MPLS PART 5 - LDP NEIGHBORSHIP FORMATION

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

 

If an LSR is one hop from its neighbor, it is directly connected to its neighbor.

-          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 Unsolicited: An LSR advertises label mappings to peers without being asked to.

• Downstream on Demand: An LSR advertises label mappings to a peer only when the peer asks for them.

Nondirectly Connected MPLS LDP Sessions

If the LSR is more than one hop from its neighbor, it is nondirectly connected to its neighbor.

-          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.

The active LSR mandates the protocol that is used for a targeted session. The passive LSR uses the protocol of the received targeted Hello messages.