If I'm advertising a route let's 10.20.0.0/16 to go to router X (i.e in AS1234), and then I advertise that 10.20.10.0 needs to go to router Z (AS4321), will the BGP routers in the MPLS send a packet with a destination of 10.20.10.4 to router "Z" or will it match the 10.20.0./16 route?
Hopefully I've made myself clear, if not please let me know.
Is 10.20.10.0 with a /.24 subnet mask. If so then a longest match will always be used but that is assuming that all things about the routes advertised are equal. BGP has a lengthy path selection process so to say for sure it will use 10.20.10.0 would be a bit premature, but if all other things about the routes are the same then it will pick the longest match.
If I may, I just want to make a slight clarification on what you wrote.
"BGP has a lengthy path selection process so to say for sure it will use 10.20.10.0 would be a bit premature, but if all other things about the routes are the same then it will pick the longest match."
You're right that we need the OP to tell us the length of the mask for the 10.20.10.0 network. However, the BGP route selection process in which BGP attributes are compared is performed when comparable routes are received by a BGP speaker. By "comparable", I mean routes that have the SAME prefix length. This is true because, like other routing protocols, BGP will use the "prefix length first" rule to select the best match. So, if the 10.20.10.0 prefix is a /24, it will automatically be selcted for a destination that falls in both the /16 and /24 prefix advertisements -- before any BGP route selection algorithm is applied.
Thanks for the replies. Yes you are correct the 10.20.10.0 network has a /24 prefix. Since I don't have access to the MPLS network I'm going to assume that they are equal. If they are equal does the match stand i.e it will choose the /24 path before it choosed the /16 path?
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