Information Technology Reference
In-Depth Information
Finally, the
service-policy
service-policy-name
command links to another policy map (voice, not shown) that is used to configure
LLQ for voice traffic.
LAN QoS
Although bandwidth is usually plentiful in the LAN, it can still be important to configure switches to ensure that voice traffic receives
required QoS. In particular, it is important to establish a trust boundary, appropriately classify/mark traffic as close to its source as
possible, and police excess or unwanted traffic so that it does not cause problems in the network. It can also be important to configure
QoS in the LAN because, in the absence of QoS configuration, voice quality will be affected even if there is momentary congestion.
The configuration of a trust boundary depends on what types of devices are connected at the access layer in a LAN; specifically,
whether connected devices can be considered trusted endpoints, untrusted endpoints, or conditionally trusted endpoints:
■
Trusted endpoints:
These are devices, such as analog gateways, that can appropriately mark traffic and re-mark any traffic
that has been marked by any connected untrusted devices.
■
Untrusted endpoints:
These are devices such as workstations, PCs, and servers.
■
Conditionally trusted endpoints:
Cisco IP phones are trusted devices, but the PCs that might be connected to them via a PC
port are not. Cisco IP phones also are often moved between ports on access switches, and therefore, it is an arduous task to
statically configure ports to which IP phones are connected as being trusted.
Therefore, Cisco IP phones have the capability to exchange Cisco Discovery Protocol (CDP) messages with the Cisco switch
to which they are connected and, on the basis of this exchange, the switch can extend trust to the phone and trust traffic
received from the phone. The Cisco IP phone can re-mark any traffic received from a connected PC to CoS 0.
CDP is a Cisco proprietary protocol that was designed for neighbor discovery. It is a Layer 2 protocol that sends multicast
advertisements (with the multicast destination address 01:00:0C:CC:CC:CC) using a Subnetwork Access Protocol (SNAP)
encapsulation. These advertisements are sent by default every 60 seconds, and there is a default hold time of 180 seconds. (These
timers can be modified using the
cdp timer
and
cdp holdtime
commands.) There are two versions of CDP—CDP Version 1 and CDP
Version 2—with CDPv2 offering certain enhancements over CDPv1. CDP advertisements can contain information such as device
