Unanswered Question
Feb 9th, 2009

I have an esoteric question -

here's my setup:


the 2940 is just being used as a fancy media converter, fiber in, copper out, all the packets wizzing through on vlan 2.

you can check arps, rarps, macs, the 2940 is totally ignorant of the 6509 and 2652ospf routings.

could the 2940 STILL be used to QOS the packets wizzing through it between the two routers?

I have this problem too.
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bmcginn Mon, 02/09/2009 - 19:49

Yep, indeed it could.

The 2940 Data Sheet ought to help you in that.


As it stands, it seems you are using only vlan 2? I assume that means your switchports that are connecting to your routers are access-ports only? And my assumption is that your routers are marking the traffic etc? Do you tag the packets before they leave the router??

I don't think you can utilise the qos capabilities of the switch if you don't tag the packets. The Ethernet frame header has the destination mac address, source mac address, ethertype and CRC field (excluding the payload) and there isn't any place for QoS bits.

If you trunk the connections or at least vlan tag the packets from the routers to the switch, then the 802.1Q vlan tag has the priority bits (3 of them) that can be used for COS.

Also, the cisco switches don't generally trust the QoS markings on received packets, so you would have to configure the switch to trust the markings.

Have a squiz on cisco.com for QoS on switches. you can probably run MQC (modular qos for the cli) and have service policies etc that can be attached to the vlan on the switch, however you can simply use the 4 queues in the hardware of the switch for layer 2 QoS (utilising the CoS markings from the 802.1q header).

I've rabbited on enough. Here's a good link for QoS on the 2940s


I hope that helps you get what you're after!



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