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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.
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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
 
Receives BPDUs
 
Sends BPDU
 
Handles data frames
 
Blocked
 
yes
 
no
 
Root
 
Designated
 
To illustrate the operation of the `Spanning Tree Protocol`, let us consider the simple network topology in the figure below.
 
A simple Spanning tree computed in a switched Ethernet network
 
Assume that `Switch4` is the first to boot. It sends its own `BPDU = <4,0,4,1>` (resp. `BPDU = <4,0,4,2>`) on port 1 (resp. port 2). When `Switch1` boots, it sends `BPDU = <1,0,1,1>`. This `BPDU` is received by `Switch4`, which updates its `BPDU` and root priority vector tables and computes a new `BPDU = <1,3,4,1>` (resp. `BPDU = <1,3,4,2>`) on port 1 (resp. port 2). Indeed, there is only one root priority vector and hence, it is the best one. Port 1 of `Switch4` becomes the `Root` port while its second port is still in the `Designated` state.
 
Assume now that `Switch9` boots and immediately receives `Switch1` 's `BPDU` on port 1. `Switch9` computes its own `BPDU = <1,1,9,1>` (resp. `BPDU = <1,1,9,2>`) on port 1 (resp. port 2) and port 1 becomes the `Root` port of this switch. The `BPDU` is sent on port 2 of `Switch9` and reaches `Switch4`. `Switch4` compares the priority vectors. It notices that the last computed vector (i.e., `V[2] = <1,2,9,2,2>`) is better than `V[1] = <1,3,1,1,1>`. Thus, `Switch4`'s `BPDU` is recomputed and port 2 becomes the `Root` port of `Switch4`. `Switch4` compares its new `BPDU = <1,2,4,p>` with the last `BPDU` received on each port (except for the `Root` port). Port 1 becomes a `Blocked` port on `Switch4` because the `BPDU=<1,0,1,1>` received on this port is better.
 
During the computation of the spanning tree, switches discard all received data frames, as at that time the network topology is not guaranteed to be loop-free. Once that topology has been stable for some time, the switches again start to use the MAC learning algorithm to forward data frames. Only the `Root` and `Designated` ports are used to forward data frames. Switches discard all the data frames received on their `Blocked` ports and never forward frames on these ports.
 

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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.
4 years ago
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31235 String information

Source string location
../../protocols/ethernet.rst:368
String age
4 years ago
Last updated
a month ago
Source string age
4 years ago
Translation file
locale/fr/LC_MESSAGES/protocols/ethernet.po, string 94