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Beginner

By default, Does the router mark CoS in the Ethernet Frame?

Hi, i have one question to the forum.

I have this topology. SW_LANf0/10 - f0/0-Routerf-0/1 TRUNK - TRUNKf0/5SW_MetroEth -ISP Network

If i´m using a classmap to match and mark some traffic with AF31 from the LAN, to the WAN, and i apply the service policy output in the f0/1 in the router.

By default, Does the router mark CoS in the Ethernet Frame within the 802.1q tag? I think that my frames are going out with CoS = 0 from my router to the ISP Metroethernet Switch.

Thanks a lot!!!!!!

Juan

Everyone's tags (5)
11 REPLIES 11
VIP Advisor

Re: By default, Does the router mark CoS in the Ethernet Frame?

Hello
When you enable mls qos on a switch by default cos/ dscp values are set to 0
Also you will need to trust cos/dscp/ipp on the interface as by default its not trusted.+tagged/untagged interfaces:

mls qos trust cos/dscp respectively

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml


Res
Paul

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Paul

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Enthusiast

Re:By default, Does the router mark CoS in the Ethernet Frame?

You can configure a span port in the switch and capture the traffic to verify the markings. Or verify with show commands on the router traffic is matching the policy.

If you are not setting the QoS markings on the networking equipment then I would make sure your applications are setting their own QoS markings and as mentioned above make sure you trust the ports those markings are coming from.


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Beginner

Re:By default, Does the router mark CoS in the Ethernet Frame?

Thanks both for yours answers.

I´m sure that ocurrs of the switch metroethernet side. With mls qos, the switch rewrite the marks to CoS 0, except that i utilice mls qos trust cos command.

But i need to know, if the router send out DSCPaf31/CoS3 or send out to the metro switch DSCPaf31/CoS0.

Do you know this answer?

VIP Expert

Re: By default, Does the router mark CoS in the Ethernet Frame?

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If the router's interface is a router interface (with .Q subinterfaces), as far as I know, only L3 ToS is changed, not L2 CoS.  (Unsure there's even a way to intentionally do that.)

If you need CoS tagging, it might be possible with a Ethernet switch module in an ISR or you might need a smart switch, in between the router and the MetroSwitch (assuming you can't manage the latter).

If fact, if your LAN switch has available ports and capability needed, you might loop back your router's "TRUNK" link though it.  Also depending on what your LAN switch is, you might just bypass the router.

Hall of Fame Master

Re: By default, Does the router mark CoS in the Ethernet Frame?

Yes, routers can read and set L2 CoS bits. However in practice that is almost never needed.

Beginner

Re: By default, Does the router mark CoS in the Ethernet Frame?

Hi Paolo, and all, thanks for your answers.

I know that the router can read y write the CoS mark in the ethernet frame.

But i need to know, if does the router mark CoS in the Ethernet frame, by default, if the packet is send with any L3 mark.

Imagine

class-map markaf31

match access-group 10

policy-map prueba

class markaf31

  set dscp af31

int f0/0.10

encap dot1q 10

ip add 10.10.10.1 255.255.255.0

service-policy output prueba

With this config,  does the router mark CoS in the Ethernet frame?

VIP Expert

Re: By default, Does the router mark CoS in the Ethernet Frame?

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

But i need to know, if does the router mark CoS in the Ethernet frame, by default, if the packet is send with any L3 mark.

Imagine

class-map markaf31

match access-group 10

policy-map prueba

class markaf31

  set dscp af31

int f0/0.10

encap dot1q 10

ip add 10.10.10.1 255.255.255.0

service-policy output prueba

With this config,  does the router mark CoS in the Ethernet frame?

Again, don't believe so, but try:

policy-map prueba

class markaf31

  set dscp af31

  set cos 3

Hall of Fame Master

By default, Does the router mark CoS in the Ethernet Frame?

Again, do not worry about QoS/Cos and such stuff  on LANs. It is not needed,

Thanks you for the nice rating and good luck!

VIP Expert

Re: By default, Does the router mark CoS in the Ethernet Frame?

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Paolo Bevilacqua wrote:

Again, do not worry about QoS/Cos and such stuff  on LANs. It is not needed,

Although I'll concede QoS on LANs is less often needed on WANs, and will also agree a LAN may more easily or less expensively provide additional bandwidth, occasionally QoS is really needed on LANs too.

Something that is impacting today's LANs, newer TCP stacks often advertise much, much larger RWINs allowing a sender to send larger packet bursts.

When  QoS is really needed, on LANs, often you want to prioritize what gets dropped first other than the more usual (WAN) what gets sent first.

VIP Expert

Re: By default, Does the router mark CoS in the Ethernet Frame?

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

I just checked with a 2811, and Paolo is spot on (not that I doubted him, but a CBWFQ feature I don't believe I've ever used).

CBWFQ (at least under/starting-with 12.4) can match CoS and/or set CoS.  Still, unlikely CoS is set to other than zero by default, and as a new frame is generated on a L3 interface, an ingress L3 (with L2 CoS) also shouldn't be automatically copied to egress L3 (with "new" L2 CoS).

If L3 egress needs its L2 CoS set, would expect you'll need to explicitly set it.

As newer Cisco switches can often process L3 ToS, today there's often little need to deal with L2 CoS.  Using L3 ToS also provides more granularity and avoids the need for tagged L2 frames.

Beginner

Re: By default, Does the router mark CoS in the Ethernet Frame?

Thanks Joseph, i appreciate your answer, and the answers for the team.

Thanks!!!

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