Nortel Networks 450 - Manual (Page 79)

BayStack 450 10/100/1000 Series Switches
302401-D Rev 00 1-53
IEEE 802.1p Prioritizing
You can use the VLAN Configuration screens to prioritize the order in which the
switch forwards packets, on a per-port basis. For example, if messages from a
specific segment are crucial to your operation, you can set the switch port
connected to that segment to a higher priority level (by default, all switch ports are
set to Low priority). Untagged packets received by the switch on that port are
tagged according to the priority level you assign to the port (see
Figure 1-34 ).
Figure 1-34. Prioritizing Packets
The newly tagged frame is read within the switch and sent to the port’s high or low
transmit queue for disposition (see
Figure 1-35 ). The port transmit queue example
shown in Figure 1-35 applies to all ports on the BayStack 450 switch.
BS45024A
Port 6 Port 7 Port 8
Port 1
PVID = 2
Priority = 6
Po
rt
4
Po
rt
5
Port 2 Port 3
802.1Q Switch High
Low
Key
Priority
CFI
VID - User_priority
- Canonical format indicator
- VLAN identifier
Tagged member
of VLAN 2 (Port 5)
Port configuration
parameters
Port 5
transmit
queue
Untagged member
of VLAN 2
After
Before
DA
SA
Data
CRC
(*Recalculated)
Outgoing
untagged packet
(unchanged)
DA
SA
Data
CRC
DA SA Data CRC* Tag
VID = 2 Priority = 6
16 bits 3 bits 1 bit 12 bits
8100 CFI
kombk.book Page 53 Tuesday, June 29, 1999 3:25 PM