CCNA Day 20 - STP

Blocking and forwarding

What are the two ‘stable’ STP port states?

Listening and learning

What are the two ‘transitional’ STP port states?

Blocking state

What state are non-designated ports in?

Listening, Learning, and Forwarding

What STP Port states Send BPDUs?

Blocking, Listening, Learning, and Forwarding

What STP port state receives BPDUs?

Forwarding

What STP port states forward frames?

Learning and Forwarding

What STP port states learn MAC Addresses?

15 seconds

What is the default STP forward delay timer?

2 seconds

What is the default STP hello timer?

20 seconds (10*Hello timer)

What is the default STP max age timer?

Designated

STP BPDUs are only forwarded on [designated/non-designated] ports.

Can

A forwarding interface [can/can’t] move directly to a blocking state.

Can’t

A blocking interface [can/can’t] move directly to a forwarding state.

0100.0ccc.cccd

PVST+BPDUs are sent to destination MAC address:

Per VLAN Spanning Tree

What is PVST?

0180.c200.0000

Standard STP (Not PVST) BPDUs are sent to destination MAC address:

Root Bridge

The STP timers on the […] determine the timers used for all switches in the network

Portfast

Allows an STP port to move immediately to the forwarding state

BPDU Guard

If enabled on an interface, will shut down the interface if it receives a BPDU from another switch.

spanning-tree portfast

Enable portfast on a switch port:

SW1(config-if)#[…]

spanning-tree portfast default

Enable portfast on all access ports on the switch:

SW1(config)#[…]

spanning-tree bpduguard enable

Enable BPDU guard on a switch port:

SW1(config-if)# […]

spanning-tree portfast bpduguard default

Enable BPDU Guard on all Portfast-enabled interfaces:

SW1(config)#[…]

Root Guard

An optional STP feature that prevents a newly connected switch from becoming the root bridge.

Loop Guard

An optional STP feature that shuts down an interface if it stops receiving BPDUs.

spanning-tree mode pvst

Configure spanning tree in PVST mode:

SW1(config)#[…]

spanning-tree vlan [vlan-id] root primary

Configure the spanning-tree root for a VLAN:

SW1(config)#[…]

24576 or 4096 less than the current root’s priority

What is the STP priority after running spanning-tree vlan vlan-id root primary?

spanning-tree vlan [vlan-id] root secondary

Configure the spanning-tree secondary root for a VLAN:

SW1(config)#[…]

28672

What is the STP priority after running spanning-tree vlan vlan-id root secondary?

spanning-tree vlan [vlan-id] cost [cost]

Configure the STP cost of an interface:

SW1(config-if)#[…]

spanning-tree vlan [vlan-id] port-priority [priority]

Configure the STP priority of an interface:

SW1(config-if)#[…]

Listening and Learning

What states does portfast allow a port to skip?

Forwarding

What state does a portfast-enabled port immediately enter when it comes online?

Switch

Portfast should not be enabled on a port connected to a […]

spanning-tree portfast [edge]

Configure Portfast on an individual interface:

SW1(config-if)#[…]

spanning-tree portfast [edge] default

Enable portfast by default:

SW1(config)#[…]

Access

Configuring PortFast in global config mode enables it on all […] ports

show spanning-tree interface [interface-name] detail

View detailed STP information about an interface:

SW1#[…]

spanning-tree portfast [edge] trunk

Configure PortFast on a trunk port:

SW1(config-if)#[…]

Yes

Does a PortFast-enabled port send BPDUs?

All PortFast-enabled ports

When BPDU Guard is enabled by default, which ports is it active on?

Yes

Does a PortFast-enabled port send BPDUs?

It will revert to acting like a regular STP port (without portfast)

What happens when a PortFast-enabled port receives a BPDU?

BPDU Guard

What STP feature err-disables a switch port if it receives an STP BPDU?

spanning-tree bpduguard enable

Enable BPDU Guard on a port:
SW1(config-if)#[…]

spanning-tree portfast [edge] bpduguard default

Enable BPDU Guard by default:

SW1(config)#[…]

All PortFast-enabled ports

When BPDU Guard is enabled by default, which ports is it active on?

spanning-tree bpduguard disable

Disable BPDU Guard on a specific port (after it was enabled by default):

SW1(config-if)#[…]

ErrDisable Recovery

Which feature can automatically re-enable err-disabled ports?

show errdisable recovery

View the status of ErrDisable Recovery on the switch:

SW1#[…]

300 Seconds

What is the default ErrDisable Recovery Interval?

errdisable recovery interval [seconds]

Modify the ErrDisable Recovery interval:

SW1(config)#[…]

errdisable recovery cause [cause]

Enable ErrDisable Recovery for a particular cause:

SW1(config)#[…]

BPDU Filter

Which STP feature prevents a switchport from sending BPDUs?

spanning-tree bpdufilter enable

Enable BPDU Filter on a port:

SW1(config-if)#[…]

spanning-tree portfast [edge] bpdufilter default

Enable BPDU Filter by default:

SW1(config)#[…]

spanning-tree bpdufilter disable

Disable BPDU Filter on a port (after it has been enabled by default):

SW1(config-if)#[…]

The port ignores the BPDU

What happens if a port with BPDU Filter enabled at the interface level receives a BPDU?

PortFast and BPDU Filter are disabled and the port operates as a normal STP port

What happens if a port with BPDU Filter enabled at the global level receives a BPDU?

All PortFast-enabled ports

When BPDU Filter is enabled by default, which ports is it active on?

Root Guard will disable a port if it receives a superior BPDU?

What would cause Root Guard to disable a port?

spanning-tree guard root

Enable Root Guard on a port:

SW1(config-if)#[…]

Broken (Root Inconsistent)

If a Root Guard-enabled port receives a superior BPDU, what state will it enter?

Broken

In the output of show spanning-tree, what does BKN mean?

Root Inconsistent

In the output of show spanning-tree, what does ROOT_Inc mean?

When it stops receiving superior BPDUs?

When does a port disabled by Root Guard get re-enabled?

Unidirectional Link

A network link where data transmission occurs in only one direction.

Layer 1

Issues with what layer cause Unidirectional Links?

Loop Guard

What optional STP feature protects against loops caused by unidirectional links?

Broken (Loop Inconsistent)

When a Loop Guard-enabled port’s Max Age counts down to 0, it enters this state

spanning-tree guard loop

Enable Loop Guard on a port:

SW1(config-if)#[…]

spanning-tree loopguard default

Enable Loop Guard by default:

SW1(config)#[…]

spanning-tree guard none

Disable Loop Guard on a port:

SW1(config-if)#[…]

All ports

On which ports does the command spanning-tree loopguard default enable Loop Guard?

No

Can Loop Guard and Root Guard be enabled on the same port at the same time?

When it starts receiveing BPDUs again

When will a Loop Guard-enabled port automatically recover?