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When I in my Router2 add a neighbor, the bellow is my command:

Router2(config-router)#neighbor 12.0.0.1 remote-as 100

you see, I need write two params, one is neighbor address, and the other is its AS number(one IP only can belong to one AS).

but I want to ask a question, because a router only can belong to one AS, it can only have one AS number, why there still need the remote-as number?

  • Note that some BGP implementations don't require you to configure the remote-as number. Instead you can just configure whether the neighbor is internal or external. See "getting rid of AS numbers" in blog.ipspace.net/2015/02/… – Bruno Rijsman May 16 at 14:01
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cisco documentation says that: "the configuration does not overtly define peers as iBGP or eBGP. Instead, each router examines its own ASN as defined in the router bgp command, and compares that value to the neighbor’s ASN listed in the neighbor remote-as command. If they match, the peer is iBGP; if not,the peer is eBGP."

so you should look at differences between iBGP and eBGP.

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