Usage of Loopback interfaces/ IP address management

Unanswered Question
May 6th, 2008
User Badges:
  • Bronze, 100 points or more

When using eBGP for what is the best practice with regard to IP address planning?


1) eBGP is run with loopback interface,

2) IP space advertised includes the IP subnet allocated for loopback IPs as well.

How could one tackle the issue with overlapping IP subnets? is advertised and it requires that is used for Loopback IPs (eBGP, iBGP)

But when assigned for loopback0..Cannot assign for any other router interface(overlaps with loopback0)

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4 (2 ratings)
Rick Morris Tue, 05/06/2008 - 05:22
User Badges:
  • Silver, 250 points or more

for route announcements it should not have anything to do with your assignment of IP's.

what you need to make sure of is that you use the correct netmask for assignments. You cannot overlap in this regard.

I would use a /32 for loopback assignments, then make sure you do not announce the specific IP, make sure you announce the entire subnet you own, unless you have a need not to.

You will need to subnet your block.

Or ask your ISP for IP's to use for the loopback for peering sessions.

rsgamage1 Tue, 05/06/2008 - 05:45
User Badges:
  • Bronze, 100 points or more

So when you use /22 /23 or /24 for eBGP and when you don't have a lot of room for loopback & serial assignments the best choice would be to ask upstream ISP.


Rick Morris Tue, 05/06/2008 - 05:49
User Badges:
  • Silver, 250 points or more

Most ISP's will allocate a small IP range for this purpose. This allows full use of IP use for you.

Tell them you need IP's to assign to the loopback for BGP peering. Depending on the need they will give you a /30 or a /29. Most of the time it is a /30.

Rick Morris Tue, 05/06/2008 - 10:08
User Badges:
  • Silver, 250 points or more

Did this work?

I am curious to know if the ISP allocated the IP or if you had to subnet your own block. I just want to know for my own sake and not telling people wrong.

Craig Norborg Tue, 05/06/2008 - 05:26
User Badges:
  • Bronze, 100 points or more goes from -> You cannot assign a loopback as on the same router that is directly connected to this subnet...

Usually with loopbacks, you'll take a range of IP's (lets say the and assign all your loopbacks from that as /32's. But you can't use that range anywhere else.

Either that or assign the as loopback and then assign the instead, but you can't have overlapping ranges directly connected to the router!!


This Discussion