loopback ip address selection

Answered Question
Jun 17th, 2008

Hi,

I need to configure loopback0 for an eBGP router-id instead of using Vlan1's ip address. I know how to do this but my question is, what criteria should I use for selecting what IP goes onto loopback0? This is a production 7609 router we use as a customer edge device.

Can the loopback address be part of an existing subnet on another configured up interface or does it have part of a subnet that isn't used anywere else? I know I can use a loopback for a host of other duties but I don't want to make a mistake with the IP and then have to change it later. I have a routable subnet I've been using for serial links (/30), would this be a good candidate?

Thanks in advance!

Bill

I have this problem too.
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Correct Answer by 4rmorris about 8 years 5 months ago

We typically keep a block of addresses (8 or 16 depending on the size of the office) for loopback IPs on routers and layer 3 switches. This allows us to summarise the entire block instead of advertising host routes around.

On top of that, we set aside a /24 to use for loopbacks on our CE routers. This block is summarised at out data centre to keep the table small, but the /32 routes are maintained in the MPLS cloud for reachability checks and peering.

The router definitely won't let you put a /32 on a loopback if it's part of a larger subnet on another interface. If you use your /30, you get 4 loopbacks that you can use in that office or elsewhere, depending on how you choose to advertise/summarise that block. Without knowing more details about your situation, that may be your best bet.

Regards,

Ryan Morris

CCIE #18953

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Correct Answer
4rmorris Tue, 06/17/2008 - 15:09

We typically keep a block of addresses (8 or 16 depending on the size of the office) for loopback IPs on routers and layer 3 switches. This allows us to summarise the entire block instead of advertising host routes around.

On top of that, we set aside a /24 to use for loopbacks on our CE routers. This block is summarised at out data centre to keep the table small, but the /32 routes are maintained in the MPLS cloud for reachability checks and peering.

The router definitely won't let you put a /32 on a loopback if it's part of a larger subnet on another interface. If you use your /30, you get 4 loopbacks that you can use in that office or elsewhere, depending on how you choose to advertise/summarise that block. Without knowing more details about your situation, that may be your best bet.

Regards,

Ryan Morris

CCIE #18953

bbuhlman Wed, 06/18/2008 - 14:55

Thanks Ryan,

I picked an unused /24 that we still had available. I'll use the whole thing for loopbacks like you did. Makes sense.

Thanks again for the advice.

Bill

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