DSCP Tagging Packets out of a Cisco 4506 Sup IV

Posted on 2010-01-08
Last Modified: 2012-05-08
These should be some easy points. ;-)

I dont have access to a 4500 Sup IV to test but I want to verify that we can tag DSCP outgoing on a physical interface or a VLAN.

We need to tag all outgoing packets with a certain DSCP value.

Im thinking something like.

policy-map tagdscp
class class-default
set ip dscp 45

int fa3/1
service policy output tagdscp

int vlan 20
service policy output tagdscp

I just want to make sure the 4500 Sup IV will tag all of the traffic.  Ive tested on an ISR and a 3550 (worked fine on the ISR doesnt seen to be working on the 3550 (I know you cant do vlans on the 3550 but it doesnt seem to work on the physical interface either) but Im not in a position where I can take traces to verify).
Question by:th3w01f
    LVL 23

    Accepted Solution

    yep the 4500 series supports DSCP so you will be fine. The commands you are using look good. With the 3550 you might be having issues because qos is not enabled. Use "mls qos" to enable. For the 4506 i believe you only need to enter "qos" to enable.
    "From a QoS perspective, some of the relevant features of the C4500/4948 include conditional trust, an elegant Per-Port/Per-VLAN policer implementation, DSCP-to-Queue mapping, 4Q1T or 1P3Q1T queuing support. and an advanced congestion algorithm (Dynamic Buffer Limiting or DBL). "
    LVL 6

    Expert Comment

    Here is the QoS Configuration guide reference for the 3500.

    The entire 3550 Configration Guide is here:

    On the 4500, there are performance issues on many of the ports.  All of the older line cards with lots of GigE ports for the Sup IV are 8:1 oversubscribed, meaning that regardless of how the QoS is configured, there is a limit of 1Gbps per group of 8 GE ports.  This traffic will silently drop and not show up except as errors and port overruns.

    The 10/100 ports do not support flow control and MUST have DBL enabled for things to work properly and DBL was seriously broken in software versions around 12.1(25).  Be sure your software is more recent.  <1 year old...

    They are very good switches and should handle this well, but you need to avoid several of the various pitfalls as on any complex system...

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