Change BGP ISP on internet router

[asdvnw]

Hi Everyone,

I have a situation regarding changing ISP with new BGP peering.

We are currently peering with 1 ISP, receiving only a BGP default route and advertised 2 networks prefixes out to the world.

We are preparing to test with new ISP for BGP peering to see if it established and received the

default-route

Once the BGP peering with new ISP established, is there is a way to advertise our same 2 networks prefixes to new ISP without disrupting the current production BGP traffic of old ISP?

What is the best adn safest way to avoid this issue? Any suggestion? 

Thank you very much!

byme88

[Harold Ritter]

Hi @asdvnw ,

If you bring the new BGP session up, you will receive and accept the default route and advertise your 2 prefixes by default. If you just want to bring up the session to see if it works ok, you might want to filter the routes you received and send from/to that session. You basically need to block all inbound and outbound routes. When you are ready to move that new session in production, you will just need to remove the filters and you will be all set.

Regards,

[MHM Cisco World · Accepted solution]

I think if you can use

as-prepend

then both ISP will advertise the same prefix but short path will win (via OLD ISP), 
then you can remove the OLD.
this my opinion. 

[paul driver]

Hello

---

@asdvnw wrote:

is there is a way to advertise our same 2 networks prefixes to new ISP without disrupting the current production BGP traffic of old ISP?

What is the best adn safest way to avoid this issue? Any suggestion? 

---

Yes there is, but you don't mention if the "new " ISP connection will be connecting to the same rtr as the existing ISP connection, of if you will be peering to the same ISP or a different one,  So at this time I assume it will be the same rtr and different ISP, Therefore with a little traffic engineering on your wan rtr you should be able to accomplish having the primary ISP still being the egress/ingress points for all traffic whilst soak testing the new secondary ISP bgp connection and not incurring any outage for your users.

This could be accomplished by using bgp weight  & as-path prepending.

Weight attribute (higher value preferred) will apply to all received prefixes from each bgp peering, thus by appending the higher value to the ISP primary peer all egress traffic will take this path

As-Path Prepend will set additional ASN path information to all egress updates advertised towards the ISP so to influence ingress traffic towards your ASN, applying this to the "new" ISP peer will advertise your prefixes with a longer AS path sequence thus being a less preferred path option via that new ISP.

 

Example WAN rtr: 
router bgp 1
neighbor 1.1.1.2 remote-as 2
neighbor 1.1.1.2 description Primary ISP
address-family ipv4
neighbor 1.1.1.2 activate
network 10.10.10.0 mask 255.255.255.0
network 10.20.20.0 mask 255.255.255.0




Add "new" isp connection

route-map RM-PREPEND permit 10
set as-path prepend 1 1 1

router bgp 1
address-family ipv4
neighbor 1.1.1.2 weight 50000
exit
neighbor 11.11.11.2 remote-as 3
neighbor 11.11.11.2 description New secondary ISP
neighbor 11.11.11.2 shutdown
address-family ipv4
neighbor 11.11.11.2 activate
neighbor 11.11.11.2 weight 40000
neighbor 11.11.11.2 route-map RM-PREPEND out
exit-address-family

clear bgp ipv4 unicast * soft

router bgp 1
no neighbor 11.11.11.2 shutdown

[asdvnw]

Thank you for the response, I am testing new ISP BGP with a brand new router.

[paul.driver · Accepted solution]

.

[paul driver · Accepted solution]

Hello
So if this new rtr is to eventually replace the old rt you can just bring up the new rtr and peer it with the "NEW" ISP but do not advertise any prefixes towards that new ISP and dont connect the new rtr into your existing network, Just leave it "soak testing" for a few days and when you are ready to use that new RTR and ISP, you can simply relocate the lan connection over to the new rtr and begin to  advertise your site prefixes.

Alternatively if you are wanting to use both old and new rtrs and both ISP circuits  then this does complicate things a little regards the setup but it is very much applicable to do, if this is the case please confirm?

[asdvnw · Accepted solution]

Hi Paul,

Thanks for the response, this is exactly the scenario I am facing. Thanks for the post.

asdvnw

[paul driver · Accepted solution]

Hello @asdvnw 
Your welcome and thanks for the feedback, much appreciated.