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New Member

Multilayer routed ports . Are they always untagged (no sub-int)?

Hi,

I have a basic question about multilayer switching and routed ports / Intervlan routing.

I know that I can setup a vlan 100 and 200 interface, and have switchport trunks trunking both vlans from another switch.

        With ip routing enbled, the 3550 will do intervlan routing.

Now, what if I have  routed ports.

       Say I have 2 ports, 10.1.100.1 and 10.1.200.1 corrosponding to the 2 vlans.

       Now, on a router, I could create sub-interfaces, set encapsulation and accept trunks from other switches

       to do inter-vlan router on a stick.

       SO the question is, do ROUTED PORTS on a 3550 always UNTAGGED ?

Thanks.

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Multilayer routed ports . Are they always untagged (no sub-i

Hello,

Once you configure a physical interface as routed port, it does not belong to any VLAN. So, for example, if you connect fa 0/24 to VLAN 10, then it becomes a host on VLAN 10. As far as traffic within the switch is concerned, it will go untagged from the individual layer 3 ports to the routed port (just like a router). So, no question of tagging when you configure the routed ports.

Hope this helps.

Regards,

NT

4 REPLIES
Cisco Employee

Re: Multilayer routed ports . Are they always untagged (no sub-i

Hello,

If I understand you right, you are looking for some information about the layer 3 (virtual) interfaces on the switches. You can treat those interfaces as any other hosts on the VLAN. All their traffic goes untagged within the VLAN on the same switch. But if that traffic is going across to another switch through a trunk, then it will be tagged just like any other host traffic. If the layer 3 interface is for the native VLAN, then it will be untagged on the trunk (just like any other host traffic from native vlan). From that perspective, the layer 3 port is also another host in the VLAN.

Hope this helps.

Regards,

NT

New Member

Re: Multilayer routed ports . Are they always untagged (no sub-i

Hi,

No, I was not talking about the "interface vlan" (SVI) itself, but

intrerface fa 0/24

  no switchport


  switchport ip address 10.1.100.1 255.255.255.0

Now, what vlan does fa 0/24 belong to ?

  If this was a router, I could create fa 0/24.100 and fa 0/24.200, and setup fa 0/24 for trunking with another switch.

   I guess I cannot have a trunk on fa 0/24 once its not a switchport.

So what you say does make sense. Fa 0/24 is another host on the vlan.

But would fa 0/24 always be VLAN 1? Because there is no way to set the vlan ?

    Or am I missing something...

Cisco Employee

Re: Multilayer routed ports . Are they always untagged (no sub-i

Hello,

Once you configure a physical interface as routed port, it does not belong to any VLAN. So, for example, if you connect fa 0/24 to VLAN 10, then it becomes a host on VLAN 10. As far as traffic within the switch is concerned, it will go untagged from the individual layer 3 ports to the routed port (just like a router). So, no question of tagging when you configure the routed ports.

Hope this helps.

Regards,

NT

New Member

Re: Multilayer routed ports . Are they always untagged (no sub-i

Some more searching seems to suggest that the IOS puts the routed ports in its own internal vlan.

   Not sure if that is 100% correct though.

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