Chapter 2 - Spanning Tree Protocol

What is the primary purpose of Spanning Tree Protocol (STP) in a redundant Layer 2 topology?

To build a loop-free topology by temporarily blocking traffic on redundant ports

Why can't Layer 2 forwarding loops be resolved the way Layer 3 loops are?

Because there is no time-to-live (TTL) mechanism for Layer 2 forwarding, so looped frames circulate indefinitely

List the five iterations/versions of Spanning Tree Protocol mentioned as part of its evolution.

802.1D (original), PVST, PVST+, 802.1W (RSTP), and 802.1S (MST)

Which three STP modes do modern Catalyst switches operate in, and are they backward compatible with 802.1D?

PVST+, RSTP, and MST; yes, all three are backward compatible with 802.1D

Name the six 802.1D port states.

Disabled, Blocking, Listening, Learning, Forwarding, and Broken

In 802.1D, which port state allows a port to receive BPDUs but not modify the MAC address table or forward other traffic?

Blocking

In 802.1D, which port state allows a port to send or receive BPDUs but still cannot forward other network traffic?

Listening

In 802.1D, which port state allows a port to update the MAC address table but still not forward non-BPDU traffic?

Learning

What is the final 802.1D port state in which a port both forwards traffic and updates the MAC address table?

Forwarding

What does the 802.1D 'Broken' port state indicate?

That the switch detected a configuration or operational problem on the port, causing it to discard packets while the problem persists

Roughly how long does it take an 802.1D port to go from blocking to forwarding under default timers?

About 30 seconds

Name the three port types defined by the 802.1D standard.

Root port (RP), designated port (DP), and blocking port

How many root ports per VLAN should exist on a single switch?

Only one

How many active designated ports should exist on a given link?

Only one

What destination MAC address do BPDU frames use?

01:80:c2:00:00:00

Name the two BPDU types used by STP.

Configuration BPDU and Topology Change Notification (TCN) BPDU

Which BPDU type is used to identify the root bridge, root ports, designated ports, and blocking ports?

The configuration BPDU

List the fields carried in a configuration BPDU.

STP type, root path cost, root bridge identifier, local bridge identifier, max age, hello time, and forward delay

Which BPDU type communicates a change in the Layer 2 topology to other switches?

The Topology Change Notification (TCN) BPDU

How is root path cost defined?

The combined (cumulative) interface cost along a specific path toward the root bridge

The system priority field used in root bridge elections is how many bits wide, and what is its default value?

4 bits; default value of 32,768

What does the 12-bit system ID extension in a bridge identifier represent?

The VLAN that the BPDU correlates to

A bridge's root bridge identifier is a combination of which three values?

The root bridge's system MAC address, system ID extension, and system priority

By default, how long does a bridge port retain BPDU information before assuming it is stale, and which command changes this value?

20 seconds (Max Age); spanning-tree vlan vlan-id max-age maxage

What is the default hello time interval for BPDU advertisement, and what range of values can it be configured to?

2 seconds by default; configurable from 1 to 10 seconds with spanning-tree vlan vlan-id hello-time hello-time

What is the default forward delay, and what does this timer govern?

15 seconds by default; it is the time a port spends in the listening and learning states, configurable from 4 to 30 seconds with spanning-tree vlan vlan-id forward-time forward-time

True or false: the terms 'bridge' and 'switch' are used interchangeably when discussing STP.

True — STP predates modern switches, and the original devices were called bridges

In short mode, what reference speed is used for calculating STP interface cost, and how many bits is the cost value?

20 Gbps reference speed; a 16-bit value

What is 'long mode' STP path cost, and why was it introduced?

A 32-bit cost value using a 20 Tbps reference speed, introduced because higher-speed interfaces made the original short-mode values insufficient

What is the short-mode STP cost assigned to a 1 Gbps interface?

4

What is the short-mode STP cost assigned to a 10 Gbps interface?

2

What is the short-mode STP cost assigned to a 100 Mbps interface?

19

What is the long-mode STP cost assigned to a 1 Gbps interface?

20,000

What command enables long-mode STP path cost calculation, and why should it be applied consistently?

spanning-tree pathcost method long; every switch in the Layer 2 topology should use the same setting to keep the topology consistent

When a switch first initializes and has not yet heard from any neighbors, which bridge does it assume itself to be?

The root bridge (it uses its own local bridge identifier as the root bridge identifier)

If a switch receives a neighbor's configuration BPDU that is superior to its own, what does it do?

It updates its own BPDUs to advertise the new root bridge identifier and a new root path cost reflecting the total cost to reach that root bridge

When comparing two bridge identifiers with equal priority, which switch is preferred as the more superior bridge?

The one with the lower system MAC address

Which field is empty in the output of show spanning-tree root when run on the root bridge itself?

The Root Port field, since the root bridge has no root port (all its ports are designated ports)

On SW1 (the root bridge) for VLAN 10, show spanning-tree root shows a priority of 32,778. How is this value derived?

It is the base bridge priority (32,768) plus the sys-id-ext, which equals the VLAN number (10)

Does the root path cost advertised in a BPDU include the cost of the port the BPDU is sent out of?

No — the receiving switch adds the cost of its own receiving interface to the advertised root path cost

What is the root path cost value on the root bridge itself?

Always zero

List, in order, the five tiebreaker criteria used to select a switch's root port.

1) Lowest path cost to the root; 2) lowest system priority of the advertising switch; 3) lowest system MAC address of the advertising switch; 4) lowest port priority on the advertising switch (for parallel links to the same switch); 5) lowest port number on the advertising switch

When two non-root switches are directly connected on what would otherwise be two designated ports, what must happen to prevent a loop?

One of the two ports must transition to a blocking state

Between two non-root switches whose link would otherwise have two designated ports, which switch's port is placed into blocking if their path costs to the root are tied and their system priorities are tied?

The switch with the higher system MAC address has its port blocked (the lower MAC address wins)

Which command displays STP role, state, cost, and priority for every interface on a switch, per VLAN?

show spanning-tree [vlan vlan-id]

In show spanning-tree output, what does the port type 'P2p Edge' indicate?

That STP portfast is enabled on that port

In show spanning-tree output, what does a Type field showing *TYPE_Inc - usually indicate?

A port configuration mismatch with the connected switch, often an incorrect port type or a trunk/access mode mismatch

Which command narrows STP output to a single interface across all its VLANs, and what optional keyword adds per-VLAN counters like BPDUs sent/received?

show spanning-tree interface interface-id, with the optional detail keyword

If a VLAN is missing from show spanning-tree interface output on a trunk port, what is a common cause to check?

The VLAN may be missing from that trunk port's allowed VLANs list

When a switch detects a link status change, in which direction does it send the TCN BPDU?

Out of its root port, toward the root bridge

After the root bridge receives a TCN, what does it do?

It generates a new configuration BPDU with the Topology Change flag set and floods it to all switches in the topology

When a switch receives a configuration BPDU with the Topology Change flag set, what does it do to its MAC address table aging?

It temporarily reduces the MAC address aging timer to the forward delay value (default 15 seconds) to flush stale entries

What is a side effect of temporarily shortening the MAC aging timer after a topology change?

A temporary increase in unknown unicast flooding while the MAC address table is rebuilt

After the topology-change period ends, what does the MAC address aging timer return to by default?

300 seconds

Are TCNs generated per switch or per VLAN?

Per VLAN

Which command shows how many topology changes have occurred and how long ago the last one happened?

show spanning-tree [vlan vlan-id] detail

Scenario: the link between two non-root switches fails, and one side's port toward the other was already in a blocking state. What is the impact on traffic?

None — traffic between the two switches was already flowing through the root bridge, so no forwarding path is lost (though a TCN is still generated)

Scenario: the root bridge loses its direct link to a downstream switch whose only other path was blocking. Roughly how long does that downstream switch take to reconverge, and why?

About 30 seconds — 15 seconds in listening plus 15 seconds in learning before the previously blocked port becomes the new root port

Why doesn't the root bridge itself ever generate a TCN BPDU?

Because the root bridge has no root port to send a TCN out of — it only originates configuration BPDUs with the Topology Change flag when notified by others

Scenario: a link fails between the root bridge and a non-root switch, but the remaining path to that switch runs through another switch whose port toward it is in a blocking state. What must expire before that blocking port can transition, and how long does full reconvergence take?

The STP Max Age timer (20 seconds by default) must expire before the port moves to listening; total reconvergence is about 50 seconds (20s Max Age + 15s listening + 15s learning)

What distinguishes an 'indirect' link failure from a 'direct' link failure in STP?

In an indirect failure the physical link stays up but STP communication (BPDUs) is impaired or filtered, so the failure must be detected by timer expiration rather than a Layer 1 down event

For an indirect link failure, what has to happen before the alternate port can begin transitioning toward forwarding?

The receiving switch's Max Age timer must expire, flushing the cached root port BPDU entry

What was the main scalability limitation of original 802.1D that Cisco's PVST/PVST+ addressed?

802.1D used only a single spanning-tree topology, preventing per-VLAN traffic engineering such as load balancing across redundant links

Which IEEE specification incorporated the concepts of PVST/PVST+ along with faster-convergence enhancements?

802.1W, Rapid Spanning Tree Protocol (RSTP)

RSTP reduces the number of port states from the traditional five (802.1D, excluding Broken) down to how many, and what are they?

Three: Discarding, Learning, and Forwarding

Which three traditional 802.1D states are merged into the single RSTP 'Discarding' state?

Disabled, Blocking, and Listening

Name the four RSTP port roles.

Root port, Designated port, Alternate port, and Backup port

What distinguishes an RSTP alternate port from a backup port?

An alternate port provides a different path toward the root bridge through another switch, while a backup port provides redundancy toward the same shared segment (typically via a hub)

Name the three RSTP port types used to help build the topology.

Edge port, Non-Edge port, and Point-to-point port

Which RSTP feature directly corresponds to an 'Edge port'?

STP portfast

What defines an RSTP 'Non-Edge' port?

A port that has received a BPDU

What condition must a link satisfy to be classified as an RSTP point-to-point port?

It must connect to another RSTP switch and operate at full duplex

Why is duplex status used as the quick test for RSTP point-to-point eligibility?

Because full-duplex links cannot support more than two devices on a segment, while half-duplex (shared, hub-like) segments can

If an RSTP-enabled port can only negotiate half duplex, how must it behave?

It must fall back to traditional 802.1D forwarding behavior

If an RSTP switch fails to receive an RSTP handshake from its neighbor, what does it assume, and what is the consequence?

It assumes the neighbor is not RSTP-capable and falls back to 802.1D behavior, which can introduce the traditional ~30-second transmission delay

What is the first thing two newly connected RSTP switches verify before starting synchronization?

That the link between them is point-to-point, by checking that it is full duplex

During RSTP synchronization, what do two newly connected switches exchange to each propose becoming the designated port for the segment?

A proposal carried in a configuration BPDU

During RSTP synchronization, how is it decided which of two connected switches is 'superior' and which is 'inferior'?

Using the same bridge identifier comparison as 802.1D: lowest system priority first, then lowest system MAC address

When the inferior switch in an RSTP synchronization recognizes its status, what does it do to its local port and to all of its non-edge ports?

It marks its local port as the root port and moves all of its non-edge ports to a discarding state, stopping local switching on them

After marking its root port, what does the inferior switch send back to the superior switch to signal synchronization is underway?

An agreement, carried in a configuration BPDU

Once the agreement is sent, what state do the inferior switch's root port and the superior switch's designated port move to?

Forwarding

After completing synchronization with its upstream neighbor, what does the inferior switch do next?

It repeats the same proposal/agreement synchronization process with any of its own downstream switches

Using default timers, after how many missed hello messages does RSTP consider a port's BPDU information stale?

Three consecutive hello intervals

With default timers, how long does it take RSTP to age out missing neighbor information, compared to 20 seconds for 802.1D's Max Age?

About 6 seconds

What happens if a downstream switch fails to acknowledge an RSTP proposal during synchronization?

The RSTP switch falls back to 802.1D-style behavior to avoid creating a forwarding loop