Pause Outputs from Router to SRE Module running WAAS 5.3.1

[Tammy Cox]

Why does the SRE module not indicate it has received any Pause request from the router?

Are the ge_tx_interrupts/ge_rx_interrupts on the SRE module interface of great concern?

The Pause Outputs from the router on the SMx/x interface is seen in most of the SRE modules running WAAS 5.3.1 in our fleet.

Scenario:  Cisco 2951 router (IOS 15.1(4)M3) with SRE-900 running WAAS 5.3.1

1) Seeing Pause Output frames on the SM2/0 interface of the router, meaning that the router is telling the SRE to slow down.

router#sh cont sm2/0 | i pause
  tx_pause_frame_cnt: 153247            rx_pause_frame_cnt: 0

2) Also seeing interrupts on the SM2/0 interface of the router

router#sh controller sm2/0
Interrupt statistics
--------------------
  ge_tx_interrupt: 2140056415           ge_rx_interrupt: 1850679159

3) The SRE is not receiving nor sending any Pause frames.

sre-waas#sh int giga 1/0 det | i Pause
Pause Frames Received               : 0
Pause Frames Sent                   : 0

[Jorge Bejarano · Accepted solution]

Tammy,

What behavior do you expect to see?

Has this SRE ever worked before? Is it optimizing traffic fine?

When did you start noticing this?

Do you have any other SRE with the same behavior?

Jorge

[Tammy Cox]

Jorge,

I have now looked at all 97 SRE modules in production. Router platforms are ISR G2 2911 & 2951. The 2911's have the SRE-700 and the 2951's have a SRE-900 or SRE-910.

Here is what I've found, which is irrespective to any SRE TFO overloads and irrespective to any router overruns.
1) ALL the routers that have the SRE modules send the Pause Output to the SRE modules (SM1/0 or SM2/0), indicating that they cannot handle volume of traffic
2) ALL of SRE modules have the Pause Frame Received: 0

What am I expecting to see?
The SRE module is receiving the Pause request and obeying the request.
The router interface buffer is able to handle the full 1Gig throughput.
 
Questions:
1) Is the SRE really receiving the pause request from the router and this is just a reporting bug?
2) Does the SRE in fact slow down upon receipt of the pause frame?
3) Is the SRE not receiving the pause request and therefore not slowing down?

[Jorge Bejarano]

Tammy, this is what I found:

Basically the interface is sending "pause frames" to the SM to tell it to slow down. This is
normal for a device / interface to send a "tx-pause-frame" to the sender telling it to slow down

You should see "pause frames" increasing due to an overload condition, high cpu, etc...

Proactively, you may open a Cisco TAC case but details above could be the answers they could give you.

Hope this helps!

Jorge

[Tammy Cox]

Jorge,

I will open a TAC case on this given that this is a systemic symptom in every ISR G2 router deployed with SRE module running the WAAS software.  I expect to see that the SRE module is receiving the pause frame from the router and that it will obey the request, yet the SRE module reports that is has not received a pause frame from the router.

Questions:
1) Is the SRE really receiving the pause request from the router and this is just a reporting bug?
2) Does the SRE in fact slow down upon receipt of the pause frame?
3) Is the SRE not receiving the pause request and therefore not slowing down?