Information Technology Reference
In-Depth Information
As an egress LER for 192.168.100.5/32 and 192.168.100.6/32, R5 assigns local labels for
these two prefixes and distributes them to its upstream neighbors, R3 and R4. Table 10-2
shows the label binding information on R5.
Table 10-2
Label Bindings on R5
Prefix
Local Label
Received Label
Next Hop
192.168.100.5/32
Implicit Null
None
R5
192.168.100.6/32
20
None
R6
The local label, also called in label, is assigned locally. The received label is received from
its downstream neighbor. The received label is often an outgoing label in the forwarding
path, except for the Implicit Null, as discussed previously.
Because 192.168.100.5/32 is local to R5, no outgoing label is received. With the default
behavior on frame-based links, an Implicit Null local label is assigned for the prefix. The
prefix 192.168.100.6/32 is a remote route from R6. R5 advertises a local label of 20. Because
R6 is not running MPLS, R5 does not receive a label from R6.
Next, R5 distributes the local labels to its upstream neighbors. When R3 receives the two
prefixes from R5, it assigns local labels and distributes them upstream to R2. Table 10-3
shows the label bindings on R3.
Table 10-3
Label Bindings on R3
Prefix
Local Label
Received Label
Next Hop
192.168.100.5/32
21
Implicit Null
R5
192.168.100.6/32
22
20
R5
Example 10-5 shows the output of
show mpls ldp bindings
on R3. Note that the label
binding information from LDP or TDP is stored in the Label Information Base (LIB) or Tag
Information Base (TIB). TSR stands for Tag Switch Router.
Example 10-5
LIB on R3
R3#show mpls ldp bindings
...
tib entry: 192.168.100.5/32, rev 18
local binding: tag: 21
remote binding: tsr: 192.168.100.2:0, tag: 20
remote binding: tsr: 192.168.100.5:0, tag: imp-null
tib entry: 192.168.100.6/32, rev 20
local binding: tag: 22
remote binding: tsr: 192.168.100.2:0, tag: 21
remote binding: tsr: 192.168.100.5:0, tag: 20
...
