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In RFC 4594, titled as "Configuration Guidelines for DiffServ Service Classes", it is suggested that network control packets should be tagged by DSCP 48 (CS6). Protocols like OSPF, EIGRP, ISIS and so on are under "network control" category.

1-Should I do any classification, as a network admin, to separate these protocols' packets? Don't all these protocols label their packet by DSCP 48 themselves?

2-If I don't trust their DSCP value, how can I distinguish network control packets?

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The following information is only based on Cisco equipment.

1-Should I do any classification, as a network admin, to separate these protocols' packets? Don't all these protocols label their packet by DSCP 48 themselves?

This depends a lot on the actual network equipment you are using.

Newer routers have QoS enabled by default and mark routing and network control packets automatically. Newer layer 3 switches preserve QoS markings, but do not mark traffic itself. It only forwards the packets and preserves the DSCP marking in the Best Effort queue.

Older equipment like the Cisco 6500 Series, Cisco 3560 Series etc. does not do anything unless you enable and configure QoS. The 6500 has different modules which, by default, mark traffic as CoS values, but if QoS is not enabled on the Supervisor, the ASIC itself will remark the traffic to 0 or best effort. Likewise, if the 3560 switch receives QoS marked traffic it will strip the markings and forward as best effort (Newer software and models preserve the markings).

Please remember, that QoS queueing, by default on any Cisco equipment, is not performed and therefore forwarded in accordance to FIFO.

All in all, yes you will have to do classification if you want to separate different protocols in different queues and prioritize the traffic. Do note, that QoS has no effect on your network traffic and performance, if the network is not congested.

2-If I don't trust their DSCP value, how can I distinguish network control packets?

As written, newer equipment and models preserve the QoS markings. You will have to tell the switch or router to do something if you want to queue it and distinguish between it. Otherwise it will just be forwarded like any other traffic.

You configure QoS features throughout a network to provide for end-to-end QoS delivery. The following three components are necessary to deliver QoS across a heterogeneous network:

  • QoS within a single network element, which includes queueing, scheduling, and traffic shaping features.

  • QoS signalling techniques for coordinating QoS for end-to-end delivery between network elements.

  • QoS policing and management functions to control and administer end-to-end traffic across a network.

Not all QoS techniques are appropriate for all network routers. Because edge routers and backbone routers in a network do not necessarily perform the same operations, the QoS tasks they perform might differ as well. To configure an IP network for real-time voice traffic, for example, you would need to consider the functions of both edge and backbone routers in the network, then select the appropriate QoS feature or features.

In general, edge routers perform the following QoS functions:

  • Packet classification

  • Admission control

  • Configuration management

In general, backbone routers perform the following QoS functions:

  • Congestion management

  • Congestion avoidance

More information:

https://www.cisco.com/c/en/us/td/docs/ios/12_2/qos/configuration/guide/fqos_c/qcfintro.html

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