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.
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.
