Information Technology Reference
In-Depth Information
TDP and LDP are quite similar functionally, but they do not interoperate. As an example, TDP
uses port 711, whereas LDP uses port 646 for neighbor discovery (using UDP) and session
establishment (using TCP). When both are configured on the same link with
mpls label
protocol
both
, LDP is used to establish the adjacency.
NOTE
Labels are distributed using the
downstream
distribution
method. It has two variations:
•
Unsolicited downstream
—Sometimes simply called
downstream
, in this method, a
downstream LSR sends its entire label space to the upstream LSRs. When it is ready
to forward traffic for a labeled destination, the downstream LSR assigns an incoming
(local) label to the destination and sends the label to all its upstream neighbors. This
method is used for frame-based interfaces such as PPP, POS, and Ethernet.
•
Downstream on demand
—In this method, the upstream LSR explicitly requests a
label binding for an FEC, and the downstream LSR returns a label binding for that
FEC. This method is used for cell-based interfaces (ATM).
Labels are assigned downstream, and label bindings are distributed in the downstream-to-
upstream direction. Here, downstream is per data forwarding, where the next-hop router is
the downstream router.
In either method, label values 0 through 15 are reserved for special purposes. Table 10-1
lists some of the reserved labels. Currently, labels 4 through 15 are not used.
Table 10-1
Some Reserved Labels and Their Meanings
Label Value
LDP
TDP
0
IPv4 Explicit Null
IPv4 Explicit Null
1
Router Alert
IPv4 Implicit Null
2
IPv6 Explicit Null
Router Alert
3
IPv4 Implicit Null
IPv6 Explicit Null
Explicit Null is a unique label signaled by the egress (or ultimate) LER to the penultimate
LSR (an upstream LSR that is directly connected to the egress LER) for label replacement
during forwarding. Upon receiving such a labeled packet, the LER pops the Explicit Null
label and forwards the packet based on the IPv4 header. The forwarding of labeled packets
and the POP operation are discussed in detail in the next section.
Router Alert indicates that the router must inspect the packet. When a received packet
contains this label value at the top of the label stack, the packet is processed locally. The
actual packet forwarding is determined by the label beneath it in the stack.
