QOS on a Cat 65K

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Jun 26th, 2008
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No sure if this is the correct forum but we are prepping a network for Voice and making sure all QOS requriements are set correctly setup.

We have a network of CAT65K with SUP720. We will be enabling MLS QOS on the switch which will set all ports to untrusted (unless specifically configured for trust) and reset the cos to 0. We will be applyinga specific policy map to normalise all incoming traffic and rely on default egress queuing on various blades fro egress traffic.

My question is - we have OSPF and BGP packets flowing through the switches. These will be marked with the TOS field for DCSP and I wanted to know if we need ed to reclassify these packets.

My concern is based around the fact does the switch port remap all cos and dscp mappings to 0 on an untrusted port including the internetworking and networking protocols.

Let me know your thoughts.

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n.nandrekar Fri, 06/27/2008 - 02:11
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the following link will give details on the way qos works with control packets :


The packets are originated be the router and it ises a internal header with a field called PAK_PRIORITY which is set to high among RIP,OSPF EIGRP packets. This tells the router to give higher priority during queueing.



(pls rate if helpful)

saimbt Fri, 06/27/2008 - 04:53
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Interior gateway protocols usually adequately are protected with the Cisco IOS internal PAK_PRIORITY mechanism. Exterior gateway protocols, such as BGP, are recommended to have an explicit class for IP routing with a minimal bandwidth guarantee.

Joseph W. Doherty Fri, 06/27/2008 - 04:12
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Yes, if you have any frames/packets that flow through devices with CoS/ToS markings that you wish to preserve and/or use, then you need to be careful of just blindly remapping all inbound tag frames/packets unless you completely analyze and retag outbound. The reason for having tags at either L2 or L3 is to avoid the need for each hop to fully analyze the traffic. I.e. ideally, you normally do analysis at the edge, and trust markings from that point onward. Of course, this doesn't preclude remarking if necessary.


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