Reg: BGP AS-Path-Prepend

[AKK]

Hi All,

Due to some requirement I need to prepend the approx 12 AS-PATH in BGP advertisement but in ASR router there is limit of max 10 ASpath only. Requesting if any one can help in this regard.

Regards,

AKK

[MHM Cisco World]

if the router have max limit for AS-Path prepend, and your require is more than limit number try other way for Inbound traffic,
can I know case that you need AS-Path Prepend may be I can help you.

[AKK]

Hi,

I want to know if there a way to increase the max limit of as-path prepend.

Our use case is IN DC & DR have more than 6 service provider with primary and secondary link and we want to use each link for some specific traffic to flow from DC to Branch locations and each specific traffic as failover on other 11 links based on AS-Path Prepend.

Cheers,

AKK

[MHM Cisco World]

now for each branch there are two paths,
one toward DC and other toward DR?
the Brach must select path to DC and if failed then traffic will shift to DR?
this need to AS-Prepend via DR to make path through DC is more prefer than path through DR.
until Now if that what you want it perfect, 
but are you sure that there are more than 10 SP between DC/DR and Branch?

[AKK]

We have branches with 4 different Service provider link and utlizing all the links for traffic . DC is primary in case in DC side both primary & secondary links are down then traffic should move via DR and come back to DC using Replication link.

[MHM Cisco World]

https://blog.ipspace.net/2009/02/protect-your-network-with-bgp-maxas.html

I dont know if this command is help you, 
but can you try there is bgp maxas command may be you can increase the AS-PATH and solve your issue 

[AKK]

As per my understanding it will solve problem for incoming prefix but not the outgoing traffic. My requirement is for outgoing prefix.

[MHM Cisco World]

I effect outbound AS-PATH prepend
I do small lab 

R1- R2 
and config maxas-limit in R1 and then config AS-path prepend add more than max-limit and prefix never reach R2.
this one is solve but the big issue you run IOS XR in ASR not IOS 
so until here, I could not help you so much more.
may be other help you in your issue.
Sorry.

[David Ruess]

Hello,

You could summarize on a router somewhere in your network that makes sense. Summarization in BGP by default "hides" the previous attributes so it would reduce the number of AS path attributes you have and then you could prepend then possibly.

-David

[AKK]

Hi ,

Summarization is not possible due to requirement as we need to advertise /32 routes.

-AKK

[David Ruess]

So summarize the /32 route in the summary command. Then you can still advertise the /32 and meet the requirements

[paul driver]

Hello

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@AKK wrote:

Our use case is IN DC & DR have more than 6 service provider with primary and secondary link and we want to use each link for some specific traffic to flow from DC to Branch locations and each specific traffic as failover on other 11 links based on AS-Path Prepend.

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Even with 6 ISPs it’s is not necessary to just use as-path prepending you have additional bgp attributes to traffic engineer with.
Can you ebalorate a little on the topology.