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eBGP and AS pre pending

I recently set up a new eBGP customer's circuit, who'll be doing dual-homing with another provider. This customer has a CCIE running their network, and he's complaining that, when using a BGP route view server, his AS is not being prepended when it takes it's path through our AS.

He feels that this will cause problems with incoming traffic. Here's my thought, the reason that his AS is not showing up in the route view server's trace is that his WAN IP for the eBGP session with us is part of a /30 that belongs to our ARIN assigned subnet.

His other WAN IP to the other provider is part of a /24 that was assigned to him from ARIN. Therefore, when routes travel through our network to his subnet, the last point it hits is his WAN IP that belongs to our /30.

Can someone verify my thought? If i'm incorrect, is there any way that I could have his AS show up? I don't have much experience with pre pending AS numbers in dual homed environments, but i always thought that the customer would do the prepending facing it's desired provider.

Thanks for reading / replying!

Everyone's tags (4)
Hall of Fame Master

Re: eBGP and AS pre pending


Your description of the WAN addresses and what subnet they belong to would be appropriate if the discussion were about traceroute output. But if I am understanding correctly, the question here is that when the CCIE looks in route server output he sees his prefix advertised from you but without pre-pending. That has nothing to do with what subnet is used to get to his prefix.

There are several questions and knowing answers to them would make it easier to resolve your issue:

- when he looks at the route server, does he see his prefix advertised from you and advertised from his other provider, or does the route server show only one of the advertisements?

- when you look at what he is advertising to you, does his AS number show up only once or does it show up more than once?

- is the customer using a registered public AS or is it using a private AS?

- is the customer using provider independent IP address space, or is their IP address space assigned to them by the other provider, assigned by you, or what?

In general I believe that you are correct that it is more common to have the customer do the prepending when they dual home. But there are certainly situations where the customer does not prepend and asks the provider to prepend for them. Was there any request from the customer about prepending when they negotiated with your company for Internet access and to run BGP?



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