LDP neighbor discovery and
session establishment
When
you enable MPLS LDP, the LSRs send out messages to try to find other LSRs with
which they can create LDP sessions.
The
following sections explain the differences between
1)
directly connected LDP sessions
2) nondirectly
connected LDP sessions.
Directly Connected MPLS LDP Sessions
-
The LSR sends out LDP link Hello messages as UDP
packets on port 646 to all the routers on the subnet (multicast)224.0.0.2.
-
A neighboring LSR may respond to the link Hello
message, allowing the two routers to establish an LDP session. This is called
basic discovery.
-
To initiate an LDP session between routers, the
routers determine which router will take the active role and which router will
take the passive role.
-
The router that takes the active role
establishes the LDP TCP connection session and initiates the negotiation of the
LDP session parameters. To determine the roles, the two routers compare their
transport addresses. The router with the higher IP address takes the active
role and establishes the session.
-
After the LDP TCP connection session is
established, the LSRs negotiate the session parameters, including the method of
label distribution to be used.
-
Two methods are available:
•
Downstream on Demand: An LSR advertises label mappings to
a peer only when the peer asks for them.
Nondirectly
Connected MPLS LDP Sessions
-
For these nondirectly connected neighbors, the
LSR sends out a targeted Hello message as a UDP packet, but as a unicast
message specifically addressed to that LSR.
-
The nondirectly connected LSR responds to the
Hello message and the two routers begin to establish an LDP session. This is
called extended discovery.
An MPLS LDP targeted session is a
label distribution session between routers that are not directly connected.
When you create an MPLS traffic engineering tunnel interface, you need to
establish a label distribution session between the tunnel headend and the
tailend routers. You establish nondirectly connected MPLS LDP sessions by
enabling the transmission of targeted Hello messages.
You can use the mpls ldp
neighbor targeted command to set up a targeted session when other
means of establishing targeted sessions do not apply, such as
configuring mpls ip on a traffic engineering (TE) tunnel or
configuring Any Transport over MPLS (AToM) virtual circuits (VCs). For example,
you can use this command to create a targeted session between directly
connected MPLS label switch routers (LSRs) when MPLS label forwarding
convergence time is an issue.
The mpls ldp neighbor
targeted command can improve label convergence time for directly connected
neighbor LSRs when the link(s) directly connecting them are down. When the
links between the neighbor LSRs are up, both the link and targeted Hellos
maintain the LDP session. If the links between the neighbor LSRs go down, the
targeted Hellos maintain the session, allowing the LSRs to retain labels
learned from each other. When a link directly connecting the LSRs comes back
up, the LSRs can immediately reinstall labels for forwarding use without having
to reestablish their LDP session and exchange labels.
The exchange of targeted Hello
messages between two nondirectly connected neighbors can occur in several ways,
including the following:
•
Router 1 sends targeted Hello messages carrying a response
request to Router 2. Router 2 sends targeted Hello messages in response if its
configuration permits. In this situation, Router 1 is considered to
be active and Router 2 is considered to be passive.
•
Router 1 and Router 2 both send targeted Hello messages to
each other. Both routers are considered to be active. Both, one, or
neither router can also be passive, if they have been configured to
respond to requests for targeted Hello messages from each other.
The default behavior of an LSR is to
ignore requests from other LSRs that send targeted Hello messages. You can
configure an LSR to respond to requests for targeted Hello messages by issuing
the mpls ldp discovery targeted-hello accept command.
