Spanning Tree Protocol (STP)

Question 1

Difference between Root ID and Bridge ID (STP)

Answer 1

Root ID is the ID of the Root Bridge
Bridge ID is the ID of the local bridge
If you are looking at the Root Bridge then the Rood ID and Bridge ID will be the same

Question 2

Root Costs

Answer 2

Speed Cost

10 Mbps 100
100 Mbps 19
1 Gbps 4
10 Gbps 2

Question 3

Steps of Spanning Tree Protocol (STP)

Answer 3

1) Switches elect a root brifdge by lowest beridge ID and all ports on that root bridge are set to fowarding state

2) All other switches select one of their ports to be its root port and it goes in a forwarding state
Roor Por Selection:
Lowest Root Cost
Lowest Neighbor Bridge ID
Lowest Neighboor Port ID

3) Each remaining collision domain will select one interface to be a designated port (forwarding) and one too be a non-designated port (blocking)

Question 4

What is PVST ?

Answer 4

Per VLAN Spanning Tree (PVST) is a Cisco protocol that allows a device to have multiple spanning trees, which can help improve network efficiency, security, and management. PVST prevents loops from forming in networks divided into virtual local area networks (VLANs) by allowing different root bridges to be created for each VLAN. For example, SW1 could be the root bridge for VLAN 10, and SW2 could be the root bridge for VLAN 20.

Question 5

What are Hello Bridge Protocol Data Units (BPDUs) ?

Answer 5

Hello BPDUs, also known as Configuration BPDUs, are a type of bridge protocol data unit (BPDU) that switches use to exchange information. They are the most common type of BPDU and include many details, such as the sending switch’s BID. Hello BPDUs are only sent by the root switch after the convergence of STP, which defines the root bridge and port’s roles and status. They are used for electing a road bridge, determining Porter rows and States, and the blocking arm onto the links. Hello BPDUs include the following information:

root switch ID
sender’s switch ID
sender’s root cost
Hello, MaxAge, and forward delay timers

Switch with the lowest Bridge ID (Bridge Priority + MAC Address) becomes the root bridge, every switch must have a path to root bridge and every port on the root bridge is in a forwarding state.

Question 6

What is Mac Address Flapping

Answer 6

Mac address flapping occurs when a switch receives packets from the same source MAC address into two different interfaces. The MAC address entry in the MAC address table then continually changes from port to port. This is usually an indication of a layer 2 loop.