Does tagged l2 protocol frame would be processed ?

from88 · ‎03-13-2015

Hello all, long time - no see:)

I've one question:

Imagine the router or switch gets layer 2 transport packet for example: Cdp, Lldp, or Lacp. We all know if the router gets any of these packets the packet would be proccessed, but if all of mentioned protocols would be disabled on port, so they should've been forwarded ? yes ?

Basically my question is here: if the router or switch which is cdp, and lacp enabled is getting one of these frames, but tagged with vlan. Would vlan tag be the thing which forbids the packet to be processed ? To which frames field the device is looking when where'is a question the packet should be processed or not: to the destination mac, protocol Id or vlan tag ? Thanks!!

fsebera · ‎03-13-2015

Hi Kajatonas88,

Hint: OSI model

A layer 2 device will process up to layer 2; I.E. physical and data-link layers

A layer 3 device will process up to layer 3*; I.E. physical, data-link and network layers

* If a layer 3 device (eg. a router) is configured to process layers above layer 3 than it should process up to that configured layer.

HTH

Frank

Richard Burts · ‎03-13-2015

The original poster asks an interesting question "To which frames field the device is looking when where'is a question the packet should be processed or not: to the destination mac, protocol Id or vlan tag ?" I like Frank's suggestion that we consider the OSI model and would suggest that we consider the answer in this way:

- first the router or switch considers the vlan tag. If the router or switch interface is configured for vlan tag processing (trunk port instead of access port if on switch or sub interface with dot1q if on router) then it continues to examine the frame. If the interface is not configured for vlan tag processing then the frame is dropped and there is no further examination.

- next the router or switch considers the destination mac address to determine if the frame is one that it should process (if unicast mac is this device the destination, if multicast does this device participate in that multicast group, if broadcast then yes continue to process). If the mac address does not include this interface then drop the frame and no further examination.

- next the router or switch considers the protocol id to determine whether it should process the frame.

HTH

Rick

HTH

Rick

from88 · ‎03-13-2015

thanks now i have more clarity but still.. another simmilar situation:

the port is configured as trunk with allowed vlan 10 and CDP enabled. And here to that port comes the CDP packet in double tagged (QinQ) frame with outer vlan 10 and inner vlan 5. So, because of outer vlan the packet will not be discarded, but do it will be processed or forwarded further ?;)