BGP Multipath (eBGP under vrf)

[Jay Russell Santos]

Hello guys,

Just have a question regarding bgp multipath. we have this setup on our core. two routers connected to the core and advertised the same prefix to the core. the core have this as multipath. it supposed to load balance the traffic. My question is, when we power down one of the router (core will no longer have the multipath) and bring it back up, will the core do the recalculation (convergence)? will it affect the traffic during this period?

Thanks and have a good day guys.

[Harold Ritter]

Hi @Jay Russell Santos ,

If the core router considers the two paths as equal, it will add the second path once the second router comes back up and the traffic forwarding will not be affected, other than going through two paths instead of one.

Regards,

[Jay Russell Santos]

Hi Harold,

Yes, it is equal cost. I know that it will add the path but my question is if this will have any impact on the traffic. will it do route calculations? reconvergence?

Thanks.

[Harold Ritter]

Hi @Jay Russell Santos ,

There will be no impact, if you mean traffic drop.

Regards,

[MHM Cisco World]

if the Core connect to edge router via L2 SW, the Core not immediate detect next-hop failed and withdraw the prefix to failed router. 
if the core connect directly to router then it can fast withdraw prefix. 

also this depend on if path to both edge router is elect as best path, i.e. multipath 
if not then the Core can withdraw prefix one path and recalculate the other Path. 
this can I think solve by shift all path toward second edge router and then power down the edge router. 

this my  opinion. 

[Jay Russell Santos]

hi,

its directly connected. path to both routers are tag a multipath. However, when we boot up the second router. it has traffic drop. thats why I want to understand the behaviour of bgp multipath.

Thanks.

[Harold Ritter]

Hi @Jay Russell Santos ,

Where/how do you see traffic drops. The core router should only use the first path until the second one becomes available and then start forwarding using both paths once the convergence has completed.

Regards,

[Jay Russell Santos]

we were running a continous ping while the router is booting up. there e saw some ping dropped. thats why I am thinking that the convergence might have caused it. when the new route was introduce on the core.

[MHM Cisco World]

I will run small lab, and update you with some point 

[MHM Cisco World]

Site-A and Site-B 
Site-A have two path to Site-B 
so multipath in BGP and two path in routing table 
and CEF use per-packet, 
one of path is down 
Site-A detect path down and use second path immediate, 
Site-B still use two path until it notice the neighbor is down and withdraw one path and using second one, 
so issue is in Site-B not in Site-A. 

[MHM Cisco World]

I run small lab and see the issue, the I shut down the interface f0/0 in R2 and traffic start to drop even if R1 have second path through f1/1 R3. 
the issue is return traffic from R4 still take path to R2 and here is drop, 
solution 
A- costumer site 
run iBGP between R2 and R3, where the R2 when receive the return traffic from R4 the traffic is forward to R3 not drop
B- ISP site 
there is technology called PIC edge which can help to solve this issue.

[emurray]

BDF can be used to reduce the convergence time. 

[Harold Ritter]

Hi @MHM Cisco World ,

It is normal to have traffic drop on a link down condition. What the original poster was asking is whether he should see traffic drop on a link up condition (ie. when going from one downstream path to two). My answer to him was that he should not expect to see any traffic drop in this context.

In a link down condition, the issue is that the link down needs to be detected by the router forwarding to the multiple downstream links. As @emurray mentioned, BFD can be used to detect as quickly as possible an limit the amount of packet dropped. But this was not the question asked by the OP.

Regards,

[paul driver]

Hello
Obviously if traffic was traversing the path that was withdrawn from bgp, there will be some convergence, but it will be minimal and your clients will not even notice, even as/when the path is reestablished and route is reinstalled.

[Jay Russell Santos]

Hi Paul,

router is booting up. so it's not route withdrawn rather learned again. We know its minimal but trying to understand the behaviour and impact.