Doubts about prioririty CoS and DSCP

[ciscolover]

Hi all, I have some doubts about priority...

For example I have 802.1p in a tagged port. Ok, wen a tagged traffic enter on this interface the priority for this tagged VLAN is for example CoS 6... If the paquet traverses the network without routing and without lose the tag(between trunk ports), the priority for this tagged paquet not changes...

¿But, if the paquet needs routing and lose the tag, how can I priorize it? Maybe a CoS DSCP mapping? How can I do this?

Best regards, sorry I'm not english...

[William Benson]

Once the packet leaves your network (is routed) you no longer have the ability to make sure that packet is tagged as a priority packet until it reaches its destination.

An exception to this rule would be if you are routing over a private network, like an MPLS.  Inside of your MPLS cloud you can continue to prioritize the packet until it reaches its destination with DSCP tags.  The trick here is you have to know what DSCP value to tag the traffic with.  Your MPLS provider should be able to give you DSCP values for Realtime, Mission Critical, Business and Best Effort.

You'll need to coordinate with your ISP/Network Provider, on the DSCP values they use and tag your traffic appropriately.

If your traffic is being routed to the public internet there is no way for you to tell routers outside your network that your traffic is important and needs to be pushed to the front of the line, if you could, everyone would say their traffic is the most important and that their traffic needs to be first.

[ciscolover]

Thanks William ¡¡¡¡¡

Really this doubt is for my campus network... I have one VLAN that needs be routed , in some cases, with another Vlan/s... Really is a critical traffic, when it traverses my campus network without routing I don't have any problem because the paquet is tagged and the CoS priority remains... But in another cases this traffic needs to be routed and I want to priorize it using DSCP....

This traffic is allways in my LAN/MAN network and it not traverses any WAN network...

[William Benson]

As long as you're routing across your own network then just setup your QoS policies, prioritize based on DSCP values, tag traffics as needed, and then make sure all of your QoS policies are mirrored across all your devices.  As long as you tag your traffic with the DSCP values needed when you're routing it, it will continue to have priority as it moves from one router to the next.

I have offices all over the Gulf Coast here.  We have complex internal networks that route out multiple different MPLS networks from different providers.  Each provider has their own set of DSCP values for prioritizing traffic.  I have tag my traffic with DSCP values, not only by application type, but also by destination via route-maps.  It was a beast to get setup and coordinated with my ISPs over our MPLS, but the end users definitely appreciated the final results.

Mirroring your QoS policies to every router on your network based on DSCP values will be the hard part.  Once that's setup though all you should need to do is tag your traffic before you route it, and the priority/QoS should follow that packet all the way through your network, even after it's routed.

[William Benson · Accepted solution]

Tagging the traffic you want with DSCP values is relatively simple.  You'll need to build a class-map to match your COS, then build a policy, include your class, and set DSCP values there, then apply the policy to the outbound side of your outside interface on the router.  For example this is on my router with outside int Serial0/0/0:1

class-map match-any ClassNameHere

    match cos (your CoS value here, you can also match by ACL, protocol etc., ? for more info)

policy-map PolicyName

    class ClassNameHere (class-map from above)

    set DSCP (your value here)

int Serial0/0/0:1

service-policy output PolicyName (policy name from above)

Now all traffic egressing out the outside interface of your router that matches the CoS you're using on the inside side of the router will be tagged with the DSCP values you need to route across your campus.