BGP routing / looping

[SJ K]

Hi all

In the below design - r1 is doing ebgp peering with r2 and r3

r2 and r3 is advertising 10.10.10.0/24 to r1

r1 -> r3 is the selected path to reach 10.10.10.0/24

Assuming that prefix filtering of any kind is NOT setup

q1) is it normal for r1 to be re-advertising 10.10.10.0/24 to r2 ?

Thank you

[Richard Burts · Accepted solution]

Perhaps there is something in this question that I am not understanding. But it seems to me that it is pretty straight forward. r3 will advertise 10.10.10.0/24 to r1 as an EBGP route. r1 will advertise that network to r2. r2 will receive the advertisement, will see its own AS number in the advertisement and will not process the advertisement. This is a basic loop avoidance behavior of BGP.

[MHM Cisco World]

R1 will advertise prefix learn from R3 to R2.

R2 will drop this prefix update get from R1 becuase it see our AS in AS-path attribute of perfix.

[pman · Accepted solution]

Hi,

How EBGP loop prevention work :  

[AliDoskii]

Normally BGP will drop anyhting that comes from it is own AS but there is a command to do this for you we usually use them in DMVPN to let our spokes talk to each other though HUB.

neighbor x.x.x.x allowas-in

or

neighbor x.x.x.x as-override

[SJ K]

hi all

Appreciate your response

Yeap i do understand that there will be some kind of loop avoidance in place

1) Just wondering which router actually does the loop avoidance ( which i understand from the replies that it isn't R1 )

- R1 receive same routes from 2 different routers with the same AS number and advertised prefix - decide to not re-advertise

- R1 will still re-advertise but either R2 / R3 will drop it

2) what is the tie breaker for R1 path selection if R2 and R3 has the same ASN and advertised the same prefix / path to R1 ?

Thank you

[Jon Marshall · Accepted solution]

1)  Yes it isn't R1 because R1 would advertise to R2 in your scenario and it would be R2 who drops it as it is the one seeing it's own AS in the path. 

2) All other things being equal (which it should be based on the basic information in your scenario) it would be the one that is received first - see this link for full details - 

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

Jon

[MHM Cisco World · Accepted solution]

R1

if all criteria tie then

Lowest router-id will tie breaker.

[Jon Marshall · Accepted solution]

Yes, if both advertisments were received at the same time. 

Jon

[SJ K]

thank you richard, jon, mhm and all

glad to see that you guys are still around

[Richard Burts]

You are welcome. Glad to see you still active in the community.