Virtual Switching System 1440

VSS is similar to 3750 stacking but is not a "port" and is much more robust. With VSS, both supervisors are active. The VSS link keeps both supervisors in sync with each other. Updates to the mls and routing tables are sent to the other supervisor, so that each "knows" what the other "knows". One supervisor is called "active" and the other "standby" but this is a misnomer because the "active" one is the one that is running the routing protocols, and the control plane management, but both are making switching and routing decisions independent of the other supervisor. (This is why it is called VSS 1440, each supervisor has 720Gb/s of switching capacity and since both are "active" you now have 1,440Gb/s of switching capacity.)

If the supervisor in a VSS pair fails, the entire chassis is dead; but the other will keep working. If the "active" supervisor fails, the backup takes over the routing protocols and control plane functions within 200ms, with out loss of connectivity. (Routing protocols do NOT reset after a switch over) If the VSS link fails but both supervisors are still working, there is also a mechanism to detect this (that you must configure) and the active supervisor will shut down all its ports to prevent both chassis from being "active". The reason this is done is that if both supervisors are running the routing protocols and management control plane, the rest of the network will get confused because you now have two independent switches running with the same ip addresses.