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kevin.watkins
Beginner

Two BGP ASs originating the same network

I've read that two different ASs shouldn't originate the same network. I thought I read it in Halabi's book, but can't seem to find out where. Example, two different ASs have static routes to a third party, network 16.0.0.0. They both want to advertise the route to the BGP Core AS, so they both enter "network 16.0.0.0" in their BGP config.

The core sees two routes to the same network, one originating in AS1 and one in AS2. What's the reason that this is a problem, given that both AS1 and AS2 can equally reach network 16.0.0.0?

What's the reason for the rule that a given network should only originate from a single AS, if it's indeed a rule. Many thanks.

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jasyoung
Rising star

It's regarded as messy and poor practice in general, but it will technically work. Legitimate reasons for wanting to do so are rare. What do you want to accomplish?

Cisco's BGP implementation has an option to show routes it has received with inconsistent origin-ASes ('sh ip bgp inconsistent-as', amazingly enough).

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