I'm new to the vss world and I'm reading that a mec can be either a layer 2 or layer 3 type link. Is that true? Is one better than the other? And does anybody know of any documents that would discuss the pro's and con's?
Multichassis EtherChannel (MEC) is a Layer 2 multipathing technology. This form of EtherChannel allows a connected node to terminate the EtherChannel across the two physical Cisco Catalyst 6500 Series Switches that make up the VSS leading to creating simplified loop-free Layer 2 topology. Using MEC in VSS topology results in all links being active and at the same time provides for a highly available topology without the dependency of Spanning Tree Protocol. With the introduction of 12.2(33)SXI, the virtual switching system supports a maximum number of 512 MECs.
Ref can be taken from here .
Layer 2 MEC means that the Port-channel interface is a switchport (layer 2, no ip address).
Layer 3 MEC means that the Port-channel interface can be a routed interface, hence can have an ip address.
Here is another topic that explains everything :
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As the other posters have noted, yet you can do either.
As to whether one is better than the other, and the pros and cons, really not much different from any similar situation between a pair of L3 switches that support either a L2 or L3 link between them. Remember logically the VSS pair behaves as just one logical device. So, "forgetting" VSS (or 3750 type stacks), which would you chose between a pair of physical L3 switches and why?
Of course if the non-VSS device is L2 only, you wouldn't be able to configure L3 on both sides, yet you could still configure the VSS side as L3 rather than both sides being L2. In this case, again think of a physical pair, one L3 and one L2.