Packet Modification; Qos And Software Processed Packets - Cisco 4500M Software Manual

Chapter 29 Configuring QoS

Packet Modification

A packet is classified, policed, and queued to provide QoS. Packet modifications can occur during this
process:
•
•
•

QoS and Software Processed Packets

The Catalyst 4500 platform does not apply the QoS marking or policing configuration for any packets
that are forwarded or generated by the Cisco IOS software. This means that any input or output QoS
policy configured on the port or VLAN is not applied to packets if the Cisco IOS is forwarding or
generating packets.
However, Cisco IOS marks all the generated control packets appropriately and uses the internal IP DSCP
to determine the transmit queue on the output transmission interface. For IP packets, the internal IP
DSCP is the IP DSCP field in the IP packet. For non-IP packets, Cisco IOS assigns a packet priority
internally and maps it to an internal IP DSCP value.
Cisco IOS assigns an IP precedence of 6 to routing protocol packets on the control plane. As noted in
RFC 791,"The Internetwork Control designation is intended for use by gateway control originators
only." Specifically, Cisco IOS marks the following IP-based control packets: Open Shortest Path First
(OSPF), Routing Information Protocol (RIP), Enhanced Interior Gateway Routing Protocol (EIGRP)
hellos, and keepalives. Telnet packets to and from the router also receive an IP precedence value of 6.
The assigned value remains with the packets when the output interface transmits them into the network.
For Layer 2 control protocols, the software assigns an internal IP DSCP. Typically, Layer 2 control
protocol packets are assigned an internal DSCP value of 48 (corresponding to an IP precedence value of
6).
The internal IP DSCP is used to determine the transmit queue to which the packet is enqueued on the
transmission interface. See "Configuring Transmit Queues" on page 43 for details on how to configure
the DSCP to transmit queues.
The internal IP DSCP is also used to determine the transmit CoS marking if the packet is transmitted
with a IEEE 802.1q or ISL tag on a trunk interface. See "Configuring the DSCP-to-CoS Map" on page 47
for details on how to configure the DSCP to CoS mapping.
OL-6696-01
For IP packets, classification involves assigning a DSCP to the packet. However, the packet is not
modified at this stage; only an indication of the assigned DSCP is carried along. The reason for this
is that QoS classification and ACL lookup occur in parallel, and it is possible that the ACL specifies
that the packet should be denied and logged. In this situation, the packet is forwarded with its
original DSCP to the CPU, where it is again processed through ACL software.
For non-IP packets, classification involves assigning an internal DSCP to the packet, but because
there is no DSCP in the non-IP packet, no overwrite occurs. Instead, the internal DSCP is used both
for queueing and scheduling decisions and for writing the CoS priority value in the tag if the packet
is being transmitted on either an ISL or 802.1Q trunk port.
During policing, IP and non-IP packets can have another DSCP assigned to them (if they are out of
profile and the policer specifies a markdown DSCP). Once again, the DSCP in the packet is not
modified, but an indication of the marked-down value is carried along. For IP packets, the packet
modification occurs at a later stage.
Software Configuration Guide—Release 12.2(25)EW
Overview of QoS
29-15