55. Configure Trunking Encapsulations
Trunking Encapsulations
when a switch port is configured to function as a trunk port, it adds unique identification
tags - either 802.1Q tags or Inter-Switch Link (ISL) tags to the frames as they move between switches.
old switches supports both encapsulations
new cisco switches only supports 802.1Q since it is faster
encapsulation on each interface of the trunk should be thesame to avoind encapsulation mismatch
2 Trunking Encapsulations
A. ISL
InterSwitch Link
Cisco Proprietary
adds extra headers and trailers on vlan packets as mark
header to be added:
*vlan ID
*header
with additional trailer
in total, the added header and trailer is 30bytes on the frames
this only happens when a switch forwards a frame to trunk interface
this encapsulation will work only on cisco switches
B. 802.1Q / DOT1Q
IEEE standard
only the vlan ID is added on the original frame
total size of header added is 4 bytes
faster encapsulation
Configuring Trunking encapsulation
Syntax:
SW1:
conf t
int fa 0/1
switchport mode trunk //in older switches, need to set first the encapsulation
type before setting the int to trunk
switchport trunk encapsulation dot1q //newer cisco switches already set to dot1q
exit
show int fa 0/1 switchport //to check interface encapsulation
show interfaces trunk //check trunk interfaces negotiated also the encapsulation
Note: trunking encapsulation config differs on switch models