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BGP, router-id, and loopback

I have two questions about BGP and router-id that are somewhat related.  I recently got a 2nd internet circuit so that we're now multi-homed.   I am doing load sharing by policy-routing outbound and AS-path prepending to make data return on the same pipe it was sent.  Here is the setup:

ISP A -> Local segment 5.5.5.5 on myRouter int1

ISP B -> Local segment 4.4.4.4 on myRouter int2

myFirewall Outside (4.4.4.2) -> 4.4.4.1 (in ISP B's ip block) myRouter on int3

1) Not being very familiar with BGP routing, I didn't explicitly set a router-id when I turned on BGP.  So consequently the BGP router ID is ISP A's side because it is the highest ip (5.5.5.5).  I have read since that it is best to set an explicit router-id.  Is that correct?  If setting a router-id explicitly is the best practice, what would be the best choice to set it to in this case?  What are the ramifications of changing it now?

2) Soon I want to change the addresses of the FW outside to Myrouter segment from ISP B's ip block to ip's from a block that we've purchased from ARIN.  This is to prepare for an eventual move to contract with another ISP for the 2nd circuit on int2.  Our ARIN block is already being routed by both ISP A and ISP B.  Could I set a loopback address on myRouter with the new ARIN address to transition the FW-Router segment away form ISP B's ip block that way?  I have a couple of VPN boxes outside the FW that use myRouter at 4.4.4.1 as their gateway so I was hoping I could have the router reachable  for my DMZ stuff on both the current ISP B ip and a new ARIN ip for at least a transition period.  I think this question relates to the router-id question above because I understand a loopback address ip will automatically become the router-id if not explicitly set.

Any advice is appreciated.  Thanks.

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Accepted Solutions
Cisco Employee

Re: BGP, router-id, and loopback

Hello!

If not explicitely configured, the bgp router-id is the highest loopback ip, if there is one. So if you configure a first loopback, it's ip will become the bgp router-id the next time you clear bgp.

But it won't cause any problem. The router-id must not even need to be an existing ip address of the router (as long as it remains a unique identifier of course) and must not need to be network-reachable.

One last thing to know is that when you configure a manual router-id, the bgp session will reset. So don't do it when network interruptions are not tolerated .

Regards,

Herve

1 REPLY 1
Cisco Employee

Re: BGP, router-id, and loopback

Hello!

If not explicitely configured, the bgp router-id is the highest loopback ip, if there is one. So if you configure a first loopback, it's ip will become the bgp router-id the next time you clear bgp.

But it won't cause any problem. The router-id must not even need to be an existing ip address of the router (as long as it remains a unique identifier of course) and must not need to be network-reachable.

One last thing to know is that when you configure a manual router-id, the bgp session will reset. So don't do it when network interruptions are not tolerated .

Regards,

Herve

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