Nortel Networks 450 manual Ieee 802.1p Prioritizing, Prioritizing Packets

BayStack 450 10/100/1000 Series Switches

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).

Before

CRC

PVID = 2

Priority = 6

Data

Port configuration parameters

SA	Port 1	Port 2	Port 3			Tagged member
						of VLAN 2 (Port 5)
DA	802.1Q Switch			High
			Port 5
	4				5
	Port		transmit		Port	CRC*	Data	Tag	SA	DA
			queue
				Low	(*Recalculated)
	Port 6	Port 7	Port 8

Untagged member	CRC
of VLAN 2
	Data
Outgoing	SA
untagged packet DA
(unchanged)

	8100		Priority = 6	CFI	VID = 2
	16 bits		3 bits	1 bit	12 bits
			After
Key
Priority		- User_priority
CFI		- Canonical format indicator
VID		- VLAN identifier

BS45024A

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.

302401-D Rev 00 1-53