eigrp stuff

[carl_townshend]

Can anyone tell me how I would include my loopback interface in the eigrp routing process ?

[Georg Pauwen]

Hello,

the loopback interface can be added to the EIGRP routing process just like any other interface. Here is an example:

R1

interface Loopback0

ip address 1.1.1.1 255.255.255.0

!

router eigrp 1

network 1.1.1.1 0.0.0.0

Is that what you are asking ?

Regards,

GP

[dknov]

Hi,

Just one more comment. Some older IOS versions did not include wild-card option. Some included the option, but disregarded it. In both of those cases using "network 1.1.1.1 0.0.0.0" would actually go to running config as "network 1.0.0.0". If your loopback interface is the only interface in that ClassA range, then you're OK, but it might not be...

Safer approach could be to redistribute connected, but be careful that in this case your loopback prefix will have an admin distance of 170 instead of 90, which might or might not be a consideration.

David

[carl_townshend]

I thought that you have to use the 32 bit mask with the loopback interface, Also would I use the loopback interface to advertise the network my pc's are on, and im sure ive heard of a command that you put in that makes it use the loopback as the network advertisement rather than the ethernet interface , any ideas

[dknov]

Hi,

It is indeed a common best practice to use a /32 bitmask for loopback interfaces, this, however, is not mandatory.

From you description it is not clear what you're trying to achieve, can you please elaborate?

David

[carl_townshend]

Hi There

I am trying to use the loopback interface to keep the routing process alive for eigrp, is this just a case of advertising the loopback interface ip address, no matter what ip it is ? is this purely used for keeping eigrp advertising, would you use the loopback as your default gateway on your machine ?

[ruwhite]

I'm not certain if you mean you want to keep the EIGRP router id constant, or if you want to keep EIGRP advertising something at all times, or (?). If you want to set the router id so it never changes, you can also use the router-id command under EIGRP.

:-)

Russ.W

[carl_townshend]

Hi Russ

I would like to keep the router id constant, I was told that people use loopback to keep routing process alive, Can anyone post me a simple config of this in action, and how people use the loopback, whether for the default gateway or just to keep eigrp etc up !!

[olorunloba]

EIGRP selects by default, the highest ip add of the loopback interface, or the highest physical interface address as the router id. Unlike BGP though, the interface is not key in the stability of the protocol, though it must be unique across the network.

Since by default, the highest loopback interface address is selected, no extra configuration is necessary. To however configure an arbitrary address, or force a particular address use the

eigrp router-id

command.

Check

http://www.cisco.com/en/US/products/ps6441/products_command_reference_chapter09186a00804a7166.html#wp1096683

With respect to default gateways of PCs, it is best practice for the default gateway of an ip device to be on the same subnet as one of its ip address. Because the loopback interface is not physical, it is often not used as the default gateway for PCs. Rather, the ip address of the ethernet interface is used.

[ruwhite]

"Unlike BGP though, the interface is not key in the stability of the protocol, though it must be unique across the network."

Actually, this is only sort-of true.... :-) All the routers in an EIGRP network can, today, have the same router ID, and nothing bad will happen, as long as you only have internal routes (!). The moment you have external routes injected into the network, however, any router with the same ID as the redistributing router will reject the external route.

As for network stability: EIGRP never changes the router ID when the interface we took the router ID from goes down. I don't know why that is, but I've spent many hours trying to find a router that used to have the interface x.x.x.x, so I could fix rejected external routes.... You don't need to worry about "keeping EIGRP up" through forcing the router ID to stay the same all the time. It'll stay the same all the time, anyway (I just checked this in the lab, on routers running 12.3T, and the behaviour is still this way).

This all should change, someday--there are plans to include the router ID in internal routes (in fact, I think the code is already in place, but I really haven't looked to see if recent changes have included adding this, or just left space for adding this--I'll have to go check). We don't normally recommend people allow EIGRP to pull it's ID from an interface address, any longer, but rather we recommend they configure the ID manually.

HTH.

:-)

Russ.W