Mitel MiVOICE BUSINESS, Engineering Manualline

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Network Configuration Concepts
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native/default VLAN) as the data, but where priority schemes exist, the voice is treated ahead
of data.
Note that certain releases of Windows will overwrite the DSCP value that might be set within
an application and force both voice and data to DSCP 0. In this case the network equipment
may need to re-classify the DSCP values based on data type, such as UDP or RTP, or use of
TCP and UDP ports. See “MiVoice Business and 3300 IP Ports” on page 275 for more details
on ports used by the phone.
On certain combined Layer 2 and Layer 3 switches, the ports may prioritize data based on
either COS or TOS/Diffserv data. This may also force a change (unexpectedly) in the COS to
TOS mapping information based on internal mapping rules. Usually these can be reconfigured
as necessary.
The COS values run from 0 to 7. Typically 7 is the highest value, 0 the lowest. However, newer
standards and switches define a COS 2 as “best effort” with 0 and 1 as lower. Also, the default
setting on some switches might place COS 5 into the expedite queue, potentially giving this
higher priority than 6 and 7. Check these settings on the switch to ensure correct and expected
operation.
USE OF SUBNETS AND SUBNET SIZE
Creating a flat network appears to speed up transactions due to the high link speed, but Layer-
3 switches are hardware-oriented, and perform equally as well as their Layer 2 counterparts.
In the Layer 2 switch environment, data can be addressed directly to a specific port, thereby
reducing loading on links not used. However, if the Layer 2 devices cannot identify an address
or port location to use, additional protocols are needed to get the information. The additional
protocols broadcast data to every port and device, causing the loading on the network to be
almost back to that of a shared environment. The Layer 2 devices maintain a list of addresses
and port locations in internal memory. If the memory and list are small, the level of broadcasts
can also increase, since new information is rapidly aged out of the list.
A large flat network can potentially grind to halt, not because of genuine traffic loading, but
simply due to the amount of broadcast traffic that is required. Using subnets helps by segmenting
broadcast domains. The Layer 2 devices subsequently need to hold less information, and so
broadcast less often. Smaller subnets are preferable to reduce the level of broadcast traffic
within a particular network domain.
Including Layer 3 devices improves speed within communities of interest and the overall
network, and reduces the burden on the system to all broadcast traffic. It is also a requirement
for VLANs to operate correctly and provide the voice priority required when using dual-port
phones.
A subnet with more than 1024 (/22 or mask 255.255.252.0) addresses is not recommended.
Typical and recommended subnet sizes are /24 (mask 255.255.255.0) and /23 (mask
255.255.254.0).
Note: COS is Class Of Service (IEEE 802.1p), not to be confused with the telecom Class
of Service value.