[c-nsp] 6500/7600 TCAM Usage

[Mack McBride]

>From prior experience, using 100% and bad things happen.
As the device approaches a full tcam convergence will get much slower.
Additionally the table is not static so you can get bursts of routes associated with leaks.
Don't forget there are routes that are not in the BGP and OSPF tables that get inserted.
Connecteds, Statics, next hops and vlans and only cisco knows what else.  Most people
forget there is a route for every vlan even if no IPs are associated with it on the 6500 platform.

I usually use a margin of 10K above what "show ip route summary" is producing as a 'safety net'.
Once you get into that range, 'Bad things happen'.

'show mls cef summary' actually shows about 5K less on my devices but those routes are still in there.
So don't use that as what is actually getting inserted.



Mack McBride | Senior Network Architect | ViaWest, Inc.
O: 720.891.2502 | C: 303.720.2711 | mack.mcbride at viawest.com | www.viawest.com

-----Original Message-----
From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Pete Templin
Sent: Tuesday, May 31, 2016 2:53 PM
To: Gert Doering; James Bensley
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] 6500/7600 TCAM Usage

+1 on what Gert said. You'll get log entries at the 90% threshold within
a region, but the badness only happens when you tickle the 100% threshold.


On 5/31/2016 11:45 AM, Gert Doering wrote:
> Hi,
>
> On Tue, May 31, 2016 at 07:19:22PM +0100, James Bensley wrote:
>> I have asked TAC and they said the TCAM can be 100% used, not until we
>> have 1,024,000 entries in TCAM will we start of see the syslog
>> messages for failing to install a prefix. I am certain that one CAN
>> NOT use 100% of the TCAM space, I'm sure I read somewhere that at
>> around 90% utilisation we start so process switch / drop packets /
>> fail to install routes.
> You can use 100% of what you have partitoned for - so if you partion for
> 512k IPv4, you'll blow up at 512*1024 IPv4 routes (minus a few, I'd
> assume).  Been there, done that - not at 512k but at something like 200k
> on non-XLs, years ago.
>
> That "at 90% utilization bad things will happen" sounds like an urban
> legend from the BNC ethernet times...  it's TCAM, there is nothing magic
> about 90% - either a route can be poked in there, then it will work,
> or not, then all excess routes will be process switch (and subject to
> rate-limiting)
>

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