[c-nsp] BGP advertisements more specific than IGP

Ryan Rawdon ryan at u13.net
Thu Feb 28 23:40:47 EST 2013

On Feb 28, 2013, at 9:11 PM, James Urwiller wrote:

> I have a BGP multi-homed invironment that I am having problems balancing inbound traffic, besides prepends which don't seem to be helping anymore, I have heard that announcing my networks more specifically could also influence inbound traffic.  My question is, for example… If I have a /23 that I am using as a /23 in OSPF, can I announce that in BGP more specifically (2, /24's)  without having to them break it up internally as well?  What I foresee happening is this..
> Example:
> BGP:
> Network
> Network
> Network
> I would think in this scenario, the IP addresses and would not have a route in BGP, even though they are valid addresses for use when used as a /23.  Since I would be multi-homed, I would still advertise the network as the aggregate /23 on the circuit I don't want to take as much traffic, so would those IP addresses in this scenario still work, but only through the circuit I advertise as the aggregate??

The addresses will still have a valid "route" in BGP - you only "lose" those addresses on an Ethernet/broadcast segment if it was a /24 (or whatever) on-link.  

>From a routing point of view, traffic to the .0 and .255 address will still arrive just fine in the scenario you laid out above.  For example we had a /24 from ARIN and were able to use .0 as a loopback on a device inside our network (routed in our IGP [OSPF]) just fine while it was advertised in BGP as $prefix/24

Now that aside, I'm not sure if you'll run into weirdness because your IGP does not carry an equivalent or more specific route than BGP.  For example if your traffic comes in to your edges that have Null0 or other routes for the /24, they may never forward traffic inwards to wherever the /23 actually resides because the more specific /24 Null0 will always win.  I don't have firsthand experience with this scenario without going and labbing it up right now, someone else may be able to provide some more authoritative input. (well now I want to go test this - if nobody else comments on it I might come back with an answer in a day or two)


> James Urwiller
> Network Operations Manager
> CCNA 11567125
> American Broadband
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> jurwiller at americanbb.com<mailto:jurwiller at americanbb.com>
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