• A. PVST
• B. RSTP
• C. STP
• D. MVST
• E. MPVST

Explanation:

Let’s go through the image shown below. All three switches are connected to each other and we need to ensure that spanning tree creates a loop free topology. So we have to block the links depending on which switch is the root bridge.

Let’s says that we create a VLAN 10 inbetween switch1 and switch 2. If we consider link Fa0/14 only between SW1 and SW2 then it is a loop free link but we still have to run spanning tree and spanning tree is run for each VLAN.

Taking another example where we have VLAN 20 comprising of SW1, SW2 & SW3. Here we have to calculate spanning tree for each vlan in order to make it loop free. This is because the physical topology is different than the logical topology. This is what is called as Per Vlan Spanning tree. Please see that this is a Cisco proprietary protocol.

ISL Trunking is used here to allow VLAN traffic for some VLANs while others are blocked. PVST can also do load balancing by forwarding some VLANs on one trunk and other VLANs on other trunk. As it runs one STP per VLAN, it will have more overheads.

Incorrect Answers:

B. RSTP: RSTP is defined in IEEE 802.1w. For the reason as STP can’t provide faster convergence, so this STP-evolution from one point of view, but not separate instance of each VLAN as well. For example, if you connect a host to a switch, it goes through the process of listening & learning and will take approx. 30 seconds to turn into forwarding. If we enable the feature called port fast (Cisco Proprietary) the edge port will immediately turn into a forwarding port thus by saving 30 seconds. In short, if you are using RSTP, then you do not need to enable portfast on the switch and this is the way through which we speed up convergance on the access ports.

C. STP: Defined in IEEE 802.1D, this is the original standard that provided a loop-free topology in a network with redundant links. It is enabled by default on all Cisco Catalyst switches. STP stops broadcast storms, Multiple frame copies & Database Instability.

D. MSTP: defined in IEEE 802.1s, maps multiple VLANs into the same spanning-tree instance. MSTP implemented by Cisco is often referred to as Multiple Spanning Tree (MST). Here we can define set of VLANs to be mapped. For ex, if I have 20 VLANs and I am using PVST then I will have 20 STP running. In MSTP, we can run multiple instances by grouping 20 VLANs (1 to 20) into 2 instances by grouping them (1 to 10) and from (11 to 20).

e. MPVST: No such protocol exists.