/25's prefixes announced into global routing table?

Mon Jun 24 13:22:39 UTC 2013

Le 22/06/2013 00:27, Jakob Heitz a écrit :
> There are techniques to fix that. For example, Simple Virtual Aggregation
> http://tools.ietf.org/html/rfc6769

The principle behind this RFC is that RAM (RIB) is cheap, CAM (FIB) is
not. But it's mostly intended for SDN developpments.

You need a full RIB on every (current - non SDN) BGP speakers in only
one scenario : beeing a transit provider. If you're not, there's no
point in keeping (and least to install) all routes.

But let's say you have enough RAM to store a very large de-aggregated
RIB, and a smaller TCAM. The best path selection takes place in RIB, and
selected best path can be installed as long as you have free space in CAM.

Let's compress (agregate) the table prior to installation, by creating
agregates over adjacent prefixes having only the same next-hop and
origin AS, and you may fit  a virtual-full-view, perfectly matching your
routing policy, with far less physical routes to install.

This will probably not be backported to "obsolete" routers, but it would
be natural to operate in such manners for an SDN control plane.

It *could* be deployed on older hardware using some customized BGP
speakers : use a software BGP speaker as a route-server /
route-reflector, get all your eBGP peers to hook to this speaker instead
of your ASBRs, and have the controller apply dynamic compression to the
route table (dynamic meaning it has to be a localy optimized agregate
different for every routers in your network) and feed it to your ASBRs
through iBGP.

There would be different levels of agregation : destructive or not,
matching the exact policy or cutting some slack to fit in smaller TCAMs.

For instance, having AS paths in the local RIB in your ASBRs is required
for netflow/ipfix AS agregates. You may not care about AS beeing too far
from your network, thus generating agregates over multiple adjacent
prefixes originating form different AS is not an issue : if you agregate
every prefixes matching the same 3-4 (deduplicated) AS in the path,
let's use the last as origin, and agregate all the adjacent blocks in a
signle prefix, as long as your best-path selection concluded in using
the same next-hop for all of them.

If you want a more agressive compression, then you may decide not to
fully respect the routing policy, and agregate over a minor block (let's
say a few /24 have a different next hop than their common less-specific
/12), discard the specificity and enjoy shooting /24s off your CAM.

If such a software existed (it's not yet available AFAIK, I wrote some
code to try the concept but it's still far from beeing usable), you may
actually run a real network with less than 30k route entries in your CAM...

-- 
Jérôme Nicolle
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