CBWFQ - Routing Protocol Traffic

[merryllem]

Greetings All,

Just have a quick clarification for the community experts. Are routing protocol traffic (OSPF\EIGRP\BGP messages) automatically given a higher priority in a CBWFQ configuration or does it have to be manually defined and prioritized?

Thanks in Advance!

[Joseph W. Doherty · Accepted solution]

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Posting

Normally locally (to device) sourced routing protocol are marked with DSCP CS6, and they often are given special priority within the device.

From Cisco's Enterprise QoS Solution Reference Network Design Guide:

By default, Cisco IOS software (in accordance with RFC 791 and RFC 2474) marks Interior  Gateway Protocol (IGP) traffic such as Routing Information Protocol  (RIP/RIPv2), Open Shortest Path First (OSPF), and Enhanced Interior  Gateway Routing Protocol (EIGRP) to DSCP CS6. However, Cisco IOS  software also has an internal mechanism for granting internal priority  to important control datagrams as they are processed within the router.  This mechanism is called PAK_PRIORITY.

As datagrams are processed though the router and down to the interfaces,  they are internally encapsulated with a small packet header, referred  to as the PAKTYPE structure. Within the fields of this internal header  there is a PAK_PRIORITY flag that indicates the relative importance of  control packets to the internal processing systems of the router.  PAK_PRIORITY designation is a critical internal Cisco IOS software  operation and, as such, is not administratively configurable in any way.

Note that Exterior  Gateway Protocol (EGP) traffic such as Border Gateway Protocol (BGP)  traffic is marked by default to DSCP CS6 but does not receive such  PAK_PRIORITY preferential treatment and may need to be explicitly  protected in order to maintain peering sessions.

When addressing the QoS needs of IP Routing traffic, Cisco recommends the following guidelines:

•IP Routing traffic should be marked to DSCP CS6; this is default behavior on Cisco IOS platforms.

•IGPs  are usually adequately protected with the Cisco IOS internal  PAK_Priority mechanism; Cisco recommends that EGPs such as BGP have an explicit class for IP routing with a minimal bandwidth guarantee.

•Cisco IOS automatically marks IP Routing traffic to DSCP CS6.

[merryllem]

This comment below validates an experience I had with video saturated link that interrupted WAN BGP session. In which I had to define BGP in a class and allocate bandwith to fix issue

"Note that Exterior  Gateway Protocol (EGP) traffic such as Border Gateway Protocol (BGP)  traffic is marked by default to DSCP CS6 but does not receive such  PAK_PRIORITY preferential treatment and may need to be explicitly  protected in order to maintain peering sessions."

Thanks

Here is another interesting read..

http://cisconinja.wordpress.com/2009/02/18/cbwfq-routing-protocols-and-max-reserved-bandwidth/

[Joseph W. Doherty]

Indeed, an interesting read.