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802.1Q Fields

Tag protocol identifier (TPID):
16-bit is field set to 0x8100 to identify the packet as an 802.1Q packet.
Priority code point (PCP):
3-bit field indicates a class of service (CoS) as part of Layer 2 quality of service (QoS) between switches.
Drop elgible indicator (DEI):
This 1-bit field indicates whether the packet can be dropped when there is bandwidth conten­tion.
VLAN identifier (VLAN ID):
This 12-bit field specifies the VLAN associated with a network packet.

Centra­lized Forwarding VS Distri­buted Forwarding

Centralized Forwarding
⇨ When a route processor (RP) engine is equipped with a forwarding engine
Distributed Forwarding
⇨ Forwarding engines are equipped in the line cards
⇨ When a packet is received on the ingress line card, it is transm­itted to the local forwarding engine. The forwarding engine performs a packet lookup, and if it determines that the outbound interface is local, it forwards the packet out a local interface. If the outbound interface is located on a different line card, the packet is sent across the switch fabric, also known as the backplane, directly to the egress line card, bypassing the RP.

VLAN Ranges

Used for 802.1P Traffic Cannot be modifi­ed/­deleted
Default VLAN Cannot be modifi­ed/­deleted
VLAN 1002 to 1005
Reserve Range Can be modifi­ed/­deleted
VLAN 2 to 1001
Normal Range Can be modifi­ed/­deleted
VLAN 1006 to 4094
Ext­ended Range Can be modifi­ed/­deleted


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