Default Layer 3 markings on routers?

[sandevsingh]

Hi, what is the default behavior of routers for Qos Marking, i.e by default do they use IPP or DSCP to mark the traffic?

[Jon Marshall]

Not sure what you are asking.

Routers are not like Catalyst switches were you have to enable QOS with the "mls qos" command and then a set of defaults are applied.

With routers you simply configure an MQC policy, within which you would define whether you were using DSCP or IPP markings so it is up to you.

Routers do however mark some of their own generated traffic such as routing updates for certain protocols with IPP markings but you would still need additional config to honour those markings.

Jon

[sandevsingh]

Thnx Jon, i agree on what you are saying. My question here is whether the router is going to look at the first 3 bits or 6 in the TOS field by default when it recieves an IP packet. Let`s not bother about the MQC now.

Sandev

[Jon Marshall]

Sandev

If you haven't configured QOS then the router isn't really going to look at any markings when a packet arrives, it will simply treat it the same as any other packet.

As far as i know a router will only start to use QOS when youh ave configured a QOS policy ie. there is no default action.

Perhaps i am misunderstanding your question ?

Jon

[sandevsingh]

Jon, all packets that arrive in a router have the TOS byte (8-bit field in the IP header). It does not matter if you configure a service policy or not on an interface, the router ALWAYS sees this filed and takes action as per the IPP or DSCP values. (For example ping/icmp has the least preference hence a low IPP value, whereas voice has dscp ef or IPP5 that takes stronger preference during congestion situations).

By doing mqc you can do customized QoS according to your requirements, like re-marking, queuing, policing, shaping etc... The help in congestion avoidance and management.

[cadet alain]

Hi,

I think Jon knows what is a QoS policy but as he said a router by default so without any policy applied will not consider the ToS bytes setting.

Regards.

Alain.

[Jon Marshall]

the router ALWAYS sees this filed and takes action as per the IPP or DSCP values. (For example ping/icmp has the least preference hence a low IPP value, whereas voice has dscp ef or IPP5 that takes stronger preference during congestion situations).

But that's the point i am making. 2 packets can arrive at the router one with DSCP AF12 and one with DSCP EF. If you haven't configured anything on the router in terms of what to do with these different markings nothing happens ie. they are treated the same.

In addition your original question was about marking traffic on a router and not about what happens to packets received by the router that are already marked. My answer was simply if you don't configure a policy to mark packets arriving at the router then nothing happens in terms of marking.

Jon

[Joseph W. Doherty]

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Posting

Jon is correct that routers, by default, do not mark or remark transit packets.  Jon is also correct, that a router may mark some of its locally sourced packets, such as routing protocol packets.  I believe, all such markings only set IP Prec.

When it comes to different treatment for transit or locally sourced packets, most often all packets are treated alike without explict configuration but there are a couple of exceptions.

For transit packets, one router behavior that actually does something differently based on ToS, by "default", is on an E1 or slower interfaces using WFQ, where WFQ uses IP Prec in its weight computation.  The other instance where a router might treat packets differently based on ToS, by "default", is if it supports SPD on an interface's input queue.  SPD, I believe, only "uses" IP Prec.

For some locally sourced packets, routers can treat them, by "default", differently using an internally assigned pak_priority.  For more information, see: http://www.cisco.com/en/US/tech/tk543/tk544/technologies_tech_note09186a0080094612.shtml