System information

ATM Label Switching
Book Title
21-434
ATM Label Switching
ATM uses label switching, a technique in which a simple label is placed in the header of each cell.
The label provides information used in transporting the cell across the next hop in the network.
Networks that do not use label switching usually require each packet (or cell) to contain the explicit
address of the final destination. ATM uses label switching because it is simpler, thereby making
faster switching possible.
Here is how label switching works:
1 A switching unit reads an incoming cell from a particular port. The incoming cell has a routing
label.
2 The switching unit uses the combination of the input port on which the cell was received and the
information in the label to determine where the cell should go next. It does this by referring to a
routing table that correlates the incoming port and label with an outgoing port and label.
3 The switch replaces the incoming label with a new outgoing label and sends the cell through the
outgoing port, which is connected to another switching device. (The new outgoing label is taken
from the routing table.)
4 This process is repeated until the cell reaches its final destination in the ATM network.
For example, suppose your network includes a switching unit called Boston. A number of data paths
go through the Boston switch. When those data paths are created, a routing table is set up within the
Boston switch. The table in the Boston switch has one entry for each data path that goes through the
switch. The entries in the table map the incoming port and label to an outgoing port and label for
each data path, as shown in Table 21-1.
Table 21-1 A Sample Routing Table for a Boston Switch
When the Boston switch receives an incoming cell on port 1 with label M, it consults the routing
table and finds that label M should be replaced with label X and that the cell should be passed out of
the Boston switch on port 7. The cell is then transported to the switch in the network that is connected
to port 7 of the Boston switch, as shown in Figure 21-5.
Port In Label In Port Out Label Out
1L 6 Z
1M 7 X
2N 7 Y