Cheatography
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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. |
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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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