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6-149
Configuring the Switch
Class of Service (CoS): Managing Bandwidth More Effectively
Configuring the Switch
Packet Enters Switch: In a tagged VLAN
Packet Exits From Switch: In an Untagged VLAN
(Prioritizing affects only the choice of outbound priority queue. The 802.1p priority tag carried by the packet when it
entered the switch is discarded along with the tagged VLAN field.)
1. Device Priority (IP Address) Policy
(IP Packets Only)
:
– If Device Priority does not apply to the packet, then packet priority defers to the ToS policy.
– If Device Priority (0 - 7) is configured and applies to a packet, the packet is assigned to the appropriate queue (high
or normal priority) of the outbound port, regardless of any other CoS-configured policy.
2. Type of Service (ToS) Policy
(IP Packets Only)
:
– If ToS option is configured to Disabled, then packet priority defers to the Protocol Priority policy.
– IP Precedence option
: Prioritizes packet (high or normal outbound queue) according to the value of the ToS
precedence bits (upper three bits of ToS field; 0 - 7); 4 - 7 = high, 0 - 3 = normal.
– Differentiated Services option
: Prioritizes packet (high or normal outbound queue) according to Priority setting (0
- 7) for packet’s ToS field codepoint. If Priority is set to No override (the default), then packet priority defers to the
Protocol Priority policy.
See “Using Type of Service (ToS) Criteria to Prioritize IP Traffic” on 6-143.
3. Protocol Priority Policy
:
– If Protocol Priority does not assign a priority to the packet, then packet priority defers to the VLAN ID policy.
– If Protocol Priority assigns a priority (0 - 7) to a packet, the packet is assigned to the appropriate queue (high or
normal priority) of the outbound port.
4. VLAN Priority Policy:
– If VLAN Priority does not assign a priority to the packet, then packet priority defers to the incoming 802.1p priority
value. (See “Incoming 802.1p Priority” in Table 6-7. Priority Criteria and Precedence on page 6-134.)
– If VLAN Priority assigns a priority (0 - 7) to a packet, the packet is assigned to the appropriate queue (high or normal
priority) of the outbound port.
(An outbound packet belonging to an untagged VLAN can be assigned to an outbound, high or normal priority queue,
but cannot be assigned an 802.1p priority because there is no tagged VLAN field in the packet.)