BGP: Migrate to new AS and new IP subnet

[dvrizhov]

Hi there. I need some help with my issue.

This is the input data for this task:

We have one AS, for example, XXXX, and one subnet X.X.X.X , one router with BGP support. It is configured with two active sessions with two ISPs.

Because of the  separation of companies, we have obtaibed new AS YYYY and new IP Y.Y.Y.Y subnet. Now we have to migrate our services from old AS to new AS. Our used hardware is one Cisco 7304.

Additionally we have another one Cisco 7304, wich not configured yet, i think we must use it for this migration.

Can anybody suggest, how can we easiest do this?

[Alexander Demin]

You will still have multihoming topology?

New AS and new subnet - are your registered objects in RIR (perhaps,RIPE) ?

[dvrizhov]

Yes, we will still have multihoming topology. We have to connect at least one more ISP for this.

Now we have new registered objects (AS and IP subnet), obtained by RIPE.

[Alexander Demin]

Perhaps, if multihoming policy, that you used previously, fits your needs then

you can simply take old config (bgp part) and use it in new environment.

Just correct ASN number, advertised network, new p2p net addresses for peering, acquired from your ISPs, policies (if used) and filtering (depends on what you used before: prefix-lists or as-path lists).

[dvrizhov]

The major problem is that the  old AS continues to work, and in parallel we have to run new AS and transfer  services to new IP adresses on new environment.

[francisco_1]

Is the old subnet still in use? Are you going to migrate services from old subnet to new subnet? if yes then you could run both new and old AS with subnets advertised while you migrate services to new!!

The other option is you could use both ASs. For example you could  reconfigure the local BGP speakers to use new AS number but advertise the old AS in BGP OPEN messages and BGP updates until you finish migrating to the new AS.

The "local-as " command instructs the local router to advertise the "" number in BGP OPEN messages instead of the AS number specified with "router bgp " command. In additon to that, all BGP prefixes advertised to the eBGP peer would have the AS number prepended in front of every BGP updates AS_PATH attributes. Thus, the external system may continue with the local system using old AS number. In addition to that, the external system will ses updates coming from the looking liek they firsy transited .

Francisco

Hope to help

[dvrizhov]

francisco_1 wrote:
Is the old subnet still in use?

Yes, the old subnet still in use and it will be in use after this migration. Each of  our companies will have its own individual AS as a  result.

francisco_1 wrote:
Are you going to migrate services from old subnet to new subnet? 

Yes we are going to migrate from old subnet to new one.

francisco_1 wrote:

The "local-as " command instructs the local router to advertise the "" number in BGP OPEN messages instead of the AS number specified with "router bgp " command. In additon to that, all BGP prefixes advertised to the eBGP peer would have the AS number prepended in front of every BGP updates AS_PATH attributes. Thus, the external system may continue with the local system using old AS number. In addition to that, the external system will ses updates coming from the looking liek they firsy transited

Yes, I can only see this way to  migrate, I wanted to get a confirmation that this is the most correct way.

Thanks to all for help. Now i need to create test model of the migration, for example, on the "Dynamips", and test it.