High CPU on 6509 when I telnet to switch

Unanswered Question
Aug 25th, 2010

I saw a similar post about high CPU on a 2960. They said the CPU is high until they telnet then it drops back to normal, well our issue is opposite.

When we telnet, Console works fine, to the switch the CPU goes right up to 100% and it stays that way even when we log out of the telnet session. What I have to do is do a show tcp brief and find the tcb number for the telnet session and run the clear tcp tcb tcb number and then the CPU goes back to normal. Has anyone seen of heard of this issue? This just happened out of the blue, no major config changes. Someone was telneted in the switch not making changes and all was good and they logged out and not too long after someone else logged in and we were alerted that the CPU was 100%, and it has been acting that way ever since.

They are two 6509's VSS running image is


Now I know we should update and maybe this would go away but this is our main switch and down time would be very difficult for what maybe considered just an annoyance and not impacting users. Just looking to see if anyone else has seen or heard of this and see if it is fixable without downtime. We are going to try and get some downtime to update the image but that won't be for a few months or so.



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Overall Rating: 5 (3 ratings)
Edison Ortiz Wed, 08/25/2010 - 06:32

Internally we've seen customers reporting a similar issue and the resolution has been moving away from the modular code or upgrading to SXH3a which is more stable than SXH3.

It seems the telnet process isn't released after logged off and it hangs hence creating the high CPU state.



gringley Thu, 09/16/2010 - 09:50

We challenged Cisco over the apparent lack of support for Modular code this month, as we could not easily take a hard reboot of the VSS in question to convert the Native.  The response we received was that Modular was slated for full support through October 2012, when it would be replaced by 'IOS XE' which would include software modularity.  From what I can see in SXI4a, there is no longer a feature parity problem between Modular and Native.  We are experiencing VLAN creation bugs in SXI3 and some other issues and thus are moving on to SXI4a.

Reza Sharifi Wed, 08/25/2010 - 08:58


According to Cisco 6500 business unit, Cisco is completely going away from the modular code for 6500 by the end of next year. I know you are using SXH, but with the release of SXJ (some times next year) there is not going to be a modular code to download.  Also, if you do a comparison between SXH and SXI using the feature navigator, you see that SXI has a lot more features than SXH.  Also, most of the VSS features are bring deployed into SXI.

Since you are running VSS, I think moving to SXI may be a good idea!



burleyman Wed, 08/25/2010 - 10:15

Thanks Edison and Reza for your posts. We are going away from the modular code and going to the latest but probably not for a few months. I was hoping that someone might have said "Oh yeah we had that happen and we ran this command and it went away..." wishful thinking I know but hey you never know.

Thanks for you help.


Leo Laohoo Wed, 08/25/2010 - 15:53

going to the latest but probably not for a few months

Some of user were rooting for the latest SXI4a but someone from Cisco told us to stay away from this version.

"we are recommending that you go with SXI3 as the IOS version. Although SXI4 has passed safe harbour, it has not had enough field exposure to capture many issues. Also there are around 5 NOS customers in the world running SXI4 and mostly on development or test boxes"

I don't share the same sentiments with the last sentence because if 5 (or more ) NOS customers are told to avoid SXI4 or SXI4a by Cisco you wouldn't risk your neck would you now?

Unfortunately because this we have no choice but to follow the recommendations.

burleyman Thu, 08/26/2010 - 05:08


Thanks for the info. I believe that is the route we are going. We will be consulting with Cisco when we do this upgrade, too much a stake not to.


Leo Laohoo Thu, 08/26/2010 - 15:19

Thanks for the ratings Mike.  Let us know if Cisco's singing the same song.  I'm interested to know what opinions they give to the SXI4/SXI4a.


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