How are the exp bits used in a 6500

[midoherty]

I am marking packets with dscp on the edge of the network but after the packets have been through the mpls core (6500's with sup720's) only the equivalent ToS value is in the header.

Anyone know of a way to preserve the full DSCP value?

[Harold Ritter]

The MPLS core might be using the uniform mode, therefore changing the value of your original DSCP.

For more information on the different modes defined by MPLS DiffServ Tunneling Modes, refer to the following URL:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t13/ftdtmode.htm

Hope this helps,

[midoherty]

Thanks for that reference it is a very useful document, however I think I may be restricted by this bug.

'With a PFC2 or a PFC3, you can either set DSCP in a packet or apply an MPLS tag to the packet, but cannot do both. You cannot set DSCP in a packet and then apply an MPLS tag to that packet. (CSCef19599)'

For more info refer to http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/122sx/ol_4164.htm

My problem is that the PE device is a 6500 and that is also where I am trying to setting the DSCP values. If I look at the service-policy applied to the ingress interface it appears to be matching and marking the packets. When I look at the PE device on the other side of the network where I have an egress policy no packets appear to be marked.

[Harold Ritter]

This DDTS refers the following workaround:

A workaround would be to configure a policy-route and set the values in the route-map.

I think it would be worth trying.

[fcotichelli]

Hi,

a simple question: does the bug CSCef19599 means that a 7600 (with Sup7203BXL-IOS 12.2(18)SXD or SXD3) cannot be an Ingress E-LSR marking incoming IP packets' DSCP Field ?

And also, did anyone successfully tested the workaround Harold suggested (the policy-route marking the entire dscp field) - with no performance degradation I hope) ?

Thanx in advance

Francesco

[midoherty]

Francesco,

I am running 12.2(17d)SXB1 so I don't know about that later version.

I haven't had a chance to test the policy-route work around as I don't have a spare 6500 with Sup720 to try it on.

Michael

[fcotichelli]

Michael,

I hope to test that workaround (and possibly its impact ) in thr next future.

Anyway with 720-3bxl and 12.2(18)sdx (no osm) a policy map (matching any and marking ip-dscp to, say, cs1) desn't work if traffic enters the 7600 as IP and leaves as mpls (regardless of the tag stack depth)!

When I opened a case on that I've been suggested to put a 2950 before entering the 7600 to do the marking action....

I'll let you know the workaround as soon as i'll test it.

Francesco

[fcotichelli]

Hi,

i could test the proposed workaround (Cisco7609, IOS version is 12.2(18)SXD3, Sup720-3BXL, NO OSM):

***********************************

ip access-list extended test

permit tcp any any

Route-map test permit 10

Match ip address test

set ip precedence critical

interface fast 3/24

ip vrf forwarding Test-Vrf

ip address x.x.x.x y.y.y.y

ip policy route-map test

*************************************

Traffic is entering the interface fast 3/24

as IP packet and leaving this PE as MPLS Packet (L3VPN: 2 labels): tcp traffic gets marked

at IpPrec 5 whilst all other traffic is left to its default.

Just to let you know, the only fields you can set are:

Router(config-route-map)#set ip?

ip ipv6

Router(config-route-map)#set ip ?

address Specify IP address

default Set default information

df Set DF bit

next-hop Next hop address

precedence Set precedence field

qos-group Set QOS Group ID

tos Set type of service field

Router(config-route-map)#

No load-tests have been made so no idea on the performance side (any news about this?).

Hope it helps

bye

francesco

[midoherty]

Francesco,

I have a 6500 with Sup720-3B in the lab for a few days. I have been able to confirm your results.

I also tried using a policy map to set the mpls exp bits, but this appears to not work either.

In trying to validate my results I found that the mpls accounting experimental does not work on the 6500 but it does work on other routers I am testing with.

If dscp is set prior to entering the 6500 this is preserved and the mpls exp bits are set based on this.

My problem now moves to the PE device where we have GRE/IPSEC tunnels to the CE routers. These are the actual slow wan links where QoS is going to matter. The mpls exp bits are copied to the GRE/IPSEC packet and the underlying ip packet retains its dscp value. But I still haven't been able to set a dscp value in the GRE/IPSEC packet.

testing continues......

[fcotichelli]

Hi

thanx for your reports, really interesting !

If I ever could test anything similar I'll let you know for sure!

Hope to hear from you soon

Cheers

Francesco