QoS Requirements of the Control Plane
Unless the network is up, QoS is irrelevant. Therefore, it is critical to provision QoS for control-plane traffic, which includes IP routing traffic and network management.
When addressing the QoS needs of IP routing traffic, the following guidelines are recommended:
IP routing traffic should be marked to DSCP CS6; this is default behavior on Cisco IOS platforms.
Interior gateway protocols usually adequately are protected with the Cisco IOS internal PAK_priority mechanism. Exterior gateway protocols, such as BGP, are recommended to have an explicit class for IP routing with a minimal bandwidth guarantee.
Cisco IOS automatically marks IP routing traffic to DSCP CS6.
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 and 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 priority to important control datagrams as they are processed through the router. This mechanism is called PAK_priority.
As datagrams are processed though the router and down to the interfaces, they internally are encapsulated with a small packet header, referred to as the PAKTYPE structure. Within the fields of this internal header is a PAK_priority flag that indicates the relative importance of control packets to the router's internal processing systems. PAK_priority designation is a critical internal Cisco IOS Software operation and, as such, is not administratively configurable in any way.
It is important to note that although exterior gateway protocol (EGP) traffic, such as Border Gateway Protocol (BGP) traffic, is marked by default to DSCP CS6, it does not receive such PAK_priority preferential treatment and might need to be protected explicitly to maintain peering sessions.
Additional information on PAK_priority can be found at http://www.cisco.com/warp/public/105/rtgupdates.html.
When addressing the QoS needs of Network-Management traffic, the following guidelines are recommended:
Network-Management traffic should be marked to DSCP CS2.
Network-Management applications should be protected explicitly with a minimal bandwidth guarantee.
Network-Management traffic is important in performing trend and capacity analyses and troubleshooting. Therefore, a separate minimal bandwidth queue can be provisioned for Network-Management traffic, which could include SNMP, NTP, Syslog, and NFS and other management applications.