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bgp question

Posted on 2002-05-20
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Last Modified: 2010-04-11
hi

we have a multihomed bgp environment with our own AS.
We have been given a /24 ip address range (which we have subnetted to use 4 subnets /26 - using the
all zero and all ones subnet)

Given that we have our own AS number and IP address range, is the AS number linked with our ip address
range? is this linked solely with the "network" command in BGP config?

see configuration below:

router bgp x
no synchronization
bgp log-neighbor-changes
network a.a.a.0
neighbor x.x.x.x remote-as 702
neighbor x.x.x.x description UUnet s2/1
neighbor x.x.x.x filter-list 1 out
neighbor y.y.y.y remote-as 5378
neighbor y.y.y.y description C&W s2/0
neighbor y.y.y.y filter-list 1 out

so whatever we put in for the network a.a.a.0

ie network a.a.a.0 is important, as this will be advertised out to other BGP As'es, for instance if
I changed this to another ip address range would this cause carnage on the internet? (not that i would!
our isps trusts us!)

We have been using neigbour y.y.y.y for 2 months. We added neigbour x.x.x.x today. How long would it
take for other BGP routers to have x.x.x.x listed as a alternative AS path (via AS 702)?? to our a.a.a.0
network address range?

Would it be possible for us to be a transit AS... i know that this is a bad idea.. but how is it configured?

Thanks
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Question by:dbrannigan
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by:mmedwid
ID: 7022504
I believe you'd only want to be a transit AS if you are an ISP.  It souns like you are an enterprise - no?

Your ISPs probably have filters up to keep you from causing too much mayhem.   But without putting the whole block into your network statement - you risk folks not being able to reach those subnets that are excluded.  
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Expert Comment

by:mmedwid
ID: 7030818
http://www.cisco.com/warp/public/459/bgpfaq_5816.shtml#6

Check out the entire faq actually.  
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Expert Comment

by:mmedwid
ID: 7030820
Oops - this is the part I meant to call your attention to...

Q. How can I verify if a BGP router is announcing its BGP networks and propagating them to the global BGP mesh?
A. To check if the IP blocks are announced to the directly connected ISP, use the following commands. The show ip bgp neighbors [address] advertise-routes command shows which messages are being sent. The show ip bgp neighbors [address] routes command shows which messages are being received.
Note: The show ip bgp neighbors [address] advertise-routes command doesn't take into account any outbound policies you may have applied. In future IOS versions the command output will be changed to reflect the outbound policies.

To verify how the IP blocks are getting propagated to the global BGP mesh via the directly connected ISP, log onto a route server  on the Internet and look for the BGP entries of the prefix in the route server.
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mmedwid earned 150 total points
ID: 7030821
And to see how you look from the perspective of the outside work...select any of the choices, select BGP and enter the block you are wondering about.

http://nitrous.digex.net/
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Author Comment

by:dbrannigan
ID: 7141026
thanks mate.. sorry for the delay in accepting your answer!
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by:mmedwid
ID: 7141805
No problem.  Glad to be of assistance!  -Michael
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