Overlapping eigrp address

[jwnetworker]

What would be the effects of the following:

router eigrp 100
network 10.1.40.0 0.0.1.255
network 10.1.42.0 0.0.0.255

Would this cause any routing issues or dropped packets?

[Giuseppe Larosa]

Hello jwnetworker,

no there is no issue because actually there is no overlapping between the two statements

router eigrp 100
network 10.1.40.0 0.0.1.255   --->  all interfaces with IP between 10.1.40.1 and 10.1.41.255
network 10.1.42.0 0.0.0.255   ---> all interfaces with IP between 10.1.42.1 and 10.1.42.255

I even think there is no issue also on the following case:

router eigrp 100
network 10.1.40.0 0.0.3.255
network 10.1.42.0 0.0.0.255

There is no ambiguity both commands make interfaces in address blocks to run EIGRP AS 100.

The router CLI parser can complain or display a warning.

In the worst case the second command is ignored/removed  because it is a subset of the first one (in second example)

Hope to help

Giuseppe

[Richard Burts]

I agree with @Giuseppe Larosa that there is no problem if the addresses did overlap. He mentions it and I would like to go a bit deeper into the function of the network command in eigrp. Some people believe that the network command specifies what to advertise and if that were the case then overlapping might cause problems (what if one network command followed one path but the other network command followed a different path?). The network command does not specify what to advertise but is used by eigrp to figure out which interfaces should run eigrp. In this case overlap is not a problem at all. If there is an overlap exactly the same interfaces will run eigrp and the overlap does not change anything. The overlap might be redundant, especially as in the second example, and not necessary. But it certainly will not cause any problems.

HTH

Rick