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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

VLAN 0
Used for 802.1P Traffic Cannot be modifi­ed/­deleted
VLAN 1
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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