Can someone explain Cisco Stackwise

Can someone tell me what is meant by "Layer 2 Neighborhood"?

I'm working on designing the logical topology for a new data center rollout. I read the Cisco Data Center Infrastructure 2.5 Design Guide and move on to the layered design. There are several options for this design, including Loop Layer 2 and Loop-Free.

The advantage of the loop design is the layer 2 neighborhood for servers that need it. The loop-free topology appears to limit the adjacency of Layer 2 "to a single pair of access switches".

I don't quite understand what that means. I would assume it is the number of layer 2 "hops" one server is away from another, but in both cases the traffic appears to be making a trip to the aggregation layer (assuming an L2 / L3 switch) to require to cross the Vlans. If it's the same VLAN, a good loop seems to be even worse, as servers on the same VLAN have to go down to the agg layer to communicate, as opposed to traveling straight to the connected switch with the loop-free example.

Can someone shed some light on my misunderstanding?


Layer 2 adjacency in Ethernet networks refers to the idea that a packet sent on a segment can reach its destination directly without traveling through a device that would modify the packet.

A simple example:
You have two computers with a cable in between; Whatever comes from one computer is essentially guaranteed to be received by the other computer. These devices have network adjacency.

More complicated:
You have two computers with an L3 switch in between, both of which are assigned the same vLAN. Here, too, what one computer sends to the other is received without question.

Breaking the neighborhood
Two computers are again present on the L3 switch, but one is connected to a port assigned to a vLAN and the other is connected to a trunk port. Now packets from the first computer arrive with a vLAN tag on the second computer (assuming a typical setup here ...)

Who cares? Good routing protocols (they usually need to be able to figure out the network topology, and that discovery can be broken by non-adjacency) as well as a variety of non-IP protocols. It is not too common for servers in a domain controller to experience these problems, but it is quite possible.

We use Layer 2 Adjacency for our VMmotion and Microsoft applications. In your examples above, the one with the L3 switch in between would actually break L2 adjacency. The way it has been explained to us and which of our L2 protocols need to work means the following. L2 adjacency means that the communicating devices must be in the same subnet. Example: has to communicate with his MNGT server at This would work as no L3 devices like a router or an L3 switch are going through. If you have 2 different subnets, your packet will have to push the router or L3 switch to view the routing table and decide where to send the packet. In this case your package would be changed. The source & destination IPs would stay the same, but when transmitted over L3 you will notice that your packet L2 information is the routing protocol like HDLC. So I would answer the question by saying that L2 adjacency means that the devices must be on the same local LAN.

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