Why do we need spanning-tree?
What is a loop and how do we get one? Let me show you an example:
In the picture above we have two switches. These switches are connected to each other with a single cable so there is a single point of failure.To get rid of this single point of failure we will add another cable:
With the extra cable we now have redundancy. Unfortunately for us redundancy also brings loops. Why do we have a loop in the scenario above? Let me describe it to you:
- H1 sends an ARP request because it’s looking for the MAC address of H2. An ARP request is a broadcast frame.
- SW1 will forward this broadcast frame on all it interfaces, except the interface where it received the frame on.
- SW2 will receive both broadcast frames.
Now what does SW2 do with those broadcast frames?
- It will forward it out of every interface except the interface where it received the frame on.
- This means that the frame that was received on interface Fa0/0 will be forwarded on Interface Fa1/0.
- The frame that was received on Interface Fa1/0 will be forwarded on Interface Fa0/0.
Do you see where this is going? We have a loop! Both switches will keep forwarding over and over again until the following happens:
- You fix the loop by disconnecting one of the cables.
- One of your switches will crash because they are overburdened with traffic.
Ethernet frames don’t have a TTL (Time to Live) value so they will loop around forever. Besides ARP requests there are many frames that are broadcasted. For example whenever the switch doesn’t know about a destination MAC address it will be flooded.
How spanning-tree solves loops
Spanning-tree will help us to create a loop-free topology by blocking certain interfaces. Let’s take a look how spanning-tree works! Here’s an example:
We have three switches and as you can see we have added redundancy by connecting the switches in a triangle, this also means we have a loop here. I have added the MAC addresses but simplified them for this example:
- SW1: MAC AAA
- SW2: MAC BBB
- SW3: MAC CCC
Since spanning tree is enabled, all our switches will send a special frame to each other called a BPDU (Bridge Protocol Data Unit). In this BPDU there are two pieces of information that spanning-tree requires:
- MAC address
- Priority
The MAC address and the priority together make up the bridge ID. The BPDU is sent between switches as shown in the following picture:
Spanning-tree requires the bridge ID for its calculation. Let me explain how it works:
- First of all spanning tree will elect a root bridge; this root-bridge will be the one that has the best “bridge ID”.
- The switch with the lowest bridge ID is the best one.
- By default the priority is 32768 but we can change this value if we want.
So who will become the root bridge? In our example SW1 will become the root bridge! Priority and MAC address make up the bridge ID. Since the priority is the same on all switches it will be the MAC address that is the tiebreaker. SW1 has the lowest MAC address thus the best bridge ID and will become the root bridge.
The ports on our root bridge are always designated which means they are in a forwarding state. Take a look at the following picture:
Above you see that SW1 has been elected as the root bridge and the “D” on the interfaces stands for designated.
Now we have agreed on the root bridge our next step for all our “non-root” bridges (so that’s every switch that is not the root) will have to find the shortest path to our root bridge! The shortest path to the root bridge is called the “root port”. Take a look at my example:
I’ve put an “R” for “root port” on SW2 and SW3, their Fa0/0 interface is the shortest path to get to the root bridge. In my example I’ve kept things simple but “shortest path” in spanning tree means it will actually look at the speed of the interface. Each interface has a certain cost and the path with the lowest cost will be used. Here’s an overview of the interfaces and their cost:
- 10 Mbit = Cost 100
- 100 Mbit = Cost 19
- 1000 Mbit = Cost 4
Excellent!…we have designated ports on our root bridge and root ports on our non-root bridges, we still have a loop however so we need to shut down a port between SW2 and SW3 to break that loop. So which port are we going to shut down? The one on SW2 or the one on SW3? We’ll look again at the best bridge ID:
- Bridge ID = Priority + MAC address.
Lower is better, both switches have the same priority but the MAC address of SW2 is lower, this means that SW2 will “win this battle”. SW3 is our loser here which means it will have to block its port, effectively breaking our loop! Take a look at my example:
If you look at the link between SW2 and SW3 you can see that the Fa1/0 interface of SW3 says “A” which stands for alternate. An alternate port is blocked! Sometimes the alternate port is called the ND (Non Designated) port.By shutting down this interface we have solved our loop problem.
Because the default priority is 32768 the tie-breaker for selecting the root bridge is the MAC address. In a production network what switch do you think will be elected as the root bridge?
Your brand spanking new switch or that dirty old switch that has been used as a dust collector for the last 8 years?
The old switch probably has a lower MAC address and thus will be elected as the root bridge. Doesn’t sound like a good idea right? That’s why we can change the priority to determine what switch will become the root bridge.
Are you following me so far? Good! You just learned the basics of spanning-tree. Let’s add some more detail to this story…
Let’s continue our spanning tree story and further enhance your knowledge. If you have played with some Cisco switches before you might have noticed that every time you plugged in a cable the led above the interface was orange and after a while became green. What is happening at this moment is that spanning tree is determining the state of the interface; this is what happens as soon as you plug in a cable:
- The port is in listening mode for 15 seconds. In this phase it will receive and send BPDUs, still neither learning MAC addresses nor data transmission.
- The port is in learning mode for 15 seconds. We are still sending and receiving BPDUs but now the switch will also learn MAC addresses, still no data transmission though.
- Now we go in forwarding mode and finally we can start transmitting data!
Here’s a picture to visualize it:
Spanning-tree configuration on Cisco switches
Now you have an idea what spanning-tree is about. Let’s take a look at some Cisco switches to see how we can configure it. I will use the same topology that I showed you earlier but we have different interfaces.
This is the topology we will use. Spanning-tree is enabled by default; let’s start by checking some show commands.
SW1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000f.34ca.1000
Cost 19
Port 19 (FastEthernet0/17)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0011.bb0b.3600
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/14 Desg FWD 19 128.16 P2p
Fa0/17 Root FWD 19 128.19 P2p
The show spanning-tree command is the most important show command to remember. There’s quite some stuff here so I’m going to break it down for you!
VLAN0001
Spanning tree enabled protocol ieee
We are looking at the spanning-tree information for VLAN 1. Spanning-tree has multiple versions and the default version on Cisco switches is PVST (Per VLAN spanning-tree). This is the spanning-tree for VLAN 1
Root ID Priority 32769
Address 000f.34ca.1000
Cost 19
Port 19 (FastEthernet0/17)
Here you see the information of the root bridge. You can see that it has a priority of 32769 and its MAC address is 000f.34ca.1000. From the perspective of SW1 it has a cost of 19 to reach the root bridge. The port that leads to the root bridge is called the root port and for SW1 this is fa0/17.
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0011.bb0b.3600
This part shows us the information about the local switch, SW1 in our case. There’s something funny about the priority here….you can see it show two things:
- Priority 32769
- Priority 32768 sys-id-ext 1
The sys-id-ext value that you see is the VLAN number. The priority is 32768 but spanning-tree will add the VLAN number so we end up with priority value 32769. Last but not least we can see the MAC address of SW1 which is 0011.bb0b.3600.
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Here’s some information on the different times that spanning-tree uses:
- Hello time: every 2 seconds a BPDU is sent.
- Max Age: If we don’t receive BPDUs for 20 seconds we know something has changed in the network and we need to re-check the topology.
- Forward Delay: This timer is used for the listening and learning states. We remain in each state for the duration of the forward delay which is 15 seconds by default.
Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Fa0/14 Desg FWD 19 128.16 P2p Fa0/17 Root FWD 19 128.19 P2p
The last part of the show spanning-tree commands shows us the interfaces and their status. SW1 has two interfaces:
• Fa0/14 is a designated port and in (FWD) forwarding mode.
• Fa0/17 is a root port and in (FWD) forwarding mode.
• Fa0/17 is a root port and in (FWD) forwarding mode.
The prio.nbr you see here is the port priority that I explained earlier. We’ll play with this in a bit.
Because only non-root switches have a root-port I can conclude that SW1 is a non-root switch. I know that fa0/17 on SW1 leads to the root bridge.
Let’s take a look at SW2 to see what we find:
SW2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000f.34ca.1000
Cost 19
Port 18 (FastEthernet0/16)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0019.569d.5700
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/14 Altn BLK 19 128.16 P2p
Fa0/16 Root FWD 19 128.18 P2p
What do we see here?
Root ID Priority 32769 Address 000f.34ca.1000 Cost 19 Port 18 (FastEthernet0/16)
Here we see information about the root bridge. This information is similar to what we saw on SW1. The root port for SW2 seems to be fa0/16.
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0019.569d.5700
This is the information about SW2. The priority is the same as on SW1, only the MAC address (0019.569d.5700) is different.
Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Fa0/14 Altn BLK 19 128.16 P2p Fa0/16 Root FWD 19 128.18 P2p
This part looks interesting; there are two things we see here:
• Interface fa0/14 is an alternate port and in (BLK) blocking mode.
• Interface fa0/16 is a root port and in (FWD) forwarding mode.
• Interface fa0/16 is a root port and in (FWD) forwarding mode.
Last but not least let’s check SW3:
SW3#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000f.34ca.1000
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000f.34ca.1000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/14 Desg FWD 19 128.14 P2p
Fa0/16 Desg FWD 19 128.16 P2p
Let’s break down what we have here:
Root ID Priority 32769
Address 000f.34ca.1000
This bridge is the root
Bingo… SW3 is the root bridge in this network. We already knew that because SW1 and SW2 are both non-root but this is how we verify it by looking at SW3.
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 000f.34ca.1000
We can also see the MAC address of SW3.
Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/14 Desg FWD 19 128.14 P2p Fa0/16 Desg FWD 19 128.16 P2p
Both interfaces on SW3 are designated ports and in (FWD) forwarding mode.
You have now seen what the spanning-tree topology looks like. Why was SW3 chosen as the root bridge? We’ll have to verify the bridge ID for the answer:
SW1#show spanning-tree | begin Bridge ID
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0011.bb0b.3600
SW2#show spanning-tree | begin Bridge ID
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0019.569d.5700
SW3#show spanning-tree | begin Bridge ID
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000f.34ca.1000
The priority is the same on all switches (32768) so we have to look at the MAC addresses:
- SW1: 0011.bb0b.3600
- SW2: 0019.569d.5700
- SW3: 000f.34ca.1000
SW3 has the lowest MAC address so that’s why it became root bridge. Why was the fa0/14 interface on SW2 blocked and not the fa0/14 interface on SW1? Once again we have to look at the bridge identifier. The priority is 32768 on both switches so we have to compare the MAC address:
- SW1: 0011.bb0b.3600
- SW2: 0019.569d.5700
SW1 has a lower MAC address and thus a better bridge identifier. That’s why SW2 lost this battle and has to shut down its fa0/14 interface.
That’s it! You have now learned how spanning-tree works and how you can check the spanning-tree topology on your Cisco switches. If you enjoyed this lesson please leave a comment or share it on facebook or twitter.
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