Computer networks are inextricably linked to productivity in today’s small and medium-sized businesses. Consequently, IT administrators have to implement redundancy in their hierarchical networks. When a switch connection is lost, another link needs to quickly take its place without introducing any traffic loops. This chapter investigates how Spanning Tree Protocol (STP) logically blocks physical loops in the network and how STP has evolved into a robust protocol that rapidly calculates which ports should be blocked in a VLAN-based network. In addition, the chapter briefly explores how Layer 3 redundancy is implemented through First Hop Redundancy Protocols (FHRPs).
Spanning-Tree Concepts
Redundancy increases the availability of a network topology by protecting the network from a single point of failure, such as a failed network cable or switch. STP was developed to address the issue of loops in a redundant Layer 2 design.
Draw a Redundant Topology
In Figure 2-1, draw redundant links between the access, distribution, and core switches. Each access switch should have two links to the distribution layer with each link connecting to a different distribution layer switch. Each distribution layer switch should have two links to the core layer with each link connecting to a different core layer switch.
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Figure 2-1 Redundant Topology
Purpose of Spanning Tree
STP prevents specific types of issues in a redundant topology like the one in Figure 2-1. Specifically, three potential issues would occur if STP was not implemented. Describe each of the following issues:
MAC database instability:
Broadcast storms:
Multiple frame transmission:
You should be prepared to use a topology like Figure 2-1 to explain exactly how these three issues would occur if STP was not implemented.
Packet Tracer - Examining a Redundant Design (SN 2.1.1.5/SwN 4.1.1.5)
Spanning-Tree Operation
Because _______________(RSTP), which is documented in IEEE _______________-2004, supersedes the original STP documented in IEEE _______________-1998, all references to STP assume RSTP unless otherwise indicated.
STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a _______________. A switch port is considered _______________ when network traffic is prevented from entering or leaving that port.
STP uses the _______________(STA) to determine which switch ports on a network need to be _______________ to prevent _______________ from occurring. The STA designates a single switch as the _______________ bridge and uses it as the reference point for all subsequent calculations. Switches participating in STP determine which switch has the lowest _______________(BID) on the network. This switch automatically becomes the _______________ bridge.
A _______________ (BPDU) is a frame containing STP information exchanged by switches running STP. Each BPDU contains a BID that identifies the switch that sent the BPDU. The _______________BID value determines which switch is root.
After the root bridge has been determined, the STA calculates the shortest path to the root bridge. If there is more than one path to choose from, STA chooses the path with the lowest _______________.
When the STA has determined the “best” paths emanating from the root bridge, it configures the switch ports into distinct port roles. The port roles describe their relation in the network to the root bridge and whether they are allowed to forward traffic:
- ___________________ ports: Switch ports closest to the root bridge
- ___________________ ports: Nonroot ports that are still permitted to forward traffic on the network
- ___________________ ports: Ports in a blocking state to prevent loops
- ___________________ port: Ports that are administratively shut down
After a switch boots, it sends BPDU frames containing the switch BID and the root ID every ___ seconds. Initially, each switch identifies itself as the _______ bridge after boot.
How would a switch determine that another switch is now the root bridge?
How does the STA determine path cost?
Record the default port costs for various link speeds in Table 2-1.
Table 2-1 Port Costs
Link Speed |
Cost (Revised IEEE Specification) |
Cost (Previous IEEE Specification) |
10 Gbps |
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1 Gbps |
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100 Mbps |
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10 Mbps |
Although switch ports have a default port cost associated with them, the port cost is configurable.
To configure the port cost of an interface, enter the __________________________ command in interface configuration mode. The range value can be between _______________and _______________.
Record the commands, including the switch prompt, to configure the port cost for F0/1 as 15:
To verify the port and path cost to the root bridge, enter the _______________ privileged EXEC mode command, as shown here:
S2# __________________________________
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address c025.5cd7.ef00
Cost 15
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address c07b.bcc4.a980
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 15 128.1 P2p
Fa0/2 Altn BLK 19 128.2 P2p
Fa0/3 Desg LIS 19 128.3 P2p
Fa0/4 Desg LIS 19 128.4 P2p
Fa0/6 Desg FWD 19 128.6 P2p<output omitted>
The BID field of a BPDU frame contains three separate fields: _______________, _______________, and _______________.
Of these three fields, the _______________is a customizable value that you can use to influence which switch becomes the root bridge. The default value for this field is _______________.
Cisco enhanced its implementation of STP to include support for the extended system ID field, which contains the ID of the _______________ with which the BPDU is associated.
Because using the extended system ID changes the number of bits available for the bridge priority, the customizable values can only be multiples of _______________.
When two switches are configured with the same priority and have the same extended system ID, the switch with the lowest _______________ has the lower BID.
Identify the 802.1D Port Roles
The topologies in the next three figures do not necessarily represent an appropriate network design. However, they provide good exercise topologies for you to practice determining the STP port roles. In Figures 2-2 through 2-4, use the priority values and MAC addresses to determine the root bridge. Then label the ports with one of the following:
- RP: Root Port
- DP: Designated Port
- AP: Alternate Port
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Figure 2-2 802.1D Port Roles - Scenario 1
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Figure 2-3 802.1D Port Roles - Scenario 2
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Figure 2-4 802.1D Port Roles - Scenario 3
Lab – Building a Switched Network with Redundant Links (SN 2.1.2.10/SwN 4.1.2.10)
