VSS and QoS on Cat6500 SUP2T


I really don’t like QoS on Cisco Switches, its too complicated and totally depends on the chassis or even on the line card in chassis based switches. What made things worse is how QoS and VSS is implemented on the Cat6500 (and documented). I recently had to create a QoS design for a customer which had VSS on Cat6500 with SUP2T and 16-port 10GE line cards. The VSL Links were on the SUP using the two 10GE ports and I had to configure QoS towards the WAN on two out of the three 1GE ports on the SUP.

But it does not work that way, if you configure a VSL port, the system automatically actives QoS for that link and deactivates all QoS commands (and I think some others too) on all ports that share the same ASIC with the VSL port.

VSS Documentation

The VSS automatically configures VSL ports for trust CoS, using default CoS mappings (you cannot change the mappings on VSL ports). For switching modules that support per-ASIC configuration, the VSL configuration applies to all ports on the same ASIC (including any non-VSL ports). The VSS disables the QoS commands on VSL ports (and any non-VSL ports on the same ASIC). For example, you cannot use QoS queuing or map commands on VSL ports. To ensure that all eight QoS receive queues are enabled for the 10-Gigabit Ethernet ports on the supervisor engine, enter the platform qos 10g-only global configuration command.

In Cisco IOS Release 12.2(50)SY and later releases, when the platform qos 10g-only command is entered and only one of the two 10-Gigabit Ethernet ports on the supervisor engine is a VSL port, the non-VSL 10-Gigabit Ethernet port can be configured for QoS.

So if you have a look at the SUP2T architecture, you’ll pretty sure see the problem:

SUP2T architecture

Thanks to Cisco Live for the picture 🙂

So we had to move the VSL links from the SUP to the line cards (the router doesn’t have a 10GE int and the 10GE line cards did not support 1GE). And there is where the next “nice” thing comes into the game. The 10GE line card (WS-X6816-10T-2T) does support four VSL links in performance mode. In other words the line card is 4:1 oversubscribed and if you want to configure one or more VSL links the corresponding port groups (four port groups with four ports) have to be configured for performance mode (or deactivate the oversubscription) which means that only one out of those four ports will be active, the three others will be deactivated:

Cisco Catalyst 6500 Series 16-Port 10 Gigabit Ethernet Module

I love it 😉

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