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Community Member

BGP - routes within AS

I am changing the routing config for all our sites, We have a head office and 9 remote sites. Our line provider originally wrote the configs, but the backup links never worked, so I am re-writing them myself.

And I've come across a confusion with the BGP side of things.

Each site is in the same BGP AS! They connect to core routers which are in different AS but each of our sites is in the same AS. When I recreate this in my lab I cannot get the routes for each site to propagate. When I make the remote sites a different AS then the routes propagate fine.

I cannot see what they've done to achieve this.

Here's the BGP config for HO:

router bgp xxxxx

no synchronization

bgp router-id x.x.x.x

bgp log-neighbor-changes

network 10.123.123.176 mask 255.255.255.248

network 10.200.1.1 mask 255.255.255.255

network 10.255.200.32 mask 255.255.255.252

network x.x.x.x mask 255.255.255.252

aggregate-address 10.200.0.0 255.255.0.0 summary-only

neighbor x.x.x.x remote-as XXXX

neighbor x.x.x.x description Global

neighbor x.x.x.x default-originate

neighbor x.x.x.x soft-reconfiguration inbound

no auto-summary

!

and here's the BGP for a remote-site:

router bgp xxxxx

no synchronization

bgp router-id x.x.x.x

bgp log-neighbor-changes

network 10.1.2.0 mask 255.255.255.0

network 10.123.125.0 mask 255.255.255.248

network 10.202.1.1 mask 255.255.255.255

network 10.255.200.40 mask 255.255.255.252

network x.x.x.x mask 255.255.255.252

aggregate-address 10.202.0.0 255.255.0.0 summary-only

neighbor x.x.x.x remote-as XXXX

neighbor x.x.x.x soft-reconfiguration inbound

no auto-summary

!

In my lab I have advertised the network of the HO lan on the core routers, but that didn't do the trick.

Please help!!

Thanks for reading.

6 REPLIES

Re: BGP - routes within AS

What's your end goal exactly? BGP using the same AS is an iBGP environment. In order for you to be able to get a route directly to a remote site, I would think you'd need to set up ebgp-multihop in order to get this to work. I'd have to test it. Are you trying to get off of the iBGP scenario?

For example, let's say that all of your sites is in AS100 and the provider is AS200. The provider will have in their routing table the routes (in this case, your summaries) in their table pointing to your serial side that matches where the summary is coming from, and whatever other routes you're advertising. A summary route is hard to "fail over" to because summaries help with flapping links, etc. It also depends on how your site is laid out if a backup route makes sense. If you have only 1 exit point from your branch, then a backup route won't help.

John

HTH, John *** Please rate all useful posts ***
Hall of Fame Super Silver

Re: BGP - routes within AS

Hello Charles,

>> Each site is in the same BGP AS! They connect to core routers which are in different AS but each of our sites is in the same AS

BGP loop prevention avoids to learn BGP routes with an AS path attribute that already contains your own AS number.

So what you see when all remote sites share the same AS number is

Two possible options:

- you change so that you different private AS numbers at each remote site

- you keep them as now but you use the command

neigh x.x.x.x allowas-in

towards the eBGP session that can carry these routes with duplicated AS number in their AS path

it is not clear if your SP is giving you a L3 VPN service or simply a collection of links.

if it is a collection of L2 links (no SP routers involved) you could fix this by putting all devices in the same AS, that is moving to an iBGP cloud.

with iBGP in order to propagate routes the central site routers must act as BGP route reflector server

for each remote site you would need on Central site router:

neigh y.y.y.y route-reflector-client

this would allow propagation of routes from remote to remote via central site routers.

Hope to help

Giuseppe

Community Member

Re: BGP - routes within AS

Thanks Giuseppe,

your line

neigh xxxx allowas-in

worked a treat.

It's not what our routing company have done, they must have some config in the core, but it works a treat.

Thanks

Re: BGP - routes within AS

Hi,


However, you should be carefull with this command , it can easily introduce loop , its always recommended to set the number of next hops that allows - as in is configured: ie:

neighbor x.x.x.x allpw-as in y (where y is the number of hops allowing the as)

HTH

Mohamed

Hall of Fame Super Silver

Re: BGP - routes within AS

Hello Mohamed,

I agree about the risks of the command but the default value of y= number of occurrences of your ASN in AS path is 1, so he should be fine with default settings.

Usually a parameter has to be specified to accept routes with your own ASN prepended

Hope to help

Giuseppe

Community Member

Re: BGP - routes within AS

Thanks guys, I will check for loops carefully in my lab

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