VLAN: Trunking Troubleshooting, Modes and Protocols

ISL will encapsulate every frame targeted to the native vlan

dot1q will not encapsulate anything targeted to the native vlan

ISL – 26 byte header containing VLAN ID, 4 byte trailer for CRC check (30 bytes overhead)

larger frames become ‘giants’ exceeding 1518 bytes

smaller frames than 64 bytes = ‘runts’

dot1q adds a 4 bytes header which is inserted into frame rather then re-encapsulating

Troubleshooting Trunks

Speed and Duplex must match

ISL doesn’t use native VLAN

802.3ac extends frame length to 1522 bytes

Switches have to be in same VTP domain for a trunk to form

Explicitly set as trunk mode then nonegotiate if required

Dynamic auto will NOT form a trunk, but will accept negotiation with remote switch

Author: drbabbers

ccieme.wordpress.com - my personal journey to ccie

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