Routing

we are a small ISP and we have recently acquired a new AS number and new public ip address blocks.  what is the best way to implement the change in our M20 gateway router and internal network with the least amount of downtime to our clients?

Hi,

 

You didn't say if you own your current prefixes and will advertise both the old and new prefixes (until you migrate). You also didn't say how many peers do you have, so i assumed there are 2 peers.

You could try this:

1. create an aggregate route which you'd like to advertise to you peers;

2. create a policy which will advertise this aggregate (and your current prefixes if there is such need; if you're going to advertise these prefixes, perhaps it's wise to prepend the as-path (output policy) and lower local-pref (input policy) to make the reconfigured session less preferred just in case);

2a. Connect the newly created policy to output of the reconfigured session;

2b. Commit your configuration; even if your neighbor rejects the new aggregate it's not so important now, cause the only reason for commiting here is to switch traffic to the alternate path;

3. reconfigure bgp for one peering using 'local-as' statement, so that there is no need for global 'autonomous-system' change;

4. after the session becomes operational, check whether the aggregate (and rest of prefixes) is being advertised: '>   show route advertising-protocol bgp IP_OF_THE_PEER'

5. check using looking glass of you provider, how your prefixes looks like in it's bgp table (especially important for those which you use right now); if you feel fine about that, you could try to remove as-path prepending and local-pref lowering from reconfigured session; the traffic should switch to this new session.

 

If you provide more details such as

a) how many peers do you have?

b) are you  allowed to advertise your current prefixes using new ASN? if not, are you allowed to transit your current prefixes via your new ASN?

we could  find much fitting solution.

 

 

Have a nice day,

G.

thanks for the reply.

 

We have 3 EBGP peers.  ultimate goal is to use just the new AS number and new prefixes.  problem is we cannot transfer all clients to the new ip addresses in one big migration.  our plan is to run both AS simultaneously within one router. btw, ISPs would not accept the new prefixes using old AS.  we do not own the old prefixes.

 

Would a logical router configuration solve this problem? 

Hello,

 

As You already said, logical router can be used. In this case, this logical router peers via eBGP with the main router You currently use and advertise your new prefixes using this peering. Your main router transits  new AS and prefixes to the external BGP peers. Of course, your eBGP peers have to allow You to transit other AS ( I don't think it's a huge problem, but it probably requires some reconfiguration at your peers side so just ask them about it).

So, asuming your current ASN is 65412, the new one is 65413 and new prefix is 10.0.0.0/8, You peer would see 10.0.0.0/8 with ASPATH: 65412 65413. When this new prefix will be visible in the Internet (looking glass service is your friend) just add some test interface to logical-router, configure IP address from the new prefix range and test it 🙂

 

The other option is much more ugly, but if You migrate fast and don't want to configure too much, can be some kind of solution. You can choose one of current  BGP peers, and configure session using new ASN (one can configure separate ASN number in specific group):

# set protocols bgp group new_session local-as NEW_ASN

This way, your current ASN set by "routing-options autonomous-system" statement is ignored this particular group. The problem is that, now You have 2 eBGP peers for your current ASN, and 1 for the new one. As I said, it's ugly but fast to configure 🙂

 

EDIT:

On more thing worth to be mentioned is that in the 2nd case You also need some kind of FBF and perhaps a routing-instance (type: forwarding) so actually I'm starting to doubt that it is the fastest way to provide what You want.

Anyway, logical-router seems to be the best option here.

 

If You have some doubts about it, just contact me offlist. I'd be happy to help You (assuming I'd now how 🙂

 

Have a nice afternoon,

 

-- 

G.

i did the fast solution, change the as number and let the ISPs advertise my network. took awhile to reach some internet addresses but doing it when client traffic is at its lowest saved me from a lot of customer complaints.