RSTP and MST Interview Questions and Answers[CCNP]
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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