In order to understand the nuts and bolts of Spanning-Tree Protocol (STP), we need to get familiar with its terminology first.
Spanning-Tree Protocol Terminology
The ports participating in STP play different roles and those roles use different states of operation.
Spanning-Tree Port Roles
- Root Port (RP) - It is a port on a non-root switch, which is the shortest (the best) path towards the root bridge. Root bridge does NOT have any root ports. (no shortest path to itself ;-))
- Designated Port (DP) - It is a port that is in the forwarding state. All ports of the root bridge are designated ports (they are never in a blocking state). BPDU frames are sent out this port.
- Non-Designated Port (NDP) - It is a port that is in a blocking state in the STP topology.
Spanning-Tree Port States
- Disabled - The port in this state does not participate in the STP operation (it is shut down).
- Blocking - The port does NOT forward any Ethernet frames, does NOT accept any Ethernet frames (discards arriving frames), does NOT learn any MAC addresses. However, the port DOES process BPDU frames received from a neighboring switch. If the port transitions to this state (blocking), it can stay blocked for 20 seconds by default (max_age).
- Listening - The port in this state CAN send and receive the BPDU frames. However, the port in this state does NOT learn any MAC addresses, and does NOT forward or process incoming frames either. All Ethernet frames are being discarded. The computation of loop free topology takes place in this state. If the port transitions to this state (listening), it can stay in this state for 15 seconds by default (forward_delay).
- Learning - The port in this state already knows its role (root port or designated port ) in the STP domain. However, the port will not forward any Ethernet frames yet. It will be learning MAC addresses from the frames arriving at the port in order to populate MAC address table. This helps avoid too much flooding when the port transition to the forwarding state. If the port transitions to this state (learning), it can stay in this state for 15 seconds by default (forward_delay).
- Forwarding - The port in this state will forward all Ethernet frames as per switch operation. Also, the port will process all incoming Ethernet frames and will actively learn MAC addresses from the arriving traffic.
Pic. 1 - STP Port Terminology
STP (IEEE 802.1d) Principles of Operation
STP will use three stages to compute loop free topology (pic. 2):
- Single root bridge election.
- Each non-root switch to select a single best port towards the root (root port).
- Each non-root switch to select a single forwarding port per segment (designated port).
Pic. 2 - STP Overview
Bridge Protocol Data Unit (BPDU)
All switches communicate with one another using special frames called BPDU. Those frames contain multiple parameters that switches are going to process in order to create and maintain loop free topology.
Root Bridge
Root bridge is the switch that has all ports working in the designated role. It will be the reference point from which the loop free topology is computed. Root bridge will impose the timers that other switches will use such as:
- hello time - how often BPDUs are going to be sent/relayed (default timer=2 seconds),
- max age - how long the configuration is valid (default timer=20 seconds),
- forward delay - how long a port should be in listening/learning state (default timer=15 seconds).
Root bridge will be announcing its presence by sending BPDU frames. Other switches will relay those frames out their designated port given the hello time. Also, the root bridge has all its ports in the designated role (forwarding).
1. Root Bridge Election
Only one switch in the layer 2 network becomes the root bridge. This is how standard was defined and is known as the Common Spanning-Tree approach (CST). Cisco changed that paradigm and introduced Per Vlan Spanning-Tree approach (PVST+). Cisco switches elect a single root switch per VLAN so, in theory each VLAN could have its own root bridge.
Root election is based on a single parameter that is found in the BPDU frame called: Bridge ID. The switch with the lowest Bridge ID becomes the root. Bridge ID has the following format:
priority.base-mac-address
Priority is configurable parameter that is used to elect the root bridge a device you want to be the root. The default value is: 32768. The lower the value is the more likely for a switch to become a root.
Base Mac Address is the unique mac address every switch has been given by the manufacturer. It is a tie breaker in case the priority on all switches is identical.
If you've understood everything so far, you're ready to look at the election process in more detail.
Pic. 3 - Root Bridge Election.
Imagine that we've just wired our topology in the pic. 3. Now, we start up all the switches and as soon as their ports transition to LISTENING state, they begin to send BPDU frames out of all active ports. In those frames both Bridge ID and Root ID parameters point to their own priority.base-mac-address value. In other words, each switch thinks it is the root bridge. It is like each switch is saying: "Hi there! This is my name (Bridge ID) and by the way I'm the root (Root ID the same as the Bridge ID value). Since they are processing the incoming BPDU's from the neighbors, SW2 and SW3 realize that SW1's Bridge ID is lower than theirs. From that point onwards, they begin to relay BPDU frames saying that SW1 as the root bridge.
In our example, SW3 upon receiving the BPDU from SW1, SW2 and SW4 compares their Bridge ID with its own and the conclusion is that SW1's Bridge ID has the lowest value (base-mac-address breaks the tie). From this point onwards, it relays the BPDU frame out of all its active ports with the following parameters:
Bridge ID = 32768.0000.3333.3333
Root ID = 32768.0000.1111.1111
Similarly, all the switches agree that SW1 is the root (their own Bridge ID is higher).
2. Root Port Selection
As soon as the root has been elected, all non-root switches begin to calculate which port is the best (the least cost) towards the root bridge. This port will be called the root port.
Pic. 4 - Root Port Selection
SW2, SW3 and SW4 receive BPDUs from different directions. For instance, SW2 will receive them on its port F0/1 and F0/2 (look at pic 4). The accumulative cost (the sum of the cost in the path towards the root), is taken into consideration. The lowest cost to reach the root becomes the root port.
How the cost of path is calculated?
Each speed has its arbitrarily assigned cost which is configurable. A few examples are below:
10 Mbps = 100
100 Mbps = 19
1 Gbps = 4
10 Gbps = 2
The root bridge (here SW1) is sending its BPDU frame every 2 seconds. It uses the parameter called: Root Path Cost in BPDU to advertise the cost to the root. It puts the value of '0' in it, as it is the root bridge and has no cost to itself. The frame is sent out its port F0/1 towards SW3 and F0/2 towards SW2. SW2, upon receiving it, adds the cost used to reach the sender of BPDU based on the predefined speed-to-cost value (all ports in our topology are FastEthernet=19).
Root Path Cost = 0 + 19 = 19 via F0/2
SW2 is going to advertise its best (as of now) cost out of F0/1 port towards SW3. SW3 will receive BPDU from SW1 with the Root Path Cost=0 on its F0/1 port. It will also receive BPDU from SW2 on its F0/2 interface with the Root Path Cost=19. As both ports have the cost of 19 towards those BPDU senders, the following math is done to choose the least cost path towards the root bridge:
Root Path Cost = 0 + 19 = 19 via F0/1
Root Path Cost = 19 + 19 = 38 via F0/2
It is clear that the direct connection towards root bridge via F0/1 is going to be selected as the root port.
SW3 has the least cost towards equal 19 (via F0/1 port). This cost is going to be added to Root Path Cost while it sends the BPDUs out F0/2, F0/3 and F0/4. Of course, SW2 also chooses its F0/2 port as the root port since the cost is smaller.
What if the Root Cost Path is identical?
We run into that situation on SW4. It receives BPDUs on its ports F0/1 and F0/2 with the following parameters:
Bridge ID = 32768.0000.3333.3333
Root ID = 32768.0000.1111.1111
Root Path Cost = 19
The cost clearly does not help to choose a single root port as both ports have the same cost:
19 + 19 = 38.
The following algorithm is used to determine the root port or designated port (in order):
- Prefer the lowest Root Path Cost.
- In case of the same Root Path Cost, prefer the lowest Bridge ID of the designated switch (the neighbor that sends BPDUs).
- In case of receiving BPDUs on multiple ports from the same designated switch (BPDU sender), prefer the lowest Port ID (known also as port priority) of the sender. That parameter has a default value 128 and is configurable.
- In case of all above are did not resolve the problem, prefer the lowest Port ID on which the BPDU arrives.
Equipped with that knowledge let us consider SW4 now.
- SW4 receives BPDUs on port F0/1 and F0/2. The Root Path cost is the same: 19 + 19 = 38 on both ports.
- The designated switch (SW3), is the same switch i.e. the same Bridge ID (32768.0000.3333.3333).
- The designated switch (SW3) sends BPDUs out of its F0/3 and F0/4 ports with the same priority = 128 (Port ID).
- The tie breaker is the lowest Port ID where BPDU frames arrive on SW4. Port f0/1 becomes the root port since F0/1 is lower than F0/2.
The root ports have been selected on all non-root switches (pic. 5). STP will select a single designated port (forwarding) per segment to block the redundant path towards the root bridge. This way the loop does not exist. Should any of root ports fail, it will take around 30-50 seconds to put the blocking port into forwarding state.
3. Designated Port Selection.
This procedure follows exactly the same algorithm used for root port selection.
Pic. 5 - Designate Port Selection
Since root port is the best port towards the root bridge it is going to be in the forwarding state (look at the beginning of this lesson). What is left to do, is to choose one of the ports between SW2 and SW3 as designated (forwarding) and the other as non-designated (blocked). The same applies between SW3 and SW4. Either SW3 will block its F0/4, or SW4 should block its F0/2 port.
SW3 will block its F0/2 (non-designated) and SW2 will make its F0/1 port designated (forwarding). The process will look as follows:
- Root Path Cost advertised by SW2 is 19 and so is the cost advertised by SW3.
- SW2 has lower Bridge ID (32768.0000.2222.2222) than SW3 (32768.0000.3333.3333). SW3 must block its F0/2.
And last selection is going to happen between SW3 (port F0/4) and SW4 (port F0/2).
- Root Path Cost Advertised by SW3 is 19, but SW4 advertises its cost as 38 (two hops via F0/1). SW4 blocks its port F0/2 (non-designated), the SW3 promotes its port F0/4 to designated role (forwarding).
Pic. 6 - Spanning-Tree Topology Computed
This process happens in the LISTENING state of all ports. Since the topology has been computed and does not have loops (blocking appropriate ports), it is safe to move to next states: learning and finally forwarding.
Spanning-Tree Protocol in Practice
Previous post was designed to present in a nutshell the STP operation. However, without some practice it's just academic knowledge. I think it is a good idea to look at the same concepts using real equipment. Here goes...
The below topology (pic. 1) uses redundant links which create the loops.
Pic. 1 - Network Topology
Icons designed by: Andrzej Szoblik - http://www.newo.pl
If there's one thing the administrator should do with such design, that would be configuring the root bridge. Typically, the most powerful switch in the center of the network plays that role. You do not want some access switch to be transmitting the frames between other switches. Access switches are designed to connect your computers to the network, and not to handle the majority of the traffic between the switches which root bridge must deal with.
If you do not configure root bridge yourself, the switch with the lowest MAC address becomes the root since the priority is identical on all of them by default. We do not want to leave it to a chance, do we? For simplicity reasons I have chosen to make SW1 my root bridge. There are at least two ways to configure this.
Method 1
I can manually decrement the priority on SW1 and leave the default value on the other switches. I want to make SW1 my root bridge for all the VLANs I use in my network (remember Cisco uses PVST+). The lowest priority value allowed is zero and if higher needs to be used, it must be an increment of 4096. If you type in the value that is not allowed, the system will present you with the list of values you can use.
Step 1
Check the VLANs configured.
Step 2
Make SW1 the root bridge for all the VLANs configured in the network by decrementing the default value. Here, I will use the value of '0'.
A quick verification if the command took effect is below:
The above output confirms that SW1 has been elected as the root bridge:
This bridge is the root
Familiarize yourself with the output of this command. All active ports of the switch are in designated role (forwarding state) as it is the root.
Also, notice that both Bridge ID and Root ID are the same values. I assigned priority of 0, but the system extended ID (PVST) adds VLAN number to the priority assigned. Thus, the priority 0 + (VLAN id) 500 = 500.
Priority: 500
MAC: 000b.5ff7.a080
Like mentioned before, if the priority value configured is not configured according to the allowed values, the system shows the numbers you can use:
Method 2
I can use the spanning-tree vlan root primary macro command which decrements the priority value using Cisco best practices.
Step 1
Check the VLANs configured like before.
Step 2
Make SW1 the root bridge for all the VLANs configured in the network by using the macro command.
And now comes the interesting bit. Having elected the root bridge SW1, I can predict all the rest of the process. Lesson 20 provides us with all the knowledge we need to posses to tell which ports will become root ports on SW2 and SW3 as well as which ports will be designated and which will be non-designated in our topology.
Can you do that on your own?
The base MAC addresses on SW2 and SW3 are as follows (priority is default):
SW2 MAC: 000E:83DA:7580
SW3 MAC: 000D:28BF:FD40
If you want to check what is the base mac address on your switch type in:
SW#show version | include Base
At least give it a shot before you click at the pic. 2 below to check your answers. If you cannot do it yet, do not worry. I will guide you through the process using some powerful 'show' commands.
Pic. 2 - Spanning-Tree Topology Computed
Icons designed by: Andrzej Szoblik - http://www.newo.pl
There are two loops in my network. One between SW1 and SW2 using ports F0/13 and F0/14. The other loop is formed between SW3 connections to SW1 and SW2 (ports F0/15 and F0/16).
SW2 receives BPDU frames from SW1 on F0/13 and F0/14 ports and from SW3 on its F0/16 port. A closer look at the following output can be very informative.
The above output shows clearly which machine is the root bridge (000B:5FF7:A080). SW2 chose F0/13 as it Root Port. As you recall the first thing to check to determine which is the best path towards the root bridge (root port) is the accumulative cost towards the root. SW2 has three outgoing ports towards the root bridge as shown in the next output:
The accumulative cost is calculated by adding two values:
Port path cost + designated path cost.
- Port path cost - arbitrarily set values by IEEE (the speed-to-cost table is shown in the previous lesson).
- Designated path Cost - the cost towards the root bridge advertised by the neighboring switch.
Port F0/16 can be ruled out immediately since 'port path cost' (19) + 'designated path cost' (19) amounts to: 38.
As for the two remaining candidates to become a root port (F0/13 and F0/14), the total path cost is 19 in both cases (19+0). We need to resort to the second test in our algorithm to break the tie: the lowest bridge id of the BPDU sender. Unfortunately, both ports receive BPDU frames from the same switch: SW1 (look at the previous output).
Designated Bridge has priority 500, address 000B:5FF7:A080
Next step to solve the issue is checking the port priority of the sender. But both ports F0/13 and F0/14 receive the same port priority (port id):
Designated port id is 128
The number of the port is not factored in, only the id value like shown above.
There is only one more thing that can help us determine which of these two ports should be the root port: the lowest port id of the receiver (SW2). F0/13 is lower in value than F0/14, so the former becomes the root port.
In the same way SW3 chooses its root port F0/15 as the root port since the accumulative cost using it is 19 as opposed to port F0/16 which total cost out towards the root bridge is 38.
Port F0/14 on SW2 becomes non-designated port (NDP) due to the fact, that the root bridge (SW1) has to have all the ports in designated mode which means they cannot be blocked.
The last thing to compute the STP active paths is to select the designated port between SW2 (F0/16) and SW3 (F0/16). Again, the same formula solves the issue. As both SW2 and SW3 advertise the same cost: 19, the tie breaker is going to be the lowest bridge id of the sender. In this contest, SW2 has higher bridge id (less preferred) which is: priority 33268, address 000E:83DA:758
SW3 priority being lower wins. SW3 bridge id for the same VLAN 500 looks like shown below:
priority 33268, address 000D:28BF:FD40
STP selects the layer 2 paths between the switches. In the pic. 2 I showed you also that all the ports connected to PC1, PC2 and R1 are in a designated role. This is because those ports do NOT receive BPDUs. They automatically become designated (forwarding state).
As the last thing in this lesson, I'd like to ask you two questions.
Assuming that SW1 is the root bridge:
Question1
What would you need to reconfigure in our topology (pic. 1), for SW2 to choose F0/14 as the root port for VLAN 500?
Question 2
What would you need to reconfigure in our topology (pic. 1) for SW3 to choose F0/16 as the root port for VLAN 500?
NOTICE!
The method of choosing root port/designated port in the previous lesson holds the answers to these questions. Remember about the order of operation.
The answer to question 1
Since the cost is the same towards SW1 (root), we could modify it on SW2 with the following command:
SW2(config)#interface f0/13
SW2(config-if)#spanning-tree vlan 500 cost 20
This way I have increased the cost on this port to 20, and F0/14 cost now is lower (19).
Another method could be to change the port priority on the SW1 preferring port F0/14. This is how you could do it:
SW1(config)#interface f0/14
SW1(config-if)#spanning-tree vlan 500 port-priority 64
Since, the path cost towards the root are identical on both ports, bridge id of the sender is the same switch SW1, the third thing to influence which one to use is the port priority assigned by the BPDU sender (here SW1). This is shown in the following picture taken from SW2 (show spanning-tree vlan 500 detail):
Now, the priority imposed by SW1 on SW2's F0/14 is lower: 64 compared to port F0/13 which is 128. Port F0/14 becomes the root port.
Answer to question 2
In order to change the root port on SW3 the only way to do that is to increase the cost to reach the root bridge on F0/15. For instance you could configure the following:
SW3(config)#interface f0/15
SW3(config-if)#spanning-tree vlan 500 cost 39
Since the total cost towards SW1 (root) using port F0/15 is 39 now, and using port F0/16 the cost used equals 38, this configuration will do the job.
Spanning-Tree Cisco Enhancements
My previous two posts hopefully shed some light on IEEE 802.1d protocol (yes, it is STP). There are two more things I would like to add to that picture. The first thing, deals with situations when the topology changes and how it affects the STP time of convergence. The convergence here, means the time it takes to recompute the STP tree in order to keep the loop free paths upon failure. The second thing, I'd like to bring up is the Cisco STP enhanced the STP operation to decrease the time of convergence compared to the industry standard STP.
Before we delve into the details though, I need to explain something about BPDU frames first. It is true that it is the root bridge that originates those frames and sends them out its designated ports ( downstream, every 2 seconds by default). It is also true, that all other switches (non-root bridges), propagate them downstream out of their designated ports. This way all switches receive the information as to which switch is the root bridge in the network and if it is still functional.
However, what I withheld in previous posts was the types of BPDU frames. There are three types of those:
- Configuration - the type of BPDU which the root bridge sends every 2 seconds, and other switches propagate those out of their Designated Ports (downstream).
- Topology Change Notification (TCN) - the type of BPDU that a switch will send if it detects the topology change (port going down, or TCN received). This BPDU is sent out the Root Port (upstream) towards the root bridge informing it, that the tree needs to be recomputed.
- Topology Change Acknowledgement (TCA) - the type of BPDU that is sent back to the sender of TCN BPDU, acknowledging the reception of the notification.
How do those BPDUs fit into the grand scheme of things?
The default timer of how long the entries are kept in the MAC address table is 300 seconds (5 minutes). This means, that if a host connected to a port of the switch does not speak for at least five minutes, its MAC address is removed from the CAM table. That is a way too long for the switch to re-learn computer's MAC addresses if the STP topology changes.
But why do those MAC entries have to change?
Please, consider the Pic. 1 below. By now, you should be able to tell which ports of the switches are going to learn the PC1 and PC2 MAC addresses. Go ahead, click the Pic. 1, and put down on a piece of paper the switch names and the ports that learn MAC addresses of the PC1 and PC2. That is going to be a good refresher of how switches learn MAC addresses dynamically.
Pic 1 - STP Topology.
Icons designed by: Andrzej Szoblik - http://www.newo.pl
If your answers match mine below, that means that you have mastered the lessons on bridging/switching and STP.
SW1 CAM:
F0/1 - 0000.1111.1111
F0/2 - 0000.2222.2222
SW2 CAM:
F0/1 - 0000.2222.2222
F0/2 - no mac addresses learned since the port is NDP
F0/3 - 0000.1111.1111
SW3 CAM:
F0/1 - 0000.1111.1111
F0/2 - no mac addresses learned as PC1 communicates using SW1
F0/3 - 0000.2222.2222
SW4 CAM:
F0/1 - no mac addresses learned as SW2's port F0/2 is NDP
F0/2 - 0000.1111.1111
F0/2 - 0000.2222.2222
Now, lets create a problem that causes the topology change in our network. Consider Pic. 2 which shows us why some ports must re-learn the MAC addresses of PC1 and PC2.
Pic. 2 - STP Network Problem
Icons designed by: Andrzej Szoblik - http://www.newo.pl
Given the situation, STP needs to recalculate topology since we lose active connections between SW1 and SW2. If it were not for the STP operation in such circumstances, it would take 5 minutes (300 seconds) for the switches to re-learn MAC addresses according to the situation presented in Pic. 3. The resulting topology diagram is depicted below.
Try to put down on the paper which MAC addresses should be learned on which ports of the respective switches after failure (Pic. 3).
Pic. 3 - Topology after losing the connection between SW1 and SW2.
Icons designed by: Andrzej Szoblik - http://www.newo.pl
SW1 CAM:
F0/1 - down
F0/2 - 0000.1111.1111
F0/2 - 0000.2222.2222
SW2 CAM:
F0/1 - down
F0/2 - 0000.2222.2222
F0/3 - 0000.1111.1111
SW3 CAM:
F0/1 - no MAC addresses learned
F0/2 - 0000.1111.1111
F0/3 - 0000.2222.2222
SW4 CAM:
F0/1 - 0000.1111.1111
F0/2 - 0000.2222.2222
In order to decrease the time of re-learning MAC addresses, upon failure SW1 is going to send TCN BPDU out its Root Port. Normally, the Configuration BPDU are sent out Designated Ports NOT the Root Port. But this failure prompts the switches to notify the root bridge about the topology change. That is why, they will send TCN BPDU out their Root Port. All switches, in the path of this TCN BPDU must send the TCA BPDU (acknowledgement) back to the sender and forward TCN BPDU towards the root bridge. As soon as the root bridge has been notified about the topology change, it begins to send TCN BPDUs out its Designated Ports, so other switches in the network also get notified to give them a chance to flush MAC addresses, recompute the tree and re-learn the MAC addresses according to the new topology (Pic. 3). This reduces the time of convergence from 5 minutes to about 30 - 50 seconds time, depending on the nature of the change.
You might question that and say that the default timers used here (30-50 second delay) are still inappropriate for today's networks transmitting voice, video and data. And you are quite right saying so. The mechanism is still not good enough. But remember, that those timers were designed as SAFE values (not causing the loops) given the maximum diameter of network of seven switches (hops) between the root bridge and the bottom switches. Also, remember that STP was designed when there were no multimedia transmissions being sent across the switches. Is there a solution to those timers? Of course. You may change them manually but DO NOT DO THAT unless you are very experienced with STP operation. Another option is to use some proprietary features implemented in Cisco switches.
Cisco with their STP Enhancement are able to decrease this 30-50 second timers even further allowing video, voice and data co-exist in our layer 2 networks. Keep in mind, that these enhancement are Cisco proprietary STP add-ons:
- STP Portfast (now part of standard implementation as well).
- STP Uplinkfast.
- STP Backbonefast (this one is beyond the scope of this tutorial).
Let us see how the first two can change the behavior of our sample topology.
STP Portfast feature should be configured on all EDGE ports, i.e. the ones that connect devices that do not send BPDU frames and cannot create loops. These would be your computers, servers, printers etc. What STP Portfast does, it simply skips the LISTEN and LEARN states, going directly to FORWARD state if there was TCN announced or the port in question is just brought up. Think about it. It makes no sense to flush the MAC addresses on the ports that connect the computers directly, since the topology change is not going to affect them. In the topology presented in this tutorial (Pic. 1, 2, and 3), the topology change did not affect the ports F0/3 on both SW2 and SW3 where PC1 and PC2 are connected respectively. They are still connected where they were before the topology change and their addresses are mapped to the same ports as before the change. So, there is no point of flushing the MAC address table entries on SW2 port F0/3 and SW3 port F0/3. These ports are the candidates for STP Portfast. Because STP Portfast-enabled ports go FORWARD almost immediately, it is highly recommended to use this feature on ports connected to computers in order to avoid problems of getting the IP address using DHCP services.
There are two ways of enabling STP Portfast feature.
Method 1
In the global configuration mode, type in this command:
SW1(config)#spanning-tree portfast default
All ports that are discovered as EDGE ports (more on that in my next post about Rapid STP), will have STP portfast enabled by default. You can check that using a detailed STP output regarding a port (here F0/1):
SW1#show spanning-tree interface f0/1 detail
The output shows that STP portfast has been enabled on this port (look at BPDU received = 0, candidate for portfast):
Pic. 4 - STP F0/1 Detailed Output.
Method 2
Another method is to type in the following command directly on the chosen port:
SW1(config)#interface f0/1
SW1(config-if)#spanning-tree portfast
This way, we turn on STP Portfast unconditionally (whether port does or does not receive BPDUs).
The second STP enhancement is STP Uplinkfast. This one should be configured on all ACCESS switches (the leaf switches in our topology NOT distribution ones). The feature that is enabled in the global config mode, shortens the time it takes to transition NDP port into RP role upon losing the current Root Port.
In our topology, consider SW2 that has lost its Root Port (F0/1, Pic. 2). Normally, that is without STP Uplinkfast enabled, it would take 30 seconds for the F0/2 port to transition to an RP role. Keep in mind that F0/2 does not have go to blocking state since it keeps receiving superior BPDUs with the Root Bridge ID. Thus, only 30 seconds are required by default (LISTEN+LEARN states). With STP Uplinkfast enabled, Cisco guarantee that the transition of F0/2 to forwarding state (RP role) is going to happen in under 5 seconds.
The configuration of STP Uplinkfast is done in the global config mode as shown below:
SW1(config)#spanning-tree uplinkfast
Similar, in functionality, is STP Backbonefast that could be implemented on distribution switches. However, the details of this feature are beyond the scope of this tutorial.
In my next post, I'm going to briefly present Rapid Spanning-Tree Protocol (IEEE 802.1w) and how it differs from a regular STP (IEEE 802.1d).
If you want to see the enhancement in action please, watch the video below:
more videos available at:
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