Information Technology Reference
In-Depth Information
Ta b l e 1 4 -1 4
QoS Scheme Summary
QoS
Scheme
Description
LFI
Link fragmentation and interleaving. Fragments large data packets and inter-
leaves VoIP packets between them.
LLC
Uses a single strict queue for RTP traffic. Differentiated QoS available for all
other traffic.
AutoQoS
AutoQoS is a Cisco IOS feature that enables QoS for VoIP in WAN and LAN
environments. AutoQoS significantly reduces the number of configuration
lines necessary to support VoIP in the network.
IPT Design Recommendations
The following are some best-practice recommendations when implementing IPT:
Key
To p i c
Use separate VLANs and IP subnets for IP phones and data to provide ease of man-
agement and simplified QoS configuration.
■
Use private IP addresses for IP phones subnets to allow for more security to voice
devices.
■
Place CallManager and Unity servers on filtered VLAN/IP subnets in the server ac-
cess in the data center.
■
Use IEEE 802.1Q trunking and 802.1P to allow for prioritization at Layer 2.
■
Extend QoS trust boundaries to voice devices but not to PCs and other data devices.
■
In the access layer, use multiple egress queues to provide priority queuing of RTP
voice streams.
■
Use DSCP for classification and marking.
■
Use LLQ on WAN links.
■
Use LFI on WAN links less than 768 kbps.
■
Use CAC to avoid oversubscription of circuits.
■
IEEE 802.1Q should be configured on the PoE LAN switch ports to allow a voice VLAN
for the IP phone and a data VL AN for the P C connec ted to the IP phone. The s e VL ANs
should be on separate IP subnets, and the IP phone should be an RFC 1918 private address
subnet. Furthermore, the CallManager servers should be placed on a separate IP subnet in
the data center. This lets you restrict access to the IPT environment.
IPT voice packets should be marked with a DSCP of EF (IP precedence 5), and signaling
packets should be marked with AF31 (IP precedence 3). This allows QoS schemes to give
precedence to the marked packets. LLQ takes the EF marked packets and places them in
the strict-priority queue, guaranteeing bandwidth for voice. LFI should be configured on
packets. LFI and LLQ also reduce jitter in IPT conversations.
