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IP SLA responder QoS ToS value

Hi,

I have a question regarding the SLA setup.

We have the next scenario.

A router is configured with the SLA measurments. This router is located to a Central site of a customer and is used

only for the SLA measurments (Shadow router).

The SLA measurments configuration is  based on icmp packets with a specific TOS value in order to measure the

Roundtrip delay to the different classes of the network.

Sample configuration

config)#rip sla 2

(config-ip-sla)#ticmp-echo 10.10.80.1

(config-ip-sla-echo)#request-data-size 180

(config-ip-sla-echo)#tos 160

(config)#ip sla schedule 2 start now

I have the next question:

The icmp packet with the a TOS value (e.g.TOS 160 or IPprec 101) has been generated from the Shadow router and

has as destination IP a remote site (CE). The time that this icmp echo packet reaches the remote site then does it responce

with an icmp echo reply which has the same TOS value as the icmp echo packet that recieves (i.e. 160)?

We assume that the remote site has not been configured with any qos policy for the packets with destination the SLA (shadow router).

How is possible the remote site to understand the icmp TOS field (if it is possible?) and responce with the same TOS value?

Please help me. I could not find a good answer to the web.

Thank you!

10 REPLIES 10
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Hall of Fame Cisco Employee

It depends.  The ICMP echo operation does not require an IP SLA responder.  The target can be any IP-enabled device.  That node must be capable of sending the echo reply with the same QoS settings.  How this is done will depend on the OS running on your target node.  It may be possible, but it will likely require some configuration on your target node.

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Thank you for your responce but what  additional configuration is needed to a remote site to responce to the ICMP packets ,generated by the SLA (shadow) router, with the same TOS value?

I am not intersted for other measurmetns like jitter which require the command ip sla responder to the remote sites.

I investigated this issue to the web but I could not find an answer.

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As I said, that will depend on your target OS.  You will need to contact that vendor to find out what can be done (if anything) to allow it to echo the same QoS parameters back to the sender.

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The remote end is a CISCO router e.g.1941 or 2901.

We do not use any other vendor, only CISCO!

...but how can be configured to the CISCO remote router to echo with the same QoS parameters back to the sender?

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It wasn't clear the target was a Cisco device.  For ICMP echo, the target can be any IP node.  I do not think it is possible to copy the QoS parameters from an echo request onto an echo reply.  You could set up a policy to mark all echo replies to your source router with the required QoS parameters.  You might want to ask on the WAN, Routing and Switching community about the QoS parameter copy.

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  I am also not sure how to copy the ToS value from an echo request to an echo reply.

If this is not configurable to a remote router then the delay SLA measurments to a specific class are not correct. The remote router receives the packet  with eg TOS 160 but replies to the default class. The default class has different treatment to the network in compare to EF (Voice).

QoS policy marking for the echo responces to the central SLA router does not solve the problem.

I would really appreciate any help.

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Right.  As I said, I'm not sure if one can copy the QoS parameters to the echo reply in IOS.  I would ask on the WAN, Routing and Switching community here on CSC.  Alternatively, you could configure a policy on the target device to apply DSCP EF to all echo replies going from the target router to the IP address of the IP SLA source router.  This would ensure the right class end-to-end.

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OK, I will forward my question to the Routing and Switching Community. Hopefully I could find an answer.

Policy based can not be used in our case. The SLA router generates icmp echo packets in various QoS classes e.g. EF, AF12 etc. So, the icmp echo reply to the target (SLA) router should have different DSCP values which depends on the input TOS value and not to the destination (target) IP.

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I opened a ticket for the question above to CISCO TAC .

It was proved, with captured packets (Wireshark), that the TOS value is not modified for both icmp echo request and reply packets.

The key command to remote router which responds to the icmp echo is the ip sla responder command.

I thought that the ip sla responder command can be helpful in our problem (TOS value) only to TCP & UDP packets but it also works for ICMP.

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I opened a ticket for the question above to CISCO TAC .

It was proved, with captured packets (Wireshark), that the TOS value is not modified for both icmp echo request and reply packets.

The key command to remote router which responds to the icmp echo is the ip sla responder command.

I thought that the ip sla responder command can be helpful in our problem (TOS value) only to TCP & UDP packets but it also works for ICMP.

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