Hello Ali,

VRRP is standard based and can work in a multivendor context

HSRP is Cisco proprietary.

The two protocols provide default gateway redundancy

see

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&topicID=.ee71a06&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.2cd23312

b) MST and PVST

Per vlan STP executes an STP instance for each L2 vlan that has a port (access or trunk) associated to it.

MST 802.1s standard allows to use up to 16 STP instances to represent 4094 vlans.

so in MST multiple vlans are associated to a single STP instance (each instance is rapid SPT 802.1w)

this provides scalability

Hope to help

Giuseppe