Jaycee's Networking

May 9, 2009

VLAN Trunks

Filed under: IOS — Tags: , — Jaycee @ 4:16 pm

A. Connecting VLAN Trunks With ISL

The main purpose of a trunk is to save physical interfaces.

While trunks carry traffic for may different VLANs, they are able to keep them seperate by tagging each frame with the unique identification number for the appropriate VLAN.

Because of the VLAN tagging scheme, Layer 2 frames cannot pass from one VLAN to another. So, without a router, there is no way to interconnect the VLANs.

Router on a stick (or one-armed router): A router is connected to a trunk to allow routing between the different VLANs. The router routes its packets back out onto the same physical interface that it received them through.

interface FastEthernet0/0
 no ip address
 speed 100
interface FastEthernet0/0.1
 encapsulation isl 1
 ip address
interface FastEthernet0/0.3
 encapsulation isl 574
 ip address

1. ISL is Cisco Proprietary.

2. No special configuration is necessary on the physical interface.

3. You need to create one subinterface on this physical interface for each different VLAN.

4. Each VLAN represents a different Layer 3 network, so you need to give each of the subinterfaces IP addresses from the corresponding IP subnets.

5. ISL supports VLAN ID numbers 1 – 1000.

B. Connecting VLAN Trunks With 802.1Q

interface FastEthernet1/0
 no ip address
 speed 100
interface FastEthernet1/0.1
 encapsulation dot1q 1 native
 ip address
interface FastEthernet1/0.3
 encapsulation dot1q 548
 ip address

1. 802.1Q is an IEEE open standard.

2. The native VLAN is the master VLAN assigned to the interface and it must match the native VLAN configured on the switch.

3. The native VLAN is the only VLAN whose frames do not contain an 802.1Q VLAN tag in their Layer 2 frame headers.

4. There can only be one native VLAN at a time. It’s NOT safe to assume that VLAN number 1 will always be the native VLAN even though the default natvie VLAN on many switches is VLAN number 1.

5. 802.1Q supports VLAN ID numbers 1 – 4095.


1 Comment »

  1. very good article to know the differences between ISL and 802.1q

    Comment by Narayana Reddy — May 25, 2009 @ 11:57 pm

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