Why we need VPC?

Initially when I heard of VPC, I neither understand the advantage of it nor its difference with VSS. Below I tried to explain the difference between VPC and VSS and the legacy setup where STP is being used to prevent L2 loops. But STP has many limitations which are discussed below:-

1.Suboptimal Path:-  To understand it, take a look to  the below topology where  three switches are connected to provide complete redundant path .

The problem with this design is, STP will block the port Gi0/3 of Sw-2. And hence traffic instead of taking direct route from SW-1 to SW-3, will reach to SW-3 via SW-1 and is known as suboptimal path. It adds extra hop in the path and reduces the efficiency of the network.

 2.Underutilization of uplink bandwidth:-

STP prevents the layer-2 loop by blocking the redundant path which is an advantage but in way reduces the uplink bandwidth which sometimes creates the congestion in the network.

Refer to the below diagram, traffic from SW-3 to internet has two path but due to spanning tree Gig0/3 of SW-3 is in blocking state. It will reduce the uplink bandwidth available to the SW-3.

3.Inefficiency: - Let’s assume the traffic is load share between SW-1 and SW-2 and both switches advertise the user subnet from same metric. There is no problem when the return traffic hit the SW-1 but what will happen when the very first return traffic that hits SW-2.

Does SW-2 have the mac-address of PC-1? Generally NO!

 SW-2 will send the unknown broadcast for the mac-address and if there are many users sitting in the LAN, unknown unicast will not only create the unnecessary traffic but it also impacts the CPU utilization of switches.

By using VSS in 6500, both the switches will virtually become one. One sup is active at a time which will control the data plan of both the chassis. It not remove the layer 2 loop from the network  but also remove the sub-optimal path and inefficiency problem which we had in our legacy environment.

As you can see there is neither a suboptimal path nor there is problem of reduced uplinks. It also removed the unnecessary unknown unicast issue.

But in VSS, control plane is active only on one switch whereas data plane is active on both the switches. As only one Sup is active the overall throughput is limited and other SUP capacity is gone wasted.

Advantage with VPC is not only it removes the above stated problems but also control and data plane of both the chassis are active at the same time. It increases the overall throughput of the system.

In the below design, traffic from PC-1 can directly reach PC-2 with adding any hop.

Also in the below design, traffic from PC-1 can go to internet via SW-1 or SW-2 depending open the hashing algorithm of SW-3. Also it removes the problem of unknown unicast in case of asymmetric routing as both the switch will be appearing as one.