QoS on a Cisco 3845 Router - How ??

Unanswered Question
Jun 5th, 2007

Hi All,

I have a Cisco 3845 router running "c3845-advipservicesk9-mz.124-3a" code.

Off the Cisco product pages, nothing is mentioned about this router running QoS however on the router, I can set mls qos map and that's about it.

I can't work out how to configure any "trust" commands, I can't do this from an interface nor within a policy-map, yet I can configure class-maps and apply policy-maps to interfaces.

So here are my questions:

1. Is QoS fully supported on a Cisco 3845?

2. How do you configure the equivalent of "mls qos trust dscp" on an interface.

Thanks All

I have this problem too.
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Hieu Cao Tue, 06/05/2007 - 08:35

That "trust" commands that you refer to only works in the switch, not in the router. For QoS to work correctly in the router, you'll need to setup class-map, policy-map, then apply the policy-map (outbound) to the serial interface.

Your router is currently running "advanced" code, so all QoS services for the router should be there.


Andrew Morris Tue, 06/05/2007 - 08:59

Hi There,

Thanks for that. I've done just that, set up the class-maps, policy-maps etc... and applied these outbound to a frame-relay subinterface which necessitated setting up a frame-relay map-class and incorporating the policy-map within there.

So, "trust" cannot be defined on the 3845 routers then. So be it :)


Andrew Morris

aciscolook Thu, 07/17/2008 - 23:44

i have a router that has one interface on the LAN and another going to the WAN. I simply want the LAN connected interface to 'trust' dscp values coming into the interface that have already been marked by switches in the LAN, and have those dscp values used by the service policy applied to the WAN connected interface.

Does the LAN connected interface need to be configured to 'trust' dscp values? I cannot apply the 'mls qos trust dscp' command to the router interface. Neither can I apply the 'mls qos' command globally to the router.

I'm assuming a router is a bit different from the switch, but could elaborate on this matter conceptually.


Joseph W. Doherty Fri, 07/18/2008 - 03:11

Cisco software routers (e.g. 28xx, 38xx, 72xx), usually don't modify the L3 ToS unless you configure them to do so. Whether they'll provide different service levels also usually depends on configuration except for WFQ (default on low speed WAN interface, e.g. T1/E1 or slower) which will proportion bandwidth to flows differently based on IP Precedence.


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