Portchannel Query.

cb3dwacb3dwa Member Posts: 80 ■■□□□□□□□□
Hi All

Does any one have any experience of combining layer 2 and 3 port-channels?

example, port-channel set to layer 3 with IP address and interfaces in the channel set to layer 2 and trunked?

This was put forward as an idea at my work by a CCNP, i am not convinced this will work and cannot find any information/documentation to support it. I don't want to dismiss this but cannot understand the theory behind it.

Any one heard of this? or done this before



  • networker050184networker050184 Mod Posts: 11,962 Mod
    Are you talking about doing 802.1q sub interfaces on the port-channel interface? That works, but you can't do L2 trunks and an L3 port-channel interface. There would be no point in doing this anyway.
    An expert is a man who has made all the mistakes which can be made.
  • astorrsastorrs Member Posts: 3,139 ■■■■■■□□□□
    I don't think you can mix and match L2/L3 - at least you didn't used to be able to.

    Why not just do l2 port-channel and use an interface vlan, or subinterfaces on an l3 port-channel. Why do you want to combine them (i.e., what are you trying to accomplish)?
  • cb3dwacb3dwa Member Posts: 80 ■■□□□□□□□□
    Interesting, its not sub interfaces.

    He deffo said you could do no switchport on port-channel and switchport on interface.

    Be worth asking him to lab it up :)
  • networker050184networker050184 Mod Posts: 11,962 Mod
    What would be the point of doing that anyway? It wouldn't accomplish anything.....
    An expert is a man who has made all the mistakes which can be made.
  • CodeBlue1914CodeBlue1914 Member Posts: 19 ■■□□□□□□□□
    If I am understanding your question...we have a similar setup in my office with our border routers and border switches. Our border routers have a port-channel with several sub interfaces. We have two 3750s stacked so one cable goes to the top and other goes to the bottom and these are also set as a port-channel. The port-channel on the router, like i said, has some sub-interfaces while the port-channel on the switches are set as a layer 2 trunk.

    Hope this helps
  • cb3dwacb3dwa Member Posts: 80 ■■□□□□□□□□
    What would be the point of doing that anyway? It wouldn't accomplish anything.....

    god knows i didn't understand it either.
  • BeTheNetworkBeTheNetwork Member Posts: 18 ■□□□□□□□□□
    It doesn't matter what your order-of-operations is, if you built L2 physical interfaces into the PO first, the PO will not allow you to add an IP. And, vice-versa, if you provisioned a L3 PO and tried to add the l2 physical interfaces, the channel-group command would be rejected. When you provision interfaces on a Cisco IOS device, the device will have a finite number of hardware IDB's and software IDB's (Interface Descriptor Blocks). If you provisioned L2 interfaces, then the IDB allocation would be N+1 for HWIDBs and N for SWIDB (N = Channels/VLANs). For routed interfaces, the IDB allocation is 1 HWIDB and 1 SWIDB. A L3 PO can't be setup for 1 HWIDB and 1 SWIDB when the physical interfaces are set with SW N and HW N+1.

    IDB Documentation
Sign In or Register to comment.