Spanning Tree

Question 1

How is the Switch’s Bridge ID generated?

Answer 1

Switch’s MAC Address + the Bridge Priority

Bridge Priority from 0 - 65535

Priority defaults to 32768

Question 2

In the case of a spanning tree Bridge Priority tie, which will be the Root Switch (Root Bridge)?

Answer 2

The one with the lowest MAC address.

Question 3

If a switch has multiple equal cost paths towards the Root Switch via multiple devices, how will it select the Root Port?

Answer 3

The Neighbour with the lowest Bridge ID (Switch Priority + MAC address)

Question 4

If a switch has multiple equal cost paths towards the Root Switch via the same device, i.e. multiple links between two switches, how will it select the Root Port?

Answer 4

Select the path with the lowest port priority on the other switch.
If priority tie, he lowest port number on the other switch wins.

Question 5

What are Root Ports in STP?

Answer 5

Ports with the lowest cost path back to the Root Switch.

• There will only be one root port (interface) per switch that is not the Root Switch.

Question 6

What is a STP designated port?

Answer 6

The best inbound ports to use to get to the Root Bridge on each network segment (based on cost).

1. In case of a cost tie, the port on the device with the lowest Bridge ID wins.
2. All Layer 2 network segments will have 1 designated port.
3. All ports on the Root Bridge are designated ports.
4. Designated ports will always be ports that point away from the Root Switch.

Question 7

What ports get blocked by Spanning Tree?

Answer 7

Ports that are not Root Ports, or Designatied Ports.
They are also called ‘Alternate Ports’ in Cisco Spanning Tree standards.

Question 8

What are the steps to manually identify Spanning Tree port status?

Answer 8

1. Find Root Switch (best Bridge ID)
2. All ports on Root Switch are Designated Ports
3. Find Root Ports on other switches (lowest cost to Root Switch on each device)
4. The other side of those links are going to be Designated Ports
5. On links that are left, find Designated Ports (best path back to Root Switch)
6. The rest are Blocking ‘designated ports’

Question 9

List types of industry standard Spanning Tree Protocols

(probably not in the test)

Answer 9

Open Standards

1. Spanning Tree Protocol (STP) - One Root Switch LAN, ignores VLANS, slow.
2. Rapid Spanning Tree Protocol (RSTP) - improved convergency time. Still ignores VLANS.
3. Multiple Spanning Tree Protocol (MSTP) - Supports grouping of VLANS into different spanning tree instances.

Question 10

List types of Cisco Spanning Tree Protocols

Answer 10

Cisco Standards

1. Per VLAN Spanning Tree Plus (PVST+) - Seperate Spanning Tree instance for every VLAN. This is the default on Cisco Switches. Slow.
2. Rapid Per VLAN Spanning Tree Plus (RPVST+) - improved convergency time. Still uses seperate Spanning Tree instance for every VLAN.

Question 11

Command to view Spanning Tree info:

Answer 11

SW1# show spanning-tree

Question 12

Highest or Lowest Root Bridge Priority number wins?

Answer 12

Lowest

Question 13

Command to configure a switch to be Root Switch in the enviroment?

Answer 13

Primary Switch:

CoreSwitch1(config)# spanning-tree vlan 10 root primary
CoreSwitch2(config)#spanning-tree vlan 20 root primary

Secondary Switch:

CoreSwitch1(config)# spanning-tree vlan 20 root secondary
CoreSwitch2(config)#spanning-tree vlan 10 root secondary

Question 14

What is PortFast?

Answer 14

Turns a switch port on (forwarding) before listening for Spanning Tree BPDU packets.

(can take up to 50 seonds if PortFast is not enabled)

Question 15

Command to enable PortFast

Answer 15

By Port:

SW1(config)# interface range f0/2 - 9
SW1(config-if-range)# spanning-tree portfast

All Ports by default:

SW1(config)# spanning-tree portfast default
SW1(config)# interface f0/24
SW1(config-if)# no spanning-tree portfast

Question 16

What is BPDU Guard?

Answer 16

If BPDU is recieved on a port, the port will be shut down.

This should work in conjunction with PortFast

Question 17

Commands to enable BPDU Guard

Answer 17

By Port:

SW1(config)# interface range f0/2 - 9
SW1(config-if-range)# spanning-tree bpduguard enable

All Ports by default:

SW1(config)# spanning-tree portfast bpduguard enable
SW1(config)# interface f0/24
SW1(config-if)# no spanning-tree bpduguard enable