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The `MAC address learning` algorithm allows switches to be plug-and-play. Unfortunately, the loops that arise when the network topology is not a tree are a severe problem. Forcing the switches to only be used in tree-shaped networks as hubs would be a severe limitation. To solve this problem, the inventors of Ethernet switches have developed the `Spanning Tree Protocol`. This protocol allows switches to automatically disable ports on Ethernet switches to ensure that the network does not contain any cycle that could cause frames to loop forever.
The first Ethernet relays that operated in the datalink layers were called `bridges`. In practice, the main difference between switches and bridges is that bridges were usually implemented in software while switches are hardware-based devices. Throughout this text, we always use `switch` when referring to a relay in the datalink layer, but you might still see the word `bridge`.
The Spanning Tree Protocol (802.1d)
The `Spanning Tree Protocol` (STP), proposed in [Perlman1985]_, is a distributed protocol that is used by switches to reduce the network topology to a spanning tree, so that there are no cycles in the topology. For example, consider the network shown in the figure below. In this figure, each bold line corresponds to an Ethernet to which two Ethernet switches are attached. This network contains several cycles that must be broken to allow Ethernet switches, using the MAC address learning algorithm, to exchange frames.
Spanning tree computed in a switched Ethernet network
In this network, the STP will compute the following spanning tree. `Switch1` will be the root of the tree. All the interfaces of `Switch1`, `Switch2` and `Switch7` are part of the spanning tree. Only the interface connected to `LAN B` will be active on `Switch9`. `LAN H` will only be served by `Switch7` and the port of `Switch44` on `LAN G` will be disabled. A frame originating on `LAN B` and destined for `LAN A` will be forwarded by `Switch7` on `LAN C`, then by `Switch1` on `LAN E`, then by `Switch44` on `LAN F` and eventually by `Switch2` on `LAN A`.
Switches running the `Spanning Tree Protocol` exchange `BPDUs`. These `BPDUs` are always sent as frames with destination MAC address as the `ALL_BRIDGES` reserved multicast MAC address. Each switch has a unique 64 bit `identifier`. To ensure uniqueness, the lower 48 bits of the identifier are set to the unique MAC address allocated to the switch by its manufacturer. The high order 16 bits of the switch identifier can be configured by the network administrator to influence the topology of the spanning tree. The default value for these high order bits is 32768.
The switches exchange `BPDUs` to build the spanning tree. Intuitively, the spanning tree is built by first selecting the switch with the smallest `identifier` as the root of the tree. The branches of the spanning tree are then composed of the shortest paths that allow all of the switches that compose the network to be reached. The `BPDUs` exchanged by the switches contain the following information :
the `identifier` of the root switch (`R`)
the `cost` of the shortest path between the switch that sent the `BPDU` and the root switch (`c`)
the `identifier` of the switch that sent the `BPDU` (`T`)
the number of the switch port over which the `BPDU` was sent (`p`)
In addition to the `identifier` discussed above, the network administrator can also configure a `cost` to be associated to each switch port. Usually, the `cost` of a port depends on its bandwidth and the [IEEE802.1d]_ standard recommends the values below. Of course, the network administrator may choose other values. We will use the notation `cost[p]` to indicate the cost associated to port `p` in this section.
Bandwidth
Cost
10 Mbps
2000000
100 Mbps
200000
1 Gbps
20000
2000
100 Gbps
200
The `Spanning Tree Protocol` uses its own terminology that we illustrate in the figure above. A switch port can be in three different states : `Root`, `Designated` and `Blocked`. All the ports of the `root` switch are in the `Designated` state. The state of the ports on the other switches is determined based on the `BPDU` received on each port.
The `Spanning Tree Protocol` uses the ordering relationship to build the spanning tree. Each switch listens to `BPDUs` on its ports. When `BPDU = <R,c,T,p>` is received on port `q`, the switch computes the port's `root priority vector`: `V[q] = <R,c+cost[q],T,p,q>` , where `cost[q]` is the cost associated to the port over which the `BPDU` was received. The switch stores in a table the last `root priority vector` received on each port. The switch then compares its own `identifier` with the smallest `root identifier` stored in this table. If its own `identifier` is smaller, then the switch is the root of the spanning tree and is, by definition, at a distance `0` of the root. The `BPDU` of the switch is then `<R,0,R,p>`, where `R` is the switch `identifier` and `p` will be set to the port number over which the `BPDU` is sent.
Otherwise, the switch chooses the best priority vector from its table, `bv = <R,c+cost[q'],T,p,q'>`. The port `q'`, over which this best root priority vector was learned, is the switch port that is closest to the `root` switch. This port becomes the `Root` port of the switch. There is only one `Root` port per switch (except for the `Root` switches whose ports are all `Designated`). The switch can then compute its own `BPDU` as `BPDU = <R,c',S,p>` , where `R` is the `root identifier`, `c'` the cost of the best root priority vector, `S` the identifier of the switch and `p` will be replaced by the number of the port over which the `BPDU` will be sent.
To determine the state of its other ports, the switch compares its own `BPDU` with the last `BPDU` received on each port. Note that the comparison is done by using the `BPDUs` and not the `root priority vectors`. If the switch's `BPDU` is better than the last `BPDU` of this port, the port becomes a `Designated` port. Otherwise, the port becomes a `Blocked` port.
The state of each port is important when considering the transmission of `BPDUs`. The root switch regularly sends its own `BPDU` over all of its (`Designated`) ports. This `BPDU` is received on the `Root` port of all the switches that are directly connected to the `root switch`. Each of these switches computes its own `BPDU` and sends this `BPDU` over all its `Designated` ports. These `BPDUs` are then received on the `Root` port of downstream switches, which then compute their own `BPDU`, etc. When the network topology is stable, switches send their own `BPDU` on all their `Designated` ports, once they receive a `BPDU` on their `Root` port. No `BPDU` is sent on a `Blocked` port. Switches listen for `BPDUs` on their `Blocked` and `Designated` ports, but no `BPDU` should be received over these ports when the topology is stable. The utilization of the ports for both `BPDUs` and data frames is summarized in the table below.
Port state
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This translation Propagated Empty cnp3-ebook/protocols/lan
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Propagated Empty cnp3-ebook/protocols/ethernet

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Source string location
../../protocols/ethernet.rst:353
String age
3 years ago
Source string age
3 years ago
Translation file
locale/fr/LC_MESSAGES/protocols/lan.po, string 95