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Beginner

BGP Timer

Hi,

 

I am running into a small issue with BGP, see below example setup.

 

Router A ---- Leased Line ---- Router B

 

Between the two routers we run a eBGP session. The link sometimes gets a hit, when this happens the interface on Router A will go down, but the interface on Router B will stay up. We have BGP Fall-Over configured on Router A, so when the interface goes down the route will disappear from RIB and BGP will skip the hold down timers. But this does not happen on Router B, as the interface stays up on that side. So router B will still have the 180 seconds hold timer. Is there a way for BGP to tell Router B that the session is down and it can skip hold timers, or could there be another way to achieve this? Was thinking about IPSLA and EEM or just tweak the timers for this specific neighbor, but dont think that is the cleanest solution.

 

Hope someone can help me out with this. 

 

Thanks in advance!

 

Br,

 

JP

1 ACCEPTED SOLUTION

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VIP Mentor

Hi

Yes you can use ip sla pinging remote end and eem to clear the bgp session when it goes down.
Or you can tweak timers between these 2 peers. This is the solution i would prefer because it will be cleaner. You can be a bit aggressive but not too much to avoid flapping and set it up to 10 32 for example.

Thanks
Francesco
PS: Please don't forget to rate and select as validated answer if this answered your question

View solution in original post

4 REPLIES 4
Highlighted
VIP Mentor

Hi

Yes you can use ip sla pinging remote end and eem to clear the bgp session when it goes down.
Or you can tweak timers between these 2 peers. This is the solution i would prefer because it will be cleaner. You can be a bit aggressive but not too much to avoid flapping and set it up to 10 32 for example.

Thanks
Francesco
PS: Please don't forget to rate and select as validated answer if this answered your question

View solution in original post

Highlighted
Beginner

I think you can also write an EMM script that removes the Policy Route from your BGP configuration when the BGP neighbor goes down.

Highlighted
Participant

Hi,

First of all I’d ask the leased line provider how come they do not propagate the link failure signal end-to-end (that is a must).

Maybe what you perceive as a leased line is in fact a virtual circuit with incorrect configuration if this is the case in fact then, while on it, have them also confirm that your circuit MTU will be the same regardless the path it takes through the provider’s network (in case of failures in their core)

 

Also check you have carrier delay 0 configured on interfaces at both ends of the link.

 

Regarding the options for solving this in control-plane.

The option selection depends on the HW you have at both ends, and on your business requirements.

You could for instance register BFD session with the BGP peer this way you could rely on low overhead protocol to tear the session down in a timely manner, instead of tweaking the heavy weight BGP machinery.

 

But if you’re not after sub-second failover times, then tweaking BGP timers is perfectly fine cause what we’re talking about is just a single sessions with aggressive timers, so not a big deal in terms of overall platform resources (CPU) utilization.

 

But please note that what goes hand in hand with swift failure detection is instability.

When reducing failure detection times consider enabling event dampening as well

Luckily in your setup you can deal with it on a number of levels,

At interface level you have ip event dampening or link state dampening, or just simple interface hold down timer, but please not this last one might tie the up and down hold down timer into one variable  –in that case you want to set it to 0 and use BGP route state dampening instead.

 

 

adam

 

netconsultings.com

::carrier-class solutions for the telecommunications industry::

 

adam
Highlighted

First of all, thanks for taking the time to respond to this topic and helping me out in this matter.

BFD is not something we can use in this setup, as this link is a 150Mbit link which is almost full time fully loaded. You cannot do QoS on BFD, so this will always end up in the default que, which means these packets will be dropped when the link is congested. Which will result in flaps.

On the MUX at the remote location, we disabled the feature to notify the connecting router that the link is down. This is a long story why, but was a request from the customer. It could be that this will be enabled in the future again.

For now we will use BGP timers to accomplish faster convergence time. We will monitor its behavior to see if it works as expected.

Thanks again.