Cisco NM-8B-U - HW ROUTERS L-M User Manual page 31

16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
Layer 3 IPv6 packets are dropped when received by the switch.
Note
All switches and routers across the Internet rely on the class information to provide the same forwarding
treatment to packets with the same class information and different treatment to packets with different
class information. The class information in the packet can be assigned by end hosts or by switches or
routers along the way, based on a configured policy, detailed examination of the packet, or both. Detailed
examination of the packet is expected to happen closer to the edge of the network so that the core
switches and routers are not overloaded.
Switches and routers along the path can use the class information to limit the amount of resources
allocated per traffic class. The behavior of an individual device when handling traffic in the DiffServ
architecture is called per-hop behavior. If all devices along a path provide a consistent per-hop behavior,
you can construct an end-to-end QoS solution.
Implementing QoS in your network can be a simple or complex task and depends on the QoS features
offered by your internetworking devices, the traffic types and patterns in your network, and the
granularity of control you need over incoming and outgoing traffic.
The Ethernet switch network module can function as a Layer 2 switch connected to a Layer 3 router.
When a packet enters the Layer 2 engine directly from a switch port, it is placed into one of four queues
in the dynamic, 32-MB shared memory buffer. The queue assignment is based on the dot1p value in the
packet. Any voice bearer packets that come in from the Cisco IP phones on the voice VLAN are
automatically placed in the highest priority (Queue 3) based on the 802.1p value generated by the IP
phone. The queues are then serviced on a weighted round robin (WRR) basis. The control traffic, which
uses a CoS or ToS of 3, is placed in Queue 2.
Table 6 summarizes the queues, CoS values, and weights for Layer 2 QoS on the Ethernet switch
network module.
Table 6
Queue Number
3
2
1
0
The weights specify the number of packets that are serviced in the queue before moving on to the next
queue. Voice Realtime Transport Protocol (RTP) bearer traffic marked with a CoS or ToS of 5 and Voice
Control plane traffic marked with a CoS/ToS of 3 are placed into the highest priority queues. If the queue
has no packets to be serviced, it is skipped. Weighted Random Early Detection (WRED) is not supported
on the Fast Ethernet ports.
You cannot configure port-based QoS on the Layer 2 switch ports.
Basic QoS Model
Figure 15
policing, and marking:
• Classifying distinguishes one kind of traffic from another. For more information, see the
"Classification" section on page
Queues, CoS values, and Weights for Layer 2 QoS
CoS Value
5,6,7
3,4
2
0,1
shows the basic QoS model. Actions at the ingress interface include classifying traffic,
Weight
255
64
16
1
32.
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
Feature Overview
31