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RSTP and MST Interview Questions and Answers

January 28, 2016

What are the Port Roles in RSTP?
1. Root port - It is the port on the switch that has the best root path cost to the root bridge. This is identical to 802.1D.
2. Designated port - The switch port on a network segment that has the best root path cost to the root.
3. Alternate port - A port that has an alternative path to the root, different from the path the root port takes. This path is less desirable than that of the root port.
4. Backup port - A port that provides a redundant (but less desirable) connection to a segment where another switch port already connects. If that common segment is lost, the switch might or might not have a path back to the root.

What are different port states in RSTP?
1. Discarding - Incoming frames simply are dropped; no MAC addresses are learned.This state combines the 802.1D Disabled, Blocking, and Listening states.
2. Learning - Incoming frames are dropped, but MAC addresses are learned.
3. Forwarding - Incoming frames are forwarded according to MAC addresses that have been learned.

Explain RSTP BPDU's?
RSTP distinguishes its BPDUs from 802.1D BPDUs as RSTP BPDU's version is set to 2. BPDUs are sent out every switch port at hello time intervals, regardless of whether BPDUs are received from the root. When three BPDUs are missed in a row, that neighbor is presumed to be down and all information related to the port leading to the neighbor is aged out. Also, some previously unused bits in the Message Type field are used in RSTP BPDU's.

What is Edge Ports and Point to point Ports?
Edge Port - Port on which end devices connect. Portfast is enabled on this port. As loop cannot form on this port so it can be placed immediately in forwarding state. If a BPDU is received on edge port , it looses its edge port status.
Point to point Port - These are ports that connect to other switch and becomes designated port. Full-duplex ports are considered point to point because only two switches can be present on the link.

Explain RSTP convergence in terms of proposal and agreement?
In RSTP, BPDUs are exchanged back and forth in the form of a proposal and an agreement. One switch proposes that its port becomes a designated port and if the other switch agrees it replies with an agreement message.

Explain TCN in RSTP?
In RSTP, TCN BPDU is not sent to root bridge instead the switch on which the change happens will itself send TCN BPDU to all other switches. BPDU's with their TCN bit set are sent out on all non-edge designated ports.

What is the command to change mode to RSTP?
Switch(config)# spanning-tree mode rapid-pvst

Explain MST?
Multiple Spanning Tree Protocol maps one or more vlans to single STP instance. Multiple instances of STP can be used with each instance supporting a different group of VLANs. Instance zero is by default on a switch. Any non-mapped Vlan is assigned to instance Zero.

What is MST region?
Every switch in a MST region runs MST with compatible parameters. Within the region, all switches must run the instance of MST that is defined by the following attributes:
1. MST configuration name.
2. MST configuration revision number.
3. MST instance-to-VLAN mapping table.
If two switches have the same set of attributes, they belong to the same MST region.

How two MST regions communicate?
Two MST regions communicate through CST (Common Spanning Tree).

Explain M-Record?
In MST, one switch calculates hash for particular instance and send it to other switch. Other switch will match priority in that hash with its own calculated hash and root bridge is elected.

Explain MST BPDU's?
The entire MST instance-to-VLAN mapping table is not sent in the BPDUs because the instance mappings must be configured on each switch. Instead, a digest or a hash code computed is sent. Switches comapre the received BPDU hash with its own hash.

How revision number in MST works?
The configuration revision number gives us a means of tracking changes to the MST region configuration. Each time we make changes to the configuration, we should increase the number by one. It is not increemented automatically.

What is the command to change mode to MST?
Switch(config)# spanning-tree mode mst.

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