So Philip Smith / Geoff Huston's CIDR report becomes worth a good hard look today

William Herrin bill at
Thu Aug 14 00:12:50 UTC 2014

On Wed, Aug 13, 2014 at 6:47 PM, Chris Woodfield <rekoil at> wrote:
> On Aug 13, 2014, at 1:42 PM, Randy Bush <randy at> wrote:
>> half the routing table is deagg crap.  filter it.
>> you mean your vendor won't give you the knobs to do it smartly ([j]tac
>> tickets open for five years)?  wonder why.

> Same reason no vendor has bothered to prune redundant RIB
> entries (i.e. more-specific pointing to the same NH as a covering
> route) when programming the TCAM...

Hi Chris,

Not so much, no. Pruning seemingly redundant entries from BGP is
actually impossible to do safely, or if not impossible at least no one
has demonstrated a successful algorithm that can prune even a single
entry anywhere but the BGP source node or a BGP leaf node. And there's
not much point in pruning the BGP RIB at a BGP leaf node -- DRAM to
hold the RIB once received and processed is plentiful and inexpensive.

Pruning FIB entries, on the other hand, can be done quite safely as
long as you're willing to accept the conversion of "null route" to
"don't care." Some experiments were done on this in the IETF a couple
years back. Draft-zhang-fibaggregation maybe? Savings of 30% in
typical backbone nodes looked possible. That's 30% of your TCAM

For the moment it seems to be cheaper to just build bigger TCAMs.
Cheaper for the router vendors anyway.

Bill Herrin

William Herrin ................ herrin at  bill at
Owner, Dirtside Systems ......... Web: <>
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