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290 Avaya Application Solutions IP Telephony Deployment Guide
Ethernet switches
The following recommendations apply to Ethernet switches to optimize operation with Avaya
endpoints. Theses recommendations are meant to provide the simplest configuration by
removing unnecessary features.
● Enable spanning tree fast start feature or disable spanning tree at the port level. The
Spanning Tree Protocol (STP) is a Layer 2 loop-avoidance protocol. When a device is first
connected (or reconnected) to a port that is running spanning tree, the port takes
approximately 50 seconds to cycle through the Listening, Learning, and Forwarding states.
This 50-second delay is neither necessary nor desired on ports that are connected to IP
endpoints. Instead, enable a fast start feature on these ports to put them into the
Forwarding state almost immediately. If this feature is not available, disabling spanning
tree on the port is an option that should be considered. Do not disable spanning tree on an
entire switch or VLAN. Also, Rapid Spanning Tree Protocol (802.1w) is always preferred
over STP (802.1D).
● Disable Cisco features. Cisco features that are not required by Avaya endpoints include
channeling, cdp, and proprietary (not 802.3af) inline power. These features are
nonstandard mechanisms that are relevant only to Cisco devices, and can sometimes
interfere with Avaya devices. The CatOS command set port host <mod/port>
automatically disables channeling and trunking, and enables portfast. Execute this
command first, and then manually disable cdp and Cisco proprietary (not 802.3af) inline
power. Then manually enable 802.1Q trunking as necessary.
● Properly configure 802.1Q trunking on Cisco switches. When trunking is required on a
Cisco CatOS switch that is connected to an Avaya IP Telephone, enable it for 802.1Q
encapsulation in the no-negotiate mode (set trunk <mod/port> nonegotiate
dot1q). This causes the port to become a plain 802.1Q trunk port with no Cisco
autonegotiation features. When trunking is not required, explicitly disable it, because the
default is to autonegotiate trunking.
Speed and duplex
One major issue with Ethernet connectivity is proper configuration of speed and duplex. A
significant amount of misunderstanding exists in the industry as a whole with regard to the
auto-negotiation standard. It is imperative that the speed and duplex settings be configured
properly.
A duplex mismatch condition results in a state where one side perceives a high number of
collisions, while the other side does not. This results in packet loss. Although it degrades
performance in all cases, this level of packet loss might go unnoticed in a data network because
protocols such as TCP retransmit lost packets. In voice networks, however, this level of packet
loss is unacceptable. Voice quality rapidly degrades in one direction. When voice quality
problems are experienced, duplex mismatches are the first thing to investigate.
In general, best practice is to lock down both sides of an IP connection to 100 Mbps and full
duplex. However, auto-negotiate is an acceptable practice in most circumstances.