07-29-2008 04:24 PM - edited 03-03-2019 10:57 PM
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
07-30-2008 06:35 PM
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
07-31-2008 12:49 AM
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
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