subinterfaces in VLAN

Unanswered Question
Dec 29th, 2007

i have read that while configuring inter-vlan routing on a router subinterfaces must be created for each vlan. Its confusing me.For example in my case, i.e

vlan100 >> switch >>> rtr <<< switch << vlan 110

so each switch's interface is directly attached to the router.

should i still need subinterfaces on both side (on the router)or i can use the available physical interfaces?

Any reference link will be helpful.

Thanks

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Danilo Dy Sat, 12/29/2007 - 02:34

Hi,

If your switch is L2 only, you can use VTP http://www.cisco.com/univercd/cc/td/doc/product/lan/cat5000/rel_4_2/config/vlans.htm where one router physical interface can be configured to multiple sub-interface to serve multiple VLAN or broadcast domain.

If your switch is L3, you enable "ip routing" so that you can configure interVLAN routing http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a008015f17a.shtml

Regards,

Dandy

Nagendra Kumar ... Sat, 12/29/2007 - 03:26

Hi Munawar,

In your setup, you dont need a subinterface. The baseline is: You need a layer3 device for communication between different vlans.

If you have a Layer2 switch with multiple vlan and a single router connected, you need sub interfaces with dot1q encapsulation.

For EX: A switch have vlan 1 and vlan 2 configured with trunk port connecting to router. On the router side, 2 subinterface (for each vlan) will be configured with respective ip address from the vlan subnet.

When the switch receives a frame from VLAN 1 destinated to VLAN2, it will flood the same as the destination is unknown. The router on receiving it, will look into the destination address and will encapsulate with vlan details(VLAN2 in this case) in layer 2 header and forward the same to switch.

HTH,

Nagendra

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