12-12-2005 07:47 AM - edited 03-03-2019 01:06 AM
We have a few ports that are showing about 200,000 overruns over a four day period. The ports are Gigabit ports connected to servers that have been having performance issues. Should we be concerned about the overruns, and does anyone know what might be causing them?
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12-12-2005 08:43 AM
This card shares a 1Mb buffer between groups ports (1-8, 9-16, 17-24, 25-32, 33-40, 41-48) since each block of eight ports is 8:1 oversubscribed. If any port in this range is receiving or transmitting traffic at a rate that exceeds its bandwidth or utilizing a large amount of buffers to handle bursts of traffic, the other ports in the same range of 8 may experience packet loss.
There are a couple of ways to address this.
1)Isolate any ports that may be consistently oversubscribed (SPAN destinations, servers with NFS, slower speed than the other ports in the range, etc.) to their own range of 8 ports to minimize the impact of drops to other interfaces.
2)Disable head of line blocking which will utilize the interface buffers instead of the shared buffers. This will result in only the single over utilized port having drops, but since the interface buffers (32k) are significantly smaller than the 1Mb shared buffer, there may be more lost traffic to individual ports. This is only recommended for extreme cases where slower clients or span ports cannot be moved to other line cards that offer dedicated interface buffers
6500(config)#service internal
6500(config)#interface gigabit 1/1
6500(config-if)#hol-blocking disable
%HOL Blocking is Disabled on: Gi1/1 Gi1/2 Gi1/3 Gi1/4 Gi1/5 Gi1/6 Gi1/7 Gi1/8
6500#show hol-blocking module 1
Interface Hol-Blocking
-------------- ---------------
Gi1/1 Disable
Gi1/2 Disable
Gi1/3 Disable
Gi1/4 Disable
Gi1/5 Disable
Gi1/6 Disable
Gi1/7 Disable
Gi1/8 Disable
Once this is disabled, the drops will move to the interface counters and can be seen with 'show interface gigabit
6500#show interface gigabit 1/1
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 25542147 <-----------
Additionally, this document may provide some more insight.
Buffers, Queues & Thresholds on Catalyst 6500 Ethernet Modules
http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a0080131086.shtml
12-12-2005 08:01 AM
Cause for overruns
http://www.cisco.com/warp/public/473/53.shtml#overrun
What kind of line card and software are you running? Can you paste the output of sh mod from the switch?
12-12-2005 08:14 AM
The software is Version 12.2(17d)SXB6.
Sh mod below:
Mod Ports Card Type Model Serial No.
--- ----- -------------------------------------- ------------------ -----------
5 48 SFM-capable 48 port 10/100/1000mb RJ45 WS-X6548-GE-TX SAD074304WC
Mod MAC addresses Hw Fw Sw Status
--- ---------------------------------- ------ ------------ ------------ -------
5 000d.edb4.e790 to 000d.edb4.e7bf 6.0 7.2(1) 8.3(0.110)TE Ok
Mod Online Diag Status
--- -------------------
5 Pass
12-12-2005 08:03 AM
Overruns are the number of times the receiver hardware was unable to hand received data to a hardware buffer.
This is generally caused by the input rate of traffic exceeding the receiver's ability to handle the data.
What type of module is this? More than likely this is a blocking card who is receiving more data than can be put into the shared buffer. Depending on the type of card, we may be able to mitigate this by spreading the top talkers among different port groups.
12-12-2005 08:23 AM
The module is a WS-X6548-GE-TX.
12-12-2005 08:43 AM
This card shares a 1Mb buffer between groups ports (1-8, 9-16, 17-24, 25-32, 33-40, 41-48) since each block of eight ports is 8:1 oversubscribed. If any port in this range is receiving or transmitting traffic at a rate that exceeds its bandwidth or utilizing a large amount of buffers to handle bursts of traffic, the other ports in the same range of 8 may experience packet loss.
There are a couple of ways to address this.
1)Isolate any ports that may be consistently oversubscribed (SPAN destinations, servers with NFS, slower speed than the other ports in the range, etc.) to their own range of 8 ports to minimize the impact of drops to other interfaces.
2)Disable head of line blocking which will utilize the interface buffers instead of the shared buffers. This will result in only the single over utilized port having drops, but since the interface buffers (32k) are significantly smaller than the 1Mb shared buffer, there may be more lost traffic to individual ports. This is only recommended for extreme cases where slower clients or span ports cannot be moved to other line cards that offer dedicated interface buffers
6500(config)#service internal
6500(config)#interface gigabit 1/1
6500(config-if)#hol-blocking disable
%HOL Blocking is Disabled on: Gi1/1 Gi1/2 Gi1/3 Gi1/4 Gi1/5 Gi1/6 Gi1/7 Gi1/8
6500#show hol-blocking module 1
Interface Hol-Blocking
-------------- ---------------
Gi1/1 Disable
Gi1/2 Disable
Gi1/3 Disable
Gi1/4 Disable
Gi1/5 Disable
Gi1/6 Disable
Gi1/7 Disable
Gi1/8 Disable
Once this is disabled, the drops will move to the interface counters and can be seen with 'show interface gigabit
6500#show interface gigabit 1/1
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 25542147 <-----------
Additionally, this document may provide some more insight.
Buffers, Queues & Thresholds on Catalyst 6500 Ethernet Modules
http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a0080131086.shtml
12-12-2005 10:07 AM
Thanks, this should help.
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