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Cisco router trunk concept

Posted on 2006-10-24
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Hello all,

i had found in the past a web site that explains the difference between trunk concept on cisco switches and cisco router. In my case, between a catalyst 3550 and a cisco router 3600 with fast ethernet interfaces

In fact, as far as i remember, you cannot configure a cisco router interface as a trunk port like you do it on a cisco switch.

Any comment or information about that will help a lot.

thanks
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Question by:lemaitre75
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lrmoore earned 250 total points
ID: 17799934
You're half right. You can't use the same 'set port trunk' type of commands, but you can still connect a router to a switch's trunk port and create sub-interface for each VLAN. The whole idea of a trunk is to pass vlan data, and of course data across the vlans.

Example:
Switch:

  interface fast 0/12
   switchport access vlan 2

  interface fast 0/14
   switchport access vlan 4

  interface Fast0/28
   description uplink to router
   switchport mode trunk
 
Router:
 interface Fast0/0  <== one physical interface, multiple VLAN's
  Description this is VLAN1 <== physical interface is always switches' native vlan (vlan1 by default)
  ip address 1.2.1.1 255.255.255.0
 interface Fast 0/0.2  <== logical interface for VLAN2
  Description VLAN2
  encap dot1q 2  <== look for vlan2 tags
  ip address 1.2.2.1 255.255.255.0
 interface Fast 0/0.4
  Description VLAN4
  encap dot1q 4  <== look for vlan4 tags
  ip address 1.2.4.1 255.255.255.0

Etc...
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by:lrmoore
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