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【CCNP ENCOR OCG】CHAPTER 2》Spanning Tree Protocol

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“Do I Know This Already?” Quiz

Foundation Topics

Spanning Tree Protocol Fundamentals

802.1D Port States

Spanning Tree Path Cost

Root Bridge Election

Locating Root Ports

Locating Blocked Designated Switch Ports

Verification of VLANs on Trunk Links

STP Topology Changes

Key Topics

802.1D port types

STP Key Terminology

Root Bridge Election

Locating Root Ports

STP Topology Changes

Rapid Spanning Tree Protocol

RSTP (802.1W) Port States

Building the RSTP Topology

Key Terms

configuration BPDU

hello time

Practice Test

白皮书主要涉及：802.1D

“Do I Know This Already?” Quiz

interface port cost value (original and new)

STP port states :

Foundation Topics

Spanning Tree Protocol Fundamentals

802.1D Port States

Six states: Disabled (administratively off, that is, shutdown)、Blocking-listening-learning-Forwarding、Broken

Spanning Tree Path Cost

Root Bridge Election

more preferable：bridge identifier is lower (BID: priority、MAC address), such as RID:

The priority in the configuration BPDU packets is actually the priority plus the value

of the sys-id-ext (which is the VLAN number). You can confirm this by looking at VLAN

10, which has a priority of 32,778, which is 10 higher than 32,768.

When a switch generates the BPDUs, the root path cost includes only the calculated metric

to the root and does not include the cost of the port that the BPDU is advertised out of. The

receiving switch adds the port cost for its interface on which the BPDU was received in con

junction with the value of the root path cost in the BPDU. The root path cost is always zero

on the root bridge. Figure 2-2 illustrates the root path cost as SW1 advertises the configura

tion BPDUs toward SW3 and then SW3’s configuration BPDUs toward SW5.

Locating Root Ports

1.path cost

2.system priority of the advertising switch

3.system MAC address of the advertising switch

4.port priority from the advertising switch

5. port number from the advertising switch

Locating Blocked Designated Switch Ports

0.a designated port and must not be considered an RP

1.path cost

2.system priority of the local switch

3.system MAC address of the local switch

The command show spanning-tree [vlan vlan-id] provides useful information for locating a
port’s STP state. contains "root bridge’s information" and "local bridge’s information".

These port types are expected on Catalyst switches:

■ Point-to-point (P2P): This port type connects with another network device (PC or

RSTP switch).

■ P2P edge: This port type specifies that portfast is enabled on this port.

Verification of VLANs on Trunk Links

All the interfaces that participate in a VLAN are listed in the output of the command show

spanning-tree . Using this command can be a daunting task for trunk ports that carry mul tiple VLANs. The output includes the STP state for every VLAN on an interface for every switch interface.The command show spanning-tree interface interface-id [detail] drastically reduces the output to the STP state for only the specified interface.

STP Topology Changes

In a stable Layer 2 topology, configuration BPDUs always flow from the root bridge toward
the edge switches.

The switch that detects a link status change sends a topology change notification (TCN)
BPDU toward the root bridge, out its RP.

Upon receipt of the TCN, the root bridge creates a new configuration BPDU with the Topol ogy Change flag set, and it is then flooded to all the switches.When a switch receives a con figuration BPDU with the Topology Change flag set, all switches change their MAC address
timer to the forwarding delay timer (with a default of 15 seconds). This flushes out MAC
addresses for devices that have not communicated in that 15-second window but maintains
MAC addresses for devices that are actively communicating.

The MAC address timer is then reset to normal (300 seconds by default) after the second configuration BPDU is received.

Topology changes are seen with the command show spanning-tree [vlan vlan-id] detail on a
switch bridge.

Key Topics

802.1D port types

RP、DP、Blocking ports. W hich kind of devices RP and DP respectively connective to?

root bridge or an upstream switch; downstream devices and switches

STP Key Terminology

Respective function of two types of BPDUs

Configuration BPDU for root bridge and port roles eleciton; TCN BPDU for topology changes communication.

Root Bridge Election

Criterion、show command、the priority vlalue in the configuration BPDU、calculation of the root path cost

Lowest: 1.Priority、2.MAC

show spanning-tree root

Priority + sys-id-ext (vlan)

port cost ( of interface which BPDU was received) + root path cost (in the BPDU, The root path cost is always zero)

Locating Root Ports

Criterion

Lowest: 1.path cost、2.priority、3.MAC (4.port priority、5.port number, when multiple linkes exist)

STP Topology Changes

1.TCN BPDU and configuration BPDU's sender and receiver

TCN BPDU is sent from the switch that detects a link status change to root bridge (so root bridge doesn't need to send TCN; Configuration BPDU with the Topology Change flag set is sent from root bridge to all switches.

2.What action it will do when switch receive the 2nd BPDU above?

Change MAC address timer to the forwarding delay timer

3.Show command that can see the topology change information, and 3 key infor

show spanning-tree [ vlan vlan-id] detail

topology change count、time since the last change has occurred、 interface from which the TCN originated.

Note: Blocking port can receive BPDUs.

4.convergence time of 3 direct link failure scenario (link failure between S2-S3/S1-S3/S1-S2) and 1 indirect link failure scenario (link between S1-S3)

?/30s/50s/50s

Note: it's related to the port state (up/down, interface on the link with problem) whether wait the Max Age timer or not

Rapid Spanning Tree Protocol

PVST and PVST+ were proprietary spanning protocols

RSTP (802.1W) Port States

RSTP reduces the number of port states to three: discarding/learning/forwarding (discarding combine disabled, blocking, and listening in STP port states)

Building the RSTP Topology

what action they will do when switches first connect in environment with only two switches and larger environment respectively?

1. establish a bidirec tional handshake across the shared link to identify the root bridge

2. uses a synchronization process to add a switch to the RSTP topology

Key Terms

configuration BPDU

The BPDU that is responsible for switches electing a root bridge and communicating the root path cost so that a hierarchy can be built.

hello time

The time interval for which a BPDU is advertised out of a port.

Practice Test

Which of the following best describe the BPDUs used in 802.1D?

There are two types of BPDUs.
802.1D and 802.1W use the same BPDU format.
BPDUs are sent at each 2-second hello interval, even if no BPDU is received on the root port.
Configuration BPDUs identify the root bridge.

Select 2 answers

Explanation:（14）

802.1D defines two BPDU types, the configuration BPDU and the topology change notification (TCN) BPDU. Several important fields are included in configuration BPDUs, including the root bridge identifier, timers, and cost to the root. 802.1D bridges only forward BPDUs from the root port, where 802.1W bridges generate a BPDU at each hello interval.

2.RSTP对BPDU格式进行了少量更改。 802.1D中只定义了两个标志：拓扑更改(TC)和TC确认(TCA)。但是，RSTP会使用剩余的六个标志位元组位来执行：