BGP announcements

Answered Question
Aug 5th, 2008
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I'm currently setting up a multihomed configuration for my organization. Our infrastructure is currently addressed out of our current ISP's /24. We also have direct assignment space which we plan to start addressing out of in the coming months.


My question is this: Can I just advertise the ISP's /24 out their connection, and the direct assignment /24 out the other router, or do I need to advertise both from both routers?


We will load share our connections over both 100mb circuits.


Thanks in advance for any help offered.


/rls

Correct Answer by royalblues about 8 years 11 months ago

Yes you would need to advertise the prefix via both the routers and readdress the existing ISP owned address to the one owned by you


By default, the incoming traffic would choose the link based on AS-path seen from the destination. You could use as-path prepend to make one of the connections less preferable though.


Have a look at this link

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008009456d.shtml


HTH

Narayan

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royalblues Tue, 08/05/2008 - 10:24
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You would basically need to advertise your direct IRR assigned space out both the routers for loadsharing


The ISP's /24 can still be advertised out its own connection but services using that address space wont fallback to the other ISP during a failure


HTH

Narayan

rsamuel708 Tue, 08/05/2008 - 10:55
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Let me explain a little more....we actually have a firewall sitting below what will be our bgp routers. This firewall will handle the load-sharing issue by way of a software feature in the Checkpoint software.


So all I need to know is whether it will be necessary to advertise both addresses from both still.... If I only advertised the ISP /24 from their router and our direct /24 out the other router, wouldn't both routers still have /24s in their routing table by way of the iBGP connection?


/rls

royalblues Tue, 08/05/2008 - 11:21
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Ya it would


But you should know that generally one ISP wont accept other ISP address space.


so for eg if R1 advertises ISP /24 to the ISP1 and R2 advertises ur own prefix to ISP2, and if R2 fails for some reason, the IBGP route would disappear from R1's table and anything natted to that address space will lead to R1 and fail


Narayan

rsamuel708 Tue, 08/05/2008 - 11:29
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So would the best solution be to advertise both address blocks from both routers? Both ISPs have agreed that they would advertise the space....actually only the new ISP had to agree to advertise our existing netblock from the existing ISP since we have our own direct connection.


If advertising both /24s from both places is the answer, would inbound traffic to either prefix take the path or ISP that had the "best" or most peering points, at least amount of AS hops?


/rls

Correct Answer
royalblues Tue, 08/05/2008 - 11:48
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Yes you would need to advertise the prefix via both the routers and readdress the existing ISP owned address to the one owned by you


By default, the incoming traffic would choose the link based on AS-path seen from the destination. You could use as-path prepend to make one of the connections less preferable though.


Have a look at this link

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008009456d.shtml


HTH

Narayan

rsamuel708 Tue, 08/05/2008 - 16:25
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Ok...so if I advertise both prefixes from both routers, does it make any difference at all which I advertise first? i.e. ...does the isp-owned netblock need to come first on the router connected to their circuit?



...on ISP1 router:

neighbor 66.x.x.x prefix-list OH_ANNOUNCE out

!

ip prefix-list OH_ANNOUNCE description OH Netblocks to announce

ip prefix-list OH_ANNOUNCE seq 10 permit ISP1_netblock/24

ip prefix-list OH_ANNOUNCE seq 20 permit Direct_Assignment/24



/rls

royalblues Tue, 08/05/2008 - 21:41
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That does not make any difference


HTH

Narayan

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