English French
the label for the outgoing packet
For example, consider the `label forwarding table` of a network node below.
index
outgoing interface
label
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