Matching STP in a Class Map

[Grayson Wells]

I am trying to match spanning-tree traffic in a QoS class map so I can apply quality of service to it. I have tried using the match destination address mac 0182.c200.000 command but that isn't accepted on my 3750 and 3560 switches, not sure if that is an IOS version issue or not. The mac address is the multicast destination mac for all STP traffic. I have also tried creating a mac access list to use to match traffic, but it looks like the match statement will only take a numbered IP access list. Does anyone have any ideas how I can match the STP traffic?

[cadet alain · Accepted solution]

You don't have to because it is control plane traffic and it is automatically given highest QoS values.

[Grayson Wells]

Interesting, I have never heard that before. Thanks!

[Jon Marshall]

cadetalain wrote:
You don't have to because it is control plane traffic and it is automatically given highest QoS values.

Can you explain a bit more about this.

I am aware of routing protocols being given a higher IP Precedence but i'm not sure how this works with BPDUs. BPDUs would have to use a CoS value as they are L2 only. So if you had a switch connected to another switch but not with a trunk, just a regular access port connection how would BPDU's be marked with a CoS value as without an 802.1q tag there can be no 802.1p (CoS) markings ?

Jon

[Grayson Wells]

I am new to QoS so I am not positive. However, it seems logical to me to assume that even on that access port you could apply QoS to match traffic and then prioritize based on IP precedence or DSCP. I am not sure but I think the only reason why you would need the dot1p markings is if you are trusting the CoS values passed by the trunk. Since you aren't trusting anything you just analyze the traffic, match it, and apply whatever priority you want.

My only thing is that I have never heard of BPDU's automatically being given the higher DSCP value or ip precedence tag. If that is the case then I don't need to worry about finding the appropriate match statement.

[Jon Marshall]

My only thing is that I have never heard of BPDU's automatically being given the higher DSCP value or ip precedence tag. If that is the case then I don't need to worry about finding the appropriate match statement.

Well they can't be given a DSCP or IP Prec tag because BPDUs don't have an IP header, they are L2 packets only. So they would have to use CoS markings. And the only way to mark a packet with a CoS value is if that packet has an 802.1q tag.

I'm not entirely sure what Cadetalain means which is why i asked for clarification. Some routing protocols are specifically given a higher precedence but with BPDUs as i said you can only use CoS and even then only on a trunk link. With an access port link connecting 2 switches there is no CoS.

So i'm wondering whether he meant the switch itself internally prioritizes BDPUs but the way it was written suggest a QOS/CoS value is written into the BPDU and i don't think it is. I'm not saying he is wrong, just wanted clarification really.

Jon

[Grayson Wells]

Oh yeah you are right about the IP header thing, I wasn't thinking. He mentioned something about the control-plane, but like I said, I am new to QoS so we will have to wait for him to clarify.

[azi123]

Old but gold discussion ( at least for me!). And still, I don't know how BPDU packets, like STP packet, are prioritized without CoS tag. Hope someone knows the answer!

[Mark Malone]

Control Plane Protection

Catalyst 3750-X and 3560-X switches internally support up to 16 different control plane queues. Each queue is dedicated to handling specific protocol packets and is assigned a priority level. For example, STP, routed, and logged packets are sent to three different control plane queues, which are prioritized in corresponding order, with STP having the highest priority. Each queue is allocated a certain amount of processing time based on its priority. The processing-time ratio between low-level functions and high-level functions is allocated as 1-to-2. Therefore, the control plane logic dynamically adjusts the CPU utilization to handle high-level management functions as well as punted traffic (up to the maximum CPU processing capacity). Basic control plane functions, such as the CLI, are not overwhelmed by functions such logging or forwarding of packets.

[azi123]

As a result, there is no standard field in ethernet frame for distinguishing control packets, ya? So, in this way, each vendor has its own way to handle control packets...this is ok? Don't standard protocols like STP need a through and equal behavior from different switches, so that different switches from different vendors be compatible with each other in standard protocols?

[Joseph W. Doherty]

Easy, if you understand CoS/ToS tags are just a "shortcut" to preclude deeper analysis of the frame/packet for QoS. I.e. they aren't needed for QoS processing but they do make it more efficient.

For a device generating the BPDU, it can drop the frame into an "egress" queue that has priority over others. For example, see "pak_priority" in https://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-congestion-management-queueing/18664-rtgupdates.html#intro.