I have a router with an interface programmed called Loopback99

[AidanEnos]

Why would this interface exist? It's got an ip address that is not on the network, it transmits no traffic and basically has no real purpose in life.

The network is using the WAN protocol EIGRP. From what I've found the only use for a Loopback99 interface is for OSPF. As I understand the router uses highest numerical address on the box as it's router ID in OSPF, does the same hold true for EIGRP or is it just a wasted bit of programming here?

Thanks,

 

[ccie2254]

There are several reasons why a loopback interface could be there...OSPF, as you said, uses a loopback address as its RouterID if one is present...DLSW and RSRB could use it... an unnumbered interface may use it also, because a loopback interface never goes down.
Be careful before you remove it!
Regards

 

[bkeyser]

The loopback interface will ALWAYS be up when the router is up. EIGRP will use it just like OSPF, to identify the router. One way to use the loopback address is to assign a subnet for the exclusieve use of loopbacks. Define them using a /32 mask and you can use 1 subnet for 255 routers.

 

[rsfoyles]

Don't let it bother you. It probably isn't doing anything in or to your network. It is not likely that you have an unnumbered serial interface and I have no idea what EIGRP would use it for. To the best of my recollection, I have only used it with ospf and bgp other than to give an unnumbered serial interface an address.

 

[wybnormal]

Loopback addresses are part of Cisco's best practices for OSPF and virtually any other router deployment. They add a level of stablity to OSPF and EIGRP by giving the router an interface that you can put the highest IP number on so it becomes the router ID. Since it's logical, it doesnt flap up and down which would start a convergence that is not needed.

If you happen to use DLSW, you need a loopback to give a stable remote peer IP address and local address.

You can telnet to the loopback do all maintance through it..

If you design the IP schema well enough, it can help arrange the IP to router in a logical fashion.. ie..

10.10.10.1 is core
10.10.10.2 is the next in the list and so on... it's easier then trying to remember what IP is the serial and what is the ethernet and when it's used by HP Openview, it will make MUCH more sense

Mike S
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin