Solved: BGP Neighbors...Loopbacks or Directly Connected?

JoshGarmonCiscoCCNA · ‎05-23-2016

When would you want to make your BGP neighbor their loopback as opposed to your neighbor's connected interface?

Vinit Jain · ‎05-23-2016

It depends if you are forming an IBGP neighborship or EBGP neighborship.

With IBGP, it is preferable to peer over loopback interface. This is so because, loopback is an always UP interface (unless its manually shutdown) and if a physical link goes up, there might be an alternate path via IGP to reach the loopback. Thus the BGP session will not go down vs if it is over a physical interface.

With EBGP session, usually the peerings are with provider and over a public IP. Thus the neighborship is established over physical interface. But for security reasons, the peering can be established over loopback with two conditions in mind:

- loopback to loopback/physical interface reachability is there between the two peering devices

- proper ebgp-multihop

is configure or if the peering is between two directly connected router over loopback, then

disable-connected-check

command can be configured for the neighbor.

Hope this answers your question.

Regards

Vinit

Thanks
--Vinit

View solution in original post

Vinit Jain · ‎05-23-2016

It depends if you are forming an IBGP neighborship or EBGP neighborship.

With IBGP, it is preferable to peer over loopback interface. This is so because, loopback is an always UP interface (unless its manually shutdown) and if a physical link goes up, there might be an alternate path via IGP to reach the loopback. Thus the BGP session will not go down vs if it is over a physical interface.

With EBGP session, usually the peerings are with provider and over a public IP. Thus the neighborship is established over physical interface. But for security reasons, the peering can be established over loopback with two conditions in mind:

- loopback to loopback/physical interface reachability is there between the two peering devices

- proper ebgp-multihop

is configure or if the peering is between two directly connected router over loopback, then

disable-connected-check

command can be configured for the neighbor.

Hope this answers your question.

Regards

Vinit

Thanks
--Vinit

Ritesh Thakur · ‎10-17-2017

Hi Vinit,

I have similar kind of situation while using

disable-connected-check

command.

Is it possible to form neighborship between two routers R1 and R2 , where R1 is having

neighbor

command with IP address of physical interface of R2 and

R2 have

neighbor

command with address of Loopback Interface of R1.

On R1

router bgp 100
  neighbor 192.168.12.2 remote-as 200 --> 192.168.12.2 is the Physical interface IP of R2

On R2

router bgp 200
  neighbor 1.1.1.1 remote-as 100
  neighbor 1.1.1.1 update-source Loopback0 --> 1.1.1.1 is the loopback of R1

Will it work in any ways or does disable-connected-check will help?

Ritesh Thakur · ‎10-18-2017

Hi Vinit,

Thanks for the quick reply.

I agree with you Sir but in my case I want to establish EBGP neighborship and the requirement is like

on one router its is loopback int addr and on other end router it is physical int addr onto which I am forming neighborship.If it is possible then also I do not have to use Ebgp-Multihop.

Thanks

Ritesh

trfinkenstadt · ‎05-23-2016

Loopback use case:

- iBGP neighbors with multiple direct L2 connections: This is an ideal for peering between loopbacks since they can use either ethernet interface.

- Multiple T1's between you and ISP would be an ebgp multihop scenario between loopbacks.

Connected interface:

- eBGP to your service providers.

- iBGP only one interface between routers.

Things like that.

best regards,

tim

Joseph W. Doherty · ‎05-23-2016

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Posting

The other posters have well answered how loopbacks or directly connected interfaces are typically used. However, I did want to mention, when using iBGP and loopbacks, you may want to also use

next-hop-self

to have the loopback passed to its iBGP peers as BGP's

next hop address too