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West
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Virtual circuit organization
the outgoing interface for the packet
the label for the outgoing packet
For example, consider the `label forwarding table` of a network node below.
index
outgoing interface
label
0
South
7
1
none
2
West
3
East
If this node receives a packet with `label=2`, it forwards the packet on its `West` interface and sets the `label` of the outgoing packet to `2`. If the received packet's `label` is set to `3`, then the packet is forwarded over the `East` interface and the `label` of the outgoing packet is set to `2`. If a packet is received with a label field set to `1`, the packet is discarded since the corresponding `label forwarding table` entry is invalid.
`Label switching` enables a full control over the path followed by packets inside the network. Consider the network below and assume that we want to use two virtual circuits : `R1->R3->R4->R2->R5` and `R2->R1->R3->R4->R5`.
->R2
->R5
Since a packet received with `label=1` must be forwarded to `R5` with `label=1`, `R2`'s `label forwarding table` could contain :
Two virtual circuits pass through `R3`. They both need to be forwarded to `R4`, but `R4` expects `label=1` for packets belonging to the virtual circuit originated by `R2` and `label=0` for packets belonging to the other virtual circuit. `R3` could choose to leave the labels unchanged.
->R4
->R3
The figure below shows the path followed by the packets on the `R1->R3->R4->R2->R5` path in red with on each arrow the label used in the packets.
MultiProtocol Label Switching (MPLS) is the example of a deployed networking technology that relies on label switching. MPLS is more complex than the above description because it has been designed to be easily integrated with datagram technologies. However, the principles remain. `Asynchronous Transfer Mode` (ATM) and Frame Relay are other examples of technologies that rely on `label switching`.

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cnp3-ebook / principles/network — Czech

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Source string location
../../principles/network.rst:375
String age
5 years ago
Source string age
5 years ago
Translation file
locale/cs/LC_MESSAGES/principles/network.po, string 98