Output queue drops on uplink ports of SUP720-3B

[Azannah]

I have an 8 member port channel between two Catalyst 6513s. Two of the members are on the active supervisor and two more are on the passive supervisor. The supervisors are SUP720-3Bs. The remaining 4 members are on another switching module.

Whenever I start up a test flow from host A to host B that gets assigned to one of the SUP720-3B uplink member ports, I'm experiencing packet loss. When the flow lands on one of the non-SUP720-3B members ports, no packet loss is experienced. The test flow is comprised of UDP packets using iperf.

During my test intervals, none of the links are being saturated (as far as I can tell). The link experiencing drops is receiving 23 Mbps and trasmitting 28 Mbps, which can almost entirely be attributed to my test flow.

When I start a flow with the same parameters from B to A, it runs without drops.

The configuration is fairly simple. Both sides of the port channel look like...:

interface Port-channel1
description ***
switchport
switchport trunk encapsulation isl
switchport mode trunk
no ip address

interface GigabitEthernet***
description Interconnect Path
switchport
switchport trunk encapsulation isl
switchport mode trunk
no ip address
load-interval 30
channel-group 1 mode desirable

Any guidance determining why the packets are being dropped is greatly appreciated.

Thanks!

[Joseph W. Doherty · Accepted solution]

Interface ports on different cards, including the sups, often have different underlying hardware resources and capabilities.

I recall (?) a sup720s ports aren't as good as some later line card ports.

[Azannah]

Not exactly what I was looking for, but probably accurate. I've read similar statements elsewhere online. I guess my solution will be to simply remove the SUP ports from the EtherChannel and work on replacing the 6513's all together.

[Joseph W. Doherty]

The 6515 and its sup are, I believe, end-of-life, so if you can replace all, that would be the best solution, but possibly replacing or adding even a single line card might be an interim solution.