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What is the correct way to advertise EIGRP networks?

Looking at the network advertisement under EIGRP it looks like all the network are advertised as host yet the routing table shows the correct subnet sizes when I do show ip route.  The networks are show in the running config as follows:

network 10.1.1.1 0.0.0.0

network 10.1.1.129 0.0.0.0

The routing table shows the networks correctly as shown here:

C       10.1.1.128/27 is directly connected, Vlan20
C       10.1.1.0/25 is directly connected, Vlan10

It seems to me that the correct way to advertise the networks is as follows:

network 10.1.1.1 0.0.0.127

network 10.1.1.129 0.0.0.31

With EIGRP does it make any difference in which way you enter the network advertisments and which is the best practice?

1 REPLY
Cisco Employee

What is the correct way to advertise EIGRP networks?

Hello,

This is a common question.

The mask in the network command has no effect on the advertised netmask whatsoever. In reality, the network command with the subnet and the mask merely defines a range of addresses. If a directly connected interface's address falls into one of these ranges, then the interface including its real subnet and mask is added to the routing protocol, and the real subnet/mask will be advertised.

In your particular case, the command network 10.1.1.1 0.0.0.0 defines a range of a single IP address - 10.1.1.1. If any directly connected interface has its IP address set to 10.1.1.1, then this interface including its real network will be added to EIGRP. Assuming that the interface is configured as 10.1.1.1/25, then EIGRP will be advertising the network 10.1.1.0/25, regardless of the wildcard mask present in the network statement.

Personally, I mostly prefer specifying the exact IP address of the interface and the wildcard mask 0.0.0.0 in the network statement, because of a couple of reasons:

  • It is very precise - each network command specifies an exact interface you want to add, not a possible range of interfaces
  • It is easy - you don't have to specify exact network and recalculate the subnet mask into wildcard mask
  • It is similar to the way how the IPv6 routing protocols work in that you add interfaces, not networks, to the routing process. Specifying an exact IP address is similar to individually adding each interface to the routing process.

Surely, however, specifying a dozen or more interfaces using this method can be tedious, so there are situations where a network statement specifying a larger network is outspokenly useful.

Please feel welcome to ask further!

Best regards,

Peter

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