS, or BGP.

Generally speaking, I don't recommend the use of the default network command at all. It's really far past it's usefulness, since the 0/0 route is supported by all current protocols, and it's confusing to troubleshoot and manage. Very few people expect the default route to be anything other than 0/0 any longer, so a lot of folks miss a default network even if it's in the table.





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