"2M today, 10M with no change in technology"? An informal survey.

Lincoln Dale ltd at interlink.com.au
Wed Aug 29 00:11:54 UTC 2007

> [...]
> > 2. Once the limit is reached, excess routes will fail over to software
> > switching. TAC did not specify how routes are designated as excess.

most-specific-prefixes first.  it has to be this way due to the way a TCAM
search works.

> I'm not sure if the Sup2's handle this case differently from the
> Sup720s we were using, but, in our case, when we reached the ceilign
> the routes appeared in both the routing and CEF tables but were not
> populated into the FIB.
> Translation: the route was ignored....

how old is the software you were running on your cat6k?
reason i ask is that since circa. 12.2(18)SXF9 (i.e. back in 2005), there has
been a graceful degradation back to software forwarding for those entries that
don't fit into the FIB TCAM:

 - when the h/w FIB is full (FIB exception) it goes into exception state
   where it will maintain the longest-prefix-matches by removing shortest-
   prefix-matches from the FIB TCAM first
 - it will also insert a default entry to punt lookup exceptions to software
 - software typically CAN maintain a full FIB, so entries which don't fit into
   hardware can be software forwarded in the CEF software switching path
 - when the h/w FIB is full, the following syslog message is generated:
	MLSCEF-SP-7-FIB_EXCEPTION: FIB TCAM exception, Some entries will
	 be software switched

of course, software forwarding is potentially orders-of-magnitude slower than
h/w forwarding, so how much extra headroom this gives you once you exceed the
capabilities is dependent on the amount of traffic to those prefixes that don't
fit into the h/w tables.

agree that this isn't "ideal", however Cisco has always been very specific
about the h/w FIB & adjacency table sizes on the hardware in question.
i know that vendor bashing is a sport in this list, but....

relevant bug-ids if you wanted to look up the details:
	CSCse90572 syslog message when FIB TCAM exceeds 95% utilization
	CSCsb18172 wrong packet forwarding at FIB exception

if you need further clarification, feel free to contact me off-list, my work
email address is ltd at cisco.com.



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