Question on L3 subinterface

Unanswered Question
Sep 20th, 2010

I've got a couple simple questions having to do with L3 subinterfaces.

1.  Can you do subinterfaces on the 3750 SFP ports?

2.  Can you do subinterfaces on 6500 etherchannels ports?


I have this problem too.
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Giuseppe Larosa Mon, 09/20/2010 - 12:28

Hello Jkeeffe,

generally speaking on standard C6500 ports you cannot use subinterfaces and you cannot on C3750 as well.

However, you can easily emulate a vlan based L3 subinterface by using SVIs:

the SVI is associated with the L2 broadcast domain so interface vlan100 is the SVI of L2 broadcast domain vlan 100.

To emulate a routed subinterface you just need to take care of allowing the required vlans only on the physical port

so the physical port is a L2 trunk allowing vlans 100,101,102 for example

by using

show vlan id 100

you can check that the vlan is permitted only on the desired port.

This aspect is very important as the state of SVI depends on the presence of at least an active L2 port in STP forwarding state, if multiple ports are in vlan 100 the SVI vlan 100 will not go down when the official port goes down causing routing problems for example.

This feature is called autostate and we need to take care of this behaviour.

with only one port in vlan X if that port fails SVI Vlan X goes down and this emulates a router subinterface.

Final note:

the L2 vlan must exist in order to be able to see the corresponding SVI up/up and the SVI can start its life in shutdown mode


vlan 100

interface vlan 100

ip address x.x.x.x

no shut

interface gi n/m


switchport  trunk allowed vlan 100

switchport enc dot1q

switchport mode trunk

Be aware that trunk ports by default allow all Vlans so if you are using this you need to configure all trunk ports with the list of necessary Vlans.

Hope to help



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