Vss vs.vpc. What's the difference?

1. "What are the differences between VSS and vPC?” Both are used basically to support
multi-chassis ether-channel that means we can create a port-channel whose one end
is device A, however, another end is physically connected to 2 different physical
switches which logically appears to be one switch.
There are certain differences as listed below:
-vPC is Nexus switch specific feature, however, VSS is created using 6500 series
switches
-Once switches are configured in VSS, they get merged logicaly and become one
logical switch from control plane point of view that means single control plane is
controlling both the switches in active standby manner, however, when we put nexus
switches into vPC, their control plane are still separate. Both devices are controlled
individually by their respective SUP and they are loosely coupled with each other.

2. In VSS, only one logical switch has been managed from management and
configuration point of view. That means, when the switches are put into VSS, now,
there is only one IP which is used to access the switch. They are not managed as
separate switches and all configurations are done on active switch. They are
managed similar to what we do in stack in 3750 switches, however, in vPC, the
switches are managed separately. That means both switches will have separate IP
by which they can be accessed, monitored and managed. Virtually they will appear
a single logical switch from port-channel point of view only to downstream devices.
VSS is single management and single configuration; we cannot use them for HSRP
active and standby purpose because they are no longer 2 separate boxes. In fact
HSRP is not needed, right?
One single IP can be given to L3 interface and that can be used as gateway for the
devices in that particular vlan and we will still have redundancy as being same ip
assigned on a group of 2 switches. If one switch fails, another can take over. However,
in vPC as i mentioned above devices are separately configured and managed, we
need to configure gateway redundancy same as in traditional manner.
For example: We have 2 switches in above diagram. Switch A and B, when we put
them in VSS, they will be accessed by a single logical name say X and if all are Gig
ports then interfaces will be seen as GigA01, GigA02....GigB01,GigB02 and so
on...
However, if these are configured in vPC, then they will NOT be accessed with single
logical name. They will be accessed/managed separately. Means, switch A will have
its own port only and so on B.

3. Similarly, in VSS same instances of stp,fhrp,igp,bgp etc will be used,however, in vPC
there will be separate control plane instances for stp,fhrp,igp,bgp just like they are
being used in two different switches.
In VSS, the switches are always primary and secondary in all aspects and one switch
will work as active and another as standby, however, in vPC they will be elected as
primary and secondary from virtual port-channel point of view and for all other
things, they work individually and their role of being primary/secondary regarding
vpc is also not true active standby scenario, however, it is for some particular failure
situation only. For example, if peer-link goes down in vpc, then only secondary switch
will act and bring down vpc for all its member ports.
VSS can support L3 port-channels across multiple chassis; however, vpc is used for
L2 port-channels only.
VSS supports both PAgP and LACP; however, VPC only supports LACP.
In VSS, Control messages and Data frames flow between active and standby via VSL,
however, in VPC, Control messages are carried by CFS over Peer Link and a Peer keep
alive link is used to check heartbeats and detect dual-active condition.