6500 line card and 'Head of line' blocking.

Unanswered Question
Feb 5th, 2007

I think we are over subscribing the ports on our WS-X6548-GE-TX= (Catalyst 6500 48-port fabric-enabled 10/100/1000 Module) line cards in our 6509. This 6509 is the access layer for our servers which are being migrated from 100MB connections to gig connections.

We've been experiencing odd, random slow throughput problems with many of the servers and have uncovered the fact that these line cards have an 8:1 over-subscription rate to the switch fabric.

This has been confusing to troubleshoot because the interface counters don't show dropped packets. However in the document http://cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801751d7.shtml#ASIC, under the heading 'Connectivity Problem or Packet Loss with WS-X6548-GE-TX and WS-X6148-GE-TX Modules used in a Server Farm', it talks about disabling HOL, Head of Line blocking, and if there are any drops, they will move to the interface counters so we can find the culprits that are hogging the shared path to the fabric.

My question is, if we disable HOL, will this cause any interruption of traffic to the servers that is connected to that port? Or is this transparent to any traffic that is occurring?

I have this problem too.
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dsweeny Fri, 02/09/2007 - 10:19

Disable head of line blocking (HOL) which utilizes the interface buffers instead of the shared buffers.

This results in only the single over utilized port having drops. Since the interface buffers (32 k) are significantly smaller than the 1 Mb shared buffer, there can potentially be more packet loss on the individual ports. This is only recommended for extreme cases where slower clients or SPAN ports cannot be moved to the other line cards that offer dedicated interface buffers.

*NativeIOS

Router(config)# interface gigabitethernet

Router(config-if)# hol-blocking disable

Once this is disabled, the drops move to the interface counters and can be seen with the show interface gigabit command. The other ports are no longer affected provided that they are also not individually bursting. Since it is recommended to keep HOL blocking enabled, this information can be used to find the device that overruns the buffers on the range of ports and move it to another card or an isolated range on the card so HOL blocking can be re-enabled.

* CatOS

Console> (enable) set port hol-blocking disable

Once this is disabled, the drops move to the interface counters and can be seen with the show mac command. The other ports are no longer affected provided that they are not also individually bursting. Since it is recommended to keep HOL blocking enabled, this information can be used to find the device that overruns the buffers on the range of ports and move it to another card or an isolated range on the card so HOL blocking can be re-enabled.

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Posted February 5, 2007 at 10:27 AM
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