BGP Dual homing 2 sites with Same ASN

[Skywalker]

Hello team,
So i have 2 sites running eBGP on the same ASN peering with same ISP but at different locations.
This ASN has a /22 (eg 192.168.0.0/22). The customer would like to only use 192.168.0.0/24 up to 192.168.2.0/24  on the first site (Site A) and 192.168.3.0/24 on the second site (Site B).

The problem am having though is; whenever site A goes down, i cannot reach the /24 am originating at Site B.
Please note;

• Am originating and advertising 192.168.0.0/22, 1.0/24, and 2.0/24 at Site A
• Am originating and advertising 192.168.3.0/24 at site B

i thought originating the /22 at Site A was the problem so i removed it from site A such that i only originate /24's from both sites, but instead all the /24's couldn't reach the internet.

Is there something am doing wrong, kindly assist?

Thanks and Regards,

[chrihussey · Accepted solution]

It sounds as if the ISP is only accepting the /22.

Verify you are properly advertising the networks to the ISP at both locations:

"show ip bgp neighbor x.x.x.x advertised-routes"

If you are advertising correctly, the next step would be to contact the ISP and make sure they are indeed accepting the routes. They may have filters in place.

Hope this helps

[Skywalker]

Hello chrihussey,

You could be right, the advertisements on my end are fine (i cross-checked). However i checked on route-views for the /24's and they are not reaching! There is only the /22.

Let me take it up with the ISP.

Thanks a lot.

__
Regards,

[Pawan Raut]

Do you have all these /24 route in your Site A routing table before it advertising out to customer also do you have any filter applied on BGP neighbor like route-map or prefix-list.

Could you check the route advertisement from Site A router using sh ip bgp nei <nei IP> advertised command. Also I am interested the topology of this setup and BGP configuration.

[Skywalker]

Hi Pawan,

Thanks for your response; the advertisements  towards my immediate ISP are OK, they confirmed that they are receiving the /24's am announcing to them. Though it seems like there is an ISP in the transit path that is only announcing the /22.

I have engaged them to check with their ISPs too but haven't got positive response yet.

__
Regards,

[Giuseppe Larosa]

Hello,

if the two sites use the same BGP AS number but they have no connection between them you need to add

neighbor  <ISP-EBGP-peer> allow-as-in 1 on your side

This is because the standard BGP loop avoidance rule states:

" if you see your own AS number in the received AS path attribute then drop it silently"

Depending on the vendor of the PE nodes and on their configuration also the PE nodes of the ISP may need some tuning to be allowed to send to you  prefixes that contains your own AS number

IT looks like the ISP is creating the /22 only on the PE connecting to site A

I think this is the root cause

Hope to help

Giuseppe