BGP - Multihomed , Diffrent ISP's - Want some traffic to return on backup

[jglobermudez]

Hello All,

I am new to the forums. and am no BGP expert. but I am ok with it.

Having said that here is my issue.

I have multihomed BGP to 2 diffrent ISP's. the primary and backup advertise the same subnets to the ISPS.

We are doing AS prepending to Make the backup look worse ( hence making it the backup). also my primary is in chicago and backup in new york

So here is my problem

I have some traffic that I want to route to of new york because it the destination is in NY also.

No problem I put in a static route and route out of New york ISP.

But how can I influence the ISP(s) to only route a handfull of ip's back thru the NY backup link ( when we have alreay prepended is 3 times and made it the Backup)

Like I said go easy on me

and I appreciate any and all of your suggestions

john

[sean_evershed]

You could have a look at using BGP Comminity Values. See the link below for reference:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801475b2.shtml

[milan.kulik]

Hi,

maybe you could think about advertising some small subnet as preferred over NY and NATing that handful of IPs to that small subnet?

HTH,

Milan

[Kishore Chennupati · Accepted solution]

Definetly recommend using BGP attributes. makes your life much easier. It would something like this. Lets assume your smaller subnets is 192.168.1.0/24. then create a route-map like the below and attach it to the neighbor.

Check the community value from your local ISP which would make it prefer the NY path normally by reducing the local pref. since local pref comes before the AS_Path these subnets will be routed back via NY

ip bgp-community new-format

access-list  80 permit ip 192.168.1.0  0.0.0.255

route-map SET-LESS-PREFERRED permit 10

match ip address 80

set community

route-map SET-LESS-PREFERRED permit 20

set as-path prepend

router bgp

neigh x.x.x.x. route-map SET-LESS-PREFERRED out

Please see below an example of it for Optus which is a ISP in Australia

http://www.ipbalance.com/routing/bgp/bgp-community-attributes-list/332-bgp-community-string-for-optussingtel-as7474.html

Let me know if yiou need more info on this, We have deployed BGP comminutes in out network and it works like a whilstle.

HTH

Kishore

[jglobermudez]

Hello Kishore,

Ok thanks perfect we used some Communities over here.

I know I can choose a community to have the ISP influence what Local pref they give us

I was under the impression that this was for the entire AS.

but you are saying that I can make a route map and only influence a certain subnet???

if that is indeed what you are saying than that is the swnser to my issue.

can you please clairify

Thank thank you for your time.

John

[Kishore Chennupati]

Hi John,

Of course you can use communities to influence certain subnets. We have deployed it in our network and I gave it to you based on a production working solution. If this helps to answer please rate and mark the question as answered

HTH

Kishore

[Kishore Chennupati]

Thanks for the generous rating and marking this question as answered,.  it helps others foster the learning process

[Mohamed Sobair]

John,

I have a simple Solution for you if you can do the following:

Those handful IPs you would like to recieve through NY ISP (Which looks worse path from the internet) can be subneted.

For example, you have the Class C Subnet  (1.1.1.0/24), and you are advertising this subnet through both ISPs and prepending it on NY ISP to look worse, I would just Spilit the Subnet and advertise more specific Subnet for those Handful IPs to look better from the Internet.

So, for example, I would advertise both 1.1.1.0/24 , and 1.1.1.0/25. Assuming the (1.1.1.0/25) is your Handful IPs you want to prefer through NY. this way it will always be prefered through NY, and it will give you the required redundancy if the NY ISP goes down. this Network would also be reachable through Chicago ISP since you are already advertising the Whole Block (1.1.1.0/24).

This could be a Working Solution for you,

Regards,

Mohamed