Policy based routing and Process switching

[adananm]

If a policy map is applied to an interface. Does the traffic get cef or process switched? Is the router's behaviour platform dependent?

[bbranch]

Prior to release 12.0 all PBR was process switched, after 12.0 you can enable fast switching for PBR using the "ip route-cache policy" command, but the default it is still process switched on all platforms:

http://www.cisco.com/en/US/customer/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800c75d2.html#1000980

[ruwhite]

It should be CEF switched on all software switched platforms; on hardware switched platforms it is going to be platform dependant.

Russ

[t.baranski]

This is interesting, as I've been wondering how CEF capability was added to the ancient 2500 platform and "the switching is done in software" is a sensible answer to that. I don't suppose there's a list anywhere of which platforms use which switching method (hardware/software)? Or can one generalize and say something like "all platforms introduced after (year) are hardware switched"?

[ruwhite]

The list is pretty simple, actually:

-- 6500, hardware switched in most cases, but there are some cases where it is software switched.

-- 12000, hardware switched.

-- 10000, hardware switched.

-- 7500, software switched, but distributed software switching on VIPs (each VIP switches packets in software on a local processor, so it's distributed software switching).

-- Some of the other CAT line is hardware switched, like the 5500 with some card combinations, etc.

Everthing else is software switched. The best bet is to look at the hardware platform information. For instance, the 7200 is generally software switched, but the NSE-1 has a programmable ASIC which does the switching, somewhat, the same one that does the switching in the 10000 series.

Anyway, your best bet on an explaination of software based switching is in the book Inside Cisco IOS Software; there is another book in the works to replace this book in the Cisco Press line, but it won't have the same title, I don't think. If you look up the books on Amazon that I've co-authored, you'll find the title as it currently sits, but I wouldn't imagine it will be out until sometime around June of next year, at the earliest.

Russ.W

[adananm]

Russ,

Let me make certain that I understand what you are saying, you are saying a 3600 router should use CEF to forward PBR traffic rather than a combination of Process switching and CEF.

M.

[ruwhite]

Yes, that is correct. It will be CEF software switched, in interrupt context, even though it is policy routed. Wilber Su, I think, committed this back in the 12.0S rain someplace, but I'm not certain of the exact release. I've been searching for the feature ddts number, so I could track down the actual commit date, but I can't seem to find it. I did track down the engineering documents on the feature, just not the actual commit date.

If you turn on policy based routing, with the configuration you want, in a lab, you can shove traffic at it, and use show cef not-cef-switched to determine if the traffic is actually being cef switched or not. But, for the most part, it should be.

:-)

Russ.W

[adananm]

Thanks.

This implies that you can encounter a switching issue in a particular IOS version and not have the same issue in another IOS version on the same platform.

[adananm]

Guys,

My question was meant to ask about switching in relation to route-maps applied to an interfaces rather than policy maps.

Furthermore, I presume the ip route-cache policy command creates a cache based forwarding system for PBR?