BGP Subnet Advertisement Change

[Carl Duvall]

Probably a simple question, but my BGP skills are next to nothing....apologies in advance.

Scenario:

CompanyABC owns a /22 subnet of 1.1.0.0.  The full /22 is advertised out of CorpOffice1 to ISP1.  There is a route-map configured that uses a prefix-list to advertise the subnets outbound.  Below would be an example config:

BGP:

router bgp 35555

log neighbor changes

neighbor 2.2.2.2 remote-as 5555

neighbor 2.2.2.2 activate

neighbor 2.2.2.2 soft-reconfiguration inbound
neighbor 2.2.2.2 route-map ISP1_5555_out out

Route Map:

route-map ISP1_5555_out permit 10
 match ip address prefix-list ISP1_Primary

Prefix-list:

ip prefix-list ISP1_Primary seq 10 permit 1.1.0.0/22

Lets say CompanyABC also has another office that is also serviced by ISP1.  They want to split up the /22 subnet so that the first 3 subnets (1.1.0.0,1.1.1.0, and 1.1.2.0) are still advertised from CorpOffice1, but they want to setup BGP with ISP1 at CorpOffice2 and advertise only the 1.1.3.0 subnet from there.

Questions:

Would the configuration change be as simple as changing the prefix-list at CorpOffice1 to advertise the 3 /24 subnets and configure a similar BGP setup at CorpOffice2, but only advertise the 1 /24 subnet?

Would the configuration change at CorpOffice1 require any coordination with ISP1 to make that change?

(Obviously will need to coordinate with ISP1 to setup CorpOffice2 BGP to advertise the second subnet.)

What's the downtime/impact of reconfiguring CorpOffice1?

Thanks in advance, guys.

[Akash Agrawal · Accepted solution]

Yes it can be done as you mentioned. You can follow below steps -

- Originate three /24 prefixes from CorpOffice1.

- Change prefix-list in outbound route-map  to permit three /24 prefixes.

- Ask ISP to accept these prefixes. ISP must have applied inbound route-map to have control on prefixes what you are advertising. It could be in format like "ip prefix-list LIST permit 1.1.0.0/22 le 24" or may be just permitting /22 prefix. So confirmation with ISP and change in that route-map may be required.

- Once ISP confirms that they are accepting /24 prefixes and advertising it to internet, you can stop advertising /22 prefix. To confirm whether /24 prefixes are being advertised to internet or not , you can login to any global server and check bgp route for your prefixes.

- Also as soon as you stop advertising /22 prefix, check if you are able to reach internet or not. If not, take a source based traceroute result and then start advertising /22 prefix again and troubleshoot it with ISP further.

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Regards,

Akash

[Carl Duvall]

Thanks, Akash.

So if the ISP continues to permit the /22, it would be fine if the CorpOffice side advertises individual /24 subnets?  I wasn't sure if the lists on both sides had to match up (kind of like VPN tunnel ACL's)?

Edit:  I kind of answered my own question by researching ip prefixes a little bit more. I understand that if the ISP accepts 1.1.0.0/22 le 24, it will include any subnet advertised within that up to a /24 network, correct?

Carl

[Akash Agrawal]

If ISP is just permitting /22 , router wont accept /24 prefixes. 1.1.0.0/22 le /24 will work.