 | Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |
LDP creates reliable sessions by running over TCP. You do not have to explicitly configure LDP peers, because each LSR actively discovers all other LSRs to which it is directly connected. LDP is a hard-state protocol, meaning that after the LSP is established, it is assumed to remain in place until it has been explicitly torn down. This is in contrast to RSVP-TE, which is a soft-state protocol. See RSVP-TE Messages and Sessions.
LDP uses many messages to create LSPs, classified in the following four types:
Unlike the other LDP messages, the discovery process runs over UDP. Each LSR periodically broadcasts a link hello message to the well-known UDP port, 646. Each LSR listens on this port for link hello messages from other LSRs. In this manner, each LSR learns about all other LSRs to which it is directly connected, creating link hello adjacencies. When an LSR learns about another LSR, it establishes a TCP connection to the peer on well-known TCP port 646 and creates an LDP session on top of the TCP connection.
A transport address for the local peer is advertised in LDP discovery hello messages. Interfaces that use the platform label space default to the LSR router ID for the transport address. You can use the mpls ldp discovery transport-address command to specify an arbitrary IP address as the transport address.
LDP can also discover peers that are not directly connected if you provide the LSR with the IP address of one or more peers by means of an access list. The LSR sends targeted hello messages to UDP port 646 on each remote peer. If the targeted peer responds with a targeted hello message to the initiator, a targeted hello adjacency is created and session establishment can proceed.
In certain cases, a targeted hello adjacency to directly connected peers might be useful. If an LSR receives both a link hello message and a targeted hello message from the same initiator, only a single LDP session is established between the LSRs.
By default, because all LSRs listen on the well-known port, they all attempt to create a session with the originator. You can use the mpls ldp link-hello disable command to suppress the transmission of link hello messages. Thereafter, sessions are formed only with peers contacted with targeted hello messages.
The LDP peers exchange session initialization messages that include timer values and graceful-restart parameters. An LSR responds with a keepalive message if the values in the initialization message are acceptable. If any value is not acceptable, the LSR responds instead with an error notification message, terminating the session. After a session is established, LDP peers exchange keepalive messages that verify continued functioning of the LSR. Failure to receive an expected keepalive message causes an LSR to terminate the LDP session.
Label mapping and distribution use downstream-unsolicited, independent control.
With downstream-unsolicited, independent control, an LSR creates a label binding whenever it learns a new IGP route; the LSR sends a label mapping message immediately to all of its peer LSRs—upstream and downstream—without having received a label request message from any peer. The LSR sends the label mapping message regardless of whether it has received a label mapping message from a downstream LSR. This is the label distribution method employed in a topology-driven MPLS network.
A downstream LSR can send a label withdrawal message to recall a label that it previously mapped. If an LSR that has received a label mapping subsequently determines that it no longer needs that label, it can send a label release message that frees the label for use.
| Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |