eBGP multihop and dual ISP redundancy

Answered Question
Aug 18th, 2010
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Hi all,


I have to implement a bgp-multihomed, in the beginning my routers were going to replaced the CE, but now the ISP's are saying no, we have to have the CE, which they managed.  I have my own public ASN and /24 public address so I need to control my internal routing.   So, the only option I have to implement bgp-dualhomed is to use eBGP multihop to their PE and the CE would be there just to provided access to their PE - has anyone implemented this before? are there any issues to consider, is there a nice how to link (I haven't found anything good yet), I would really appreciated it.


Thanks in advanced.


Cheers!

Correct Answer by Giuseppe Larosa about 6 years 7 months ago

Hello Mguzman4158,


I tested a similar scenario some years ago.


routers on the path (CE) have to be able to route appropriately BGP messages and user traffic by means of static routes or dynamic routing


it is possible to have a session with both the CE and the PE in the path with eBGP session with CE using BGP community no-advertise for your IP subnet but allowing to advertise the subnet in between.


on the session with the PE node you advertise your network


using static routes is probably easier


of course you need to use  ebgp-multihop option for session to PE node.


with modern IOS you can even use IP SLA and tracking objects to check PE address reachability in this way the route to PE address will disappear when the tracked object fails ( ip reachability of PE address used as BGP neighbor)


in this way you can achieve faster convergence in case of failure you just need to avoid to have a backup static route as your objective is to declare down the eBGP multihop session as soon as you detect the BGP endpoint does not answer anymore.

As the route to BGP nexthop disappear BGP next-hop tracking can declare down the session.

Again this a feature present in recent IOS versions.

In this case recovery means to use the iBGP session between your routers to send traffic to your other router that has an alive multihop eBGP session


You need to coordinate with SP in order to ensure they are doing the same.


see

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html


for BGP next hop tracking:


http://www.cisco.com/en/US/docs/ios/12_3t/12_3t14/feature/guide/gt_bnht.html


Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Wed, 08/18/2010 - 08:41
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Hello Mguzman4158,


I tested a similar scenario some years ago.


routers on the path (CE) have to be able to route appropriately BGP messages and user traffic by means of static routes or dynamic routing


it is possible to have a session with both the CE and the PE in the path with eBGP session with CE using BGP community no-advertise for your IP subnet but allowing to advertise the subnet in between.


on the session with the PE node you advertise your network


using static routes is probably easier


of course you need to use  ebgp-multihop option for session to PE node.


with modern IOS you can even use IP SLA and tracking objects to check PE address reachability in this way the route to PE address will disappear when the tracked object fails ( ip reachability of PE address used as BGP neighbor)


in this way you can achieve faster convergence in case of failure you just need to avoid to have a backup static route as your objective is to declare down the eBGP multihop session as soon as you detect the BGP endpoint does not answer anymore.

As the route to BGP nexthop disappear BGP next-hop tracking can declare down the session.

Again this a feature present in recent IOS versions.

In this case recovery means to use the iBGP session between your routers to send traffic to your other router that has an alive multihop eBGP session


You need to coordinate with SP in order to ensure they are doing the same.


see

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html


for BGP next hop tracking:


http://www.cisco.com/en/US/docs/ios/12_3t/12_3t14/feature/guide/gt_bnht.html


Hope to help

Giuseppe

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