BGP update-source is configured on only one router but still neighbourship is okay

[olly ahmed]

As far as I know BGP by default took address of outgoing interface for a neighbor as update source. If we need to configure 2 router to establish neighborship with one another using loopback interface then we need to add an extra command "neighbor x.x.x.x update-source loopback interface" on both the routers. But in my case I just configured the command on only one router and neighborship is formed without any trouble. Can anyone explain this issue. It is to mention that my lab is on GNS3.

R1#show running-config | se bgp
router bgp 1
no synchronization
bgp log-neighbor-changes
network 2.2.2.2 mask 255.255.255.255
network 7.7.7.7 mask 255.255.255.255
network 9.9.9.9 mask 255.255.255.255
network 11.11.11.0 mask 255.255.255.255
neighbor 2.2.2.2 remote-as 2
neighbor 2.2.2.2 ebgp-multihop 3
no auto-summary

R2#show running-config | se bgp
router bgp 2
no synchronization
bgp log-neighbor-changes
network 1.1.1.1 mask 255.255.255.255
network 2.2.2.2 mask 255.255.255.255
network 10.10.10.0 mask 255.255.255.240
network 11.11.11.0 mask 255.255.255.0
neighbor 1.1.1.1 remote-as 1
neighbor 1.1.1.1 ebgp-multihop 3
neighbor 1.1.1.1 update-source Loopback1
neighbor 10.10.10.2 remote-as 2
no auto-summary

Neighbor    V     AS    MsgRcvd    MsgSent    TblVer    InQ    OutQ     Up/Down    State/PfxRcd
2.2.2.2 4     2     1          38                124           12           0         0          00:09:47           7
R1#

[rais]

Is R2 advertising both 1.1.1.1 and 2.2.2.2 with /32 masks?

R2 configuration should have a static/other route to get to 1.1.1.1 and same for R1 in opposite direction. R2 BGP process shouldn't have any issues reaching out to 1.1.1.1:179. Most likely it will always be R2 establishing connection to R1.

Rais.

[olly ahmed]

--> Is R2 advertising both 1.1.1.1 and 2.2.2.2 with /32 masks?

     Yes. R2 is advertising 1.1.1.1 network so that other routers which are iBGP neighbor can use the same next-hop as R2 is getting from eBGP neighbor R1.

Is this the actual problem ? If it is then please make it clear so that I can understand clearly.

[rais]

Interesting. I thought R2 owned 1.1.1.1/32 as well. Even if R1 receives this route,  local route will prevail on R1. 

By default, R1 BGP will try to make connection to dst-addr 2.2.2.2 using outgoing i/f IP address which R2 will reject. R2, however, will initiate connection to dst-addr 1.1.1.1 and be successful in it as R2 packets will have src-addr of 2.2.2.2 that's configured on R1.

HTH

[olly ahmed]

I have a static route for 2.2.2.2/32 on R1 and 1.1.1.1/32 on R2. So what might be the case ? Is it gns3 bug or something else ?

[rais]

It's not an issue. The only problem I see here is that R1 initiated bgp connection will not be successful. R2 initiated bgp connection will succeed and bgp will enter ESTB state. BGP will work.

Thanks.

[skpooe001]

Hi Olly,

if its off interest, you can  look at whose the BGP server between the two peers by running the below command.

ISP#sh tcp brief all
TCB       Local Address           Foreign Address        (state)
657BF3C0  120.120.120.1.179       120.120.120.2.19778    ESTAB

INTERNET2(config)#do sh tcp bri all
TCB       Local Address           Foreign Address        (state)
65BA9310  120.120.120.2.19778     120.120.120.1.179      ESTAB

Rais  is spot-on.

cool!!