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Lets just say I have three routers in full mesh. R1 has an uplink to an ISP publishing 12.0.0.0/24. Behind each router takes care of a subnet of the public block for web servers connected through a switch in three different locations like so:

12.0.0.64/26 - R1 (host gateway is 12.0.0.65)

12.0.0.128/26 - R2 (host gateway is 12.0.0.129)

12.0.0.192/26 - R3 (host gateway is 12.0.0.193)

I need to use BGP to advertise internally where each subnet is at, but from what I understand, its best practice in bgp to assign the IP address for each router to a loopback. This is not going to work because the IP address for each router needs to be on the interface facing the switches for each location.

So my question is:

Do I use private /31 networks between each router so they can talk iBGP or do I have to further subnet my public range to join them all together?

Alternatively, rather than using up 2 public IP addresses for each sub-network, would it be better to use 12.0.0.0/26 and put them all on the same subnet to talk (.1,.2 and .3) leaving room for .4+ for when I add new sites?

Cheers for reading my Q :)

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    – Ron Maupin ♦
    Aug 12, 2017 at 4:09

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Maybe I missed something, but I'm not sure why you can't subnet your network to get a few /32s and /31s. It seems that you have 12.0.0.0/26 available for further subnetting?

I would allocate public /32s for loopbacks and advertise them into IGP. Links between routers are p2p, so I would address them with public /31s. iBGP sessions should survive link failure, so I would source them from loopbacks.

It's not a good idea to use private IP addresses on transit links, because it can cause problems, e.g. break PMTUD.

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  • Hi mkurek, you are right, i can subnet the first block, im just throwing ideas around to be economical. Re PMTUD, every device involved, ISP included uses MTU 9000, does this resolve any fears around PMTUD or is there something else happening here? Cheers for the reply :)
    – OJS
    Aug 13, 2015 at 0:44
  • Where you have to be careful is when a packet passes from a higher MTU network to a lower MTU network. This will cause ICMP replies to be generated, if those messages are dropped it will break PMTUD. If router interfaces have private IPs then those ICMP replies can be generated with private source IPs which increases the chance of them getting dropped before they get to where they need to go.
    – Peter Green
    Mar 26, 2016 at 1:38

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