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aamercado
Enthusiast

Same EBGP AS with allowas-in

Here's my topology:

Site A in NewJersy, USA

Router1 EBGP to primary ISP1 with AS1

Router2 EBGP to backup ISP2 with AS2

Router1 and Router2 also have an IBGP with public AS100 number to each other as well as using GLBP for loadbalancing

New Site B in London, UK is coming up and I want to do the following:

Router1 EBGP to primary ISP1 with AS1

Router2 EBGP to backup ISP3 with AS3

Router1 and Router2 also have an IBGP with public AS100 number to each other as well as using GLBP for loadbalancing

Per my research, can I use the neighbor x.x.x.x allawas-in in this scenario without any issues?

If so, What are some gotchas I need to be aware of?

TIA

2 REPLIES 2
Steve Lyons
Beginner

To configure provider edge (PE) routers to allow readvertisement of all prefixes containing duplicate autonomous system numbers (ASNs), use the neighbor allowas-in command in router configuration mode. To disable the readvertisement of the ASN of the PE router, use the no form of this command.

neighbor ip-address allowas-in [number]

no neighbor allowas-in [number]

Refer to the following link:

http://www.cisco.com/en/US/docs/ios/mpls/command/reference/mp_m4.html#wp1013322

AJAZ NAWAZ
Contributor

In this case we are overriding the loop prevention mechanism which is inherently built into the BGP protocol (i.e. eBGP). I think if you read the notes, they advise caution for this exact reason.

As best practice you should have filters in place for both sent and received prefixes. Of course there are ample ways this can be achieved with Cisco IOS. By doing so you will remove the risk of developing loops by some misconfiguration (on someones part), or a topology change (i.e. newly added networks/peers).

If you need more detail in how to do the filtering please don't hesitate to get back to us.

hth,

Ajaz