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.
-In VSS, only one logical switch has be 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 configuration 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.
-As i said, VSS is single management and single configuration, we can not use them for HSRP active and standby purpose because they are no longer 2 seperate boxes. Infact 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 GigA\0\1, GigA\0\2....GigB\0\1,GigB\0\2 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.
-Similary, 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 individualy 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 keepalive link is used to check heartbeats and detect dual-active condition.
Hope this answers your query.
Regards
Inayath
