AS-Path Prepending & Connectivity

[alistair777]

Hi,

I would like to know from those who have deployed AS-Path prepending configuration in a BGP multihoming scenario.

Does this involve any significant disruption in connectivity or is it seamless like adjusting local preference and then resetting with the bgp soft reset option?

Thanks

[paul driver · Accepted solution]

Hello

I have performed this on quite a few occasions in our DCs and on numerous remote sies that have been multi homed and have never experience any outage that resulted in any intrusive impact.

However like any other change there is a chance of misconfiguration -fat finger syndrome etc.. So I would always suggest implementation to be performed within an agreed date/time with the customer to minminise service impact ( if any) 

red

paul

[paul driver]

Hello

one thing I forgot to mention and worth noting is when you have dual ISP multi-homed links is to make sure your site doesn't become a transit path for either ISP 

This can be negated by using As-path filtering 

ip as-path access-list 10 permit ^$

router bgp xx

neighbour (isp1) x.x.x.x filter-list 10 out

neighbour (isp2) x.x.x.x filter-list 10 out

res

paul

[Masoud Pourshabanian · Accepted solution]

Hello,

Prepending is common in BGP and there will not be any significant disruption if the configuration is done correctly. There might be only disruption in current flows.

As for ressetting, new attributes will be advertise to your neighbor if you have configured soft feature.

Please take a look at this sample configuration and also the link below to see ressetting with soft option.

https://supportforums.cisco.com/discussion/12693826/internet-routing-table-storage

router bgp 65536
 address-family ipv4 unicast
  network 172.17.1.0 mask 255.255.255.0
  neighbor 192.168.1.2 remote-as 65538
  neighbor 192.168.1.2 activate
  neighbor 192.168.1.2 route-map PREPEND out
  exit-address-family
 exit
route-map PREPEND permit 10
 set as-path prepend 65536 65536

Hope it helps,

Masoud

[Ganesh Hariharan]

Hello Alstair,

For AS prepend bgp attribute implemenattion , you can have look on the below blog for more information.

AS Prepend Path

Hope it helps..

-GI

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