Information Technology Reference
In-Depth Information
IEEE 802.1P traffic is simply classified and sent to the destination; no bandwidth reserva-
tions are established.
IEEE 802.1P is a spin-off of the 802.1Q VLAN trunking standard. The 802.1Q standard
specifies a tag that appends to a MAC frame. The VLAN tag carries VLAN information.
The VLAN tag has two parts: the VLAN ID (12 bit) and Prioritization (3 bit). The Prioriti-
zation field was never defined in the VLAN standard. The 802.1P implementation defines
this Prioritization field.
802.1P establishes eight levels (3 bits) of priority similar to IP precedence. Network
adapters and switches route traffic based on the priority level. Using Layer 3 switches al-
lows you to map 802.1P prioritization to IP precedence before forwarding to routers.
Resource Reservation Protocol
Resource Reservation Protocol (RSVP) is a signaling protocol that enables end stations or
applications to obtain special QoS for their data flows. Basically, RSVP reserves band-
width for the application. RSVP does not transport application data but is rather an Inter-
net control protocol, like Internet Control Message Protocol (ICMP), Internet Group
Management Protocol (IGMP), or routing protocols. RSVP is also known as Resource
Reservation Setup Protocol. You can find the IETF charter at
www.ietf.org/html.charters/
rsvp-charter.html.
The first “standards” version of the protocol can be found in RFC 2205.
RSVP is used by a host to request specific QoS from the network for particular applica-
tion data streams or flows. RSVP requests generally result in resources being reserved in
each node along the data path.
LFI
LFI is a QoS mechanism used to reduce the serialization delay. In a multiservice network,
small VoIP packets have to compete with large data traffic packets for outbound inter-
faces. If the large data packet arrives at the interface first, the VoIP packet has to wait until
the large data packet has been serialized. When the large packet is fragmented into smaller
packets, the VoIP packets can be interleaved between the data packets. Figure 14-26
shows how LFI works. With no LFI, all VoIP packets and other small packets must wait
for the FTP data to be t ran s mit ted. With LFI, the FTP data packet is fragmented. The
queuing mechanism then can interleave the VoIP packets with the other packets and send
them out the interface.
FRF.12 is a fragmentation and interleaving mechanism specific to Frame Relay networks. It
is configured on Frame Relay permanent virtual circuits (PVC) to fragment large data
packets into smaller packets and interleave them with VoIP packets. This process reduces
the serialization delay caused by larger packets.
LLQ
As shown in Figure 14-27, LLQ provides a strict-priority queue for VoIP traffic. LLQ then
is configured with multiple queues to guarantee bandwidth for different classes of traffic.
Other traffic is WFQ'd based on its classification. With LLQ, all voice call traffic is as-
