In an imaginary Cisco world every device would be able to talk with every other device in various layers. In the actual Cisco world, some devices can talk to some devices, while they can't talk to some other devices.
I'm talking specifically about L2 Control Protocols (L2CPs), when these need to be exchanged between different devices in order to support a requirement (i.e. create a spanning-tree loop). Cisco's L2 Protocol Tunneling (L2PT) can help in accomplishing some of these cases.
So let's suppose you have a scenario like the following.
When using the simplest form of devices (L2 switches like 3750), you can just tunnel the L2CPs between devices S1 and S2 and everything will be fine. Spanning tree running on devices C1-C4 will see a loop and will block a port depending on various parameters (priority, cost, etc).
As you move ahead and start to replace the S1 or S2 device with another (usually better), you realize that the new device supports a different way of handling L2CPs, which might be "incompatible" with the old way.
Generally, you can do the following actions on L2CPs as they enter a port:
forward: frame is forwarded to another device without any change (no local processing takes place)
drop: frame is dropped
peer: frame is processed/terminated locally
tunnel: frame is tunneled to another device after changing the destination mac address (L2PT)
Tunneling is quite common is scenarios like the above, where you need to pass the L2 frame across a L2 domain, without having the intermediate devices act upon it.
You can also achieve the same result with forwarding, as long as you don't have a native L2 domain in between, because you might end up mixing local protocols with protocols that just pass over.
It's obvious that you cannot have tunneling on one side and forwarding on the other side, because exchanged frames won't be able to "talk" each other. i.e. for STP one side will tunnel the frame by changing the destination mac address from 01-00-0c-cc-cc-cd (or 01-80-c2-00-00-00) to 01-00-0c-cd-cd-d0, while the other side will just forward the frame by keeping the original destination address of 01-00-0c-cc-cc-cd (or 01-80-c2-00-00-00).
Below you'll find a list with all available options regarding the handling of L2CPs on some known platforms:
|ME-3800X||L2 switchport||l2protocol drop||l2protocol peer|
|ME-3800X||L2 service instance||l2protocol forward (1)||l2protocol peer||l2protocol tunnel|
|7600/ES||L3||l2protocol drop||l2protocol peer|
|7600/ES||L3 service instance||l2protocol forward|
|ASR9000||L2 transport||by default (2)||l2protocol tunnel|
As you can see, you cannot have L2 communication between a service instance on a 7600/ES and one of the smaller platforms, because 7600/ES doesn't support tunneling and the smaller platforms do not support forwarding. Actually, the biggest surprise to me was the lack of support of L2PT on the 7600 with the ES cards when using service instances. I had the impression that this would be the most feature rich platform.
Cisco's proposal is to use the same platform for such scenarios, because they haven't verified anything else and some platforms were built to be used in specific ways. So instead of supporting the same feature (L2PT was their idea after all) along the range of platforms, you should always replace them in pairs. And if by accident, you happen to have more S devices serving many overlapping rings, then you have to replace all of them.
I would prefer, instead of promoting new platforms or new designs, to focus on fixing the existing platforms, so they can cooperate with each other. After all, if a platform is good enough, it will get its share in the market.
Also, the online documentation is quite incomplete on this area. You have to guess what will happen in most cases. We had to open 3 different cases and involve our account team in order to clarify things and push for fixing the documentation. Not surprisingly enough, the peering functionality is another mess. I'll probably need to write another post describing all available options (which lead to different behavior) on these platforms.
1) "l2protocol forward" on ME-3800X will become available in the next major release. Thanks to Cisco for giving.us the chance to try it earlier.
2) This is the default behavior according to Xander's doc here.
3) Arie asked me to add some extra information about PW/MST/REP/PVST-AG (and all these L2 HA) scenarios. I'll try to write a new post as soon as i find enough free time to test them.