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Beginner

Process switched packets - CEF & input-queue

Hi there,

I am trying to find the reason for the high number of input-queue drops on a 2821:

  Input queue: 8/150/3790/0 (size/max/drops/flushes); Total output drops: 0

The interface concerned has CEF enabled:

IP fast switching is enabled
IP fast switching on the same interface is disabled
IP Flow switching is disabled
IP CEF switching is enabled

Reading through a few docs online I have made the following assumptions;

- the reason for the input-queue to get filled are packets for which no valid cached adj is found in the CEF cache (or a CEF unsupported feature)

- the packets within the input-queue are getting process-switched

- to find out which packets are filled within the input queue: show buffers input-interface g0/1 header

this gives us a list of UDP p161 packets (snmpget's)

- I can't find a possible reason for these UDP packets (p161) not getting CEF switched (interrupt switched) because I can find a match (valid adj) within the cache?

- running "show cef not-cef-switched" shows me these packets fall into the category: "Receive

CEF Packets passed on to next switching layer
Slot No_adj No_encap Unsupp'ted Redirect Receive Options Access Frag
RP         0         0                   0              0         6972         0           0        0

Cisco Docs mention these "receive" packets are packets destined for the router itself which I can't match...

Running "debug ip cef receive" gives us the following output:

407683: Dec 14 14:55:34.323 cet: IP-CEF: Receive packet for 145.7.25.198 (process switch)
407684: Dec 14 14:55:34.323 cet: IP-CEF: Receive packet for 145.7.25.198 (process switch)
407685: Dec 14 14:55:34.331 cet: CEF-receive: Receive packet for 145.7.25.198
407686: Dec 14 14:55:34.331 cet: CEF-Receive: Packet for 145.7.25.198 -- receive
407687: Dec 14 14:55:34.335 cet: IP-CEF: Receive packet for 145.7.25.198 (process switch)
407688: Dec 14 14:55:34.339 cet: IP-CEF: Receive packet for 145.7.25.198 (process switch)
407689: Dec 14 14:55:34.923 cet: CEF-receive: Receive packet for 145.7.25.198

As you can see, these actually are packets which get processed switched. Now I need to know what kind of packets these are?

I have already tried to do a "debug ip packet detail ACL" to see if these are packets pointed to the router itself, but the number of packets I see from the debug are much less than the number of lines which get spit out of "debug ip cef receive

I hope someone can give some poiters to get more insight in these processed switched packets....

thanks!

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VIP Expert

If the issue is unnecessary

If the issue is unnecessary process switching, I recall (?) there's a troubleshooting paper on Cisco's main site.

However, it also just might be a performance limitation of the 2821 . . .

Is the interface in question gig Ethernet, running at gig?  If so, the 2821 cannot usually support a large gig burst.

You can try increasing the input queue depth (even to maximum) and/or slow ingress (e.g. run interface at 100 Mbps) to avoid gig ingress bursts losing packets due to the CPU being unable to keep up.

4 REPLIES 4
Highlighted
VIP Expert

If the issue is unnecessary

If the issue is unnecessary process switching, I recall (?) there's a troubleshooting paper on Cisco's main site.

However, it also just might be a performance limitation of the 2821 . . .

Is the interface in question gig Ethernet, running at gig?  If so, the 2821 cannot usually support a large gig burst.

You can try increasing the input queue depth (even to maximum) and/or slow ingress (e.g. run interface at 100 Mbps) to avoid gig ingress bursts losing packets due to the CPU being unable to keep up.

Beginner

Hi Joseph,

Hi Joseph,

The if is running at 1000/full (fixed).

I would like to know if my troubleshooting steps are correct and what kind of packets are meant by "Receive" under "show cef not-cef-switched". And when I have a look at the buffer, why are these UDP snmp packets not CEF switched? This would mean that routing at this particular router could be way more efficient?

I can indeed increase the hold-queue, but then I would still have these SNMP packets processed switched...

I thought the limit of the hold-queue is 4096... is this safe on a 2821?

VIP Expert

Without doing some research,

Without doing some research, cannot comment on your troubleshooting approach.

Unable to say with 100% assurance, that there's no issue increasing hold queue to max, but recall a whitepaper, for 7200s, saying it was generally okay, as you're just trying to give the CPU a chance to keep up with an ingress burst and you cannot as easily select what traffic to drop (although if the router offers SPD, you have some minimal control).

Beginner

The thing I've not mentioned

The thing I've not mentioned before, the interfaces which handle these packets actually have "ip nat inside"  en "ip nat outside" configured. I have not find any info about NAT & CEF but could it be the case that when using NAT (including route-maps within "ip nat inside source route-map") CEF is bypassed because the router will punt all of this traffic to the CPU?

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