Information Technology Reference
In-Depth Information
ISL appends tags to each frame to identify the VLAN it belongs to. As shown in Figure 3-
16, the tag is a 30-byte header and CRC trailer that are added around the Fast Ethernet
frame. This includes a 26-byte header and 4-byte CRC. The header includes a 15-bit
VLAN ID that identifies each VLAN. The user field in the header also includes 3 bits for
the class of service (CoS).
3 Bits Used for
CoS
ISL Header
26 Bytes
Encapsulated Frame
1...24.5 KB
FCS
(4 Bytes)
Figure 3-16
ISL Frame
The IEEE 802.1Q standard trunks VLANs over Fast Ethernet and Gigabit Ethernet inter-
faces, and you can use it in a multivendor environment. IEEE 802.1q uses one instance of
STP for each VLAN allowed in the trunk. Like ISL, IEEE 802.1Q uses a tag on each frame
with a VLAN identifier. Figure 3-17 shows the IEEE 802.1Q frame. Unlike ISL, 802.1Q
uses an internal tag. IEEE 802.1Q also supports the IEEE 802.1p priority standard, which
is included in the 802.1D-1998 specification. A 3-bit Priority field is included in the
802.1Q frame for CoS.
PREAM
SFD
DA
SA
TYPE
TAG
PT
DATA
FCS
3 “Priority” Bits Used for
CoS; Values 0-7 on
802.1p/Q Ports
PRI
CFI
VLAN ID
Figure 3-17
IEEE 802.1Q Frame
The preferred location to mark traffic is as close as possible to the source. Figure 3-18
shows a segment of a network with IP phones. Most workstations send packets with CoS
or IP precedence bits (ToS) set to 0. If the workstation supports IEEE 802.1Q/p, it can
mark packets. VoIP traffic from the phone is sent with a Layer 2 CoS set to 5. The phone
also reclassifies data from the PC to a CoS/ToS of 0. With differentiated services code-
point (DSCP) at Layer 3, VoIP bearer traffic is set to Expedited Forwarding (EF) (which
implies a ToS set to 5), binary value 101110 (hexadecimal 2E). Signaling traffic is set to
DSCP AF31.
As shown in Figure 3-18, switch capabilities vary in the access layer. If the switches in this
layer are capable, configure them to accept the markings or remap them. The advanced
remap the CoS/DSCP values to different markings.
