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Questions and Answers

What type of packets do Targeted Hello messages use?

TCP packets

UDP packets (correct)

IP packets

ICMP packets

What is the purpose of a Targeted Hello message?

To identify a potential LDP peer (correct)

To establish a TCP connection

To send an Initialization message

To advertise labels

What is the role of the LSR with the larger transport address in LDP session establishment?

It waits for a TCP connection

It sends an Initialization message

It sends a Notification message

It initiates a TCP connection (correct)

What is sent by LSRA to LSRB after the TCP connection is established?

An Initialization message Signup and view all the answers

What happens if LSRB rejects some parameters in the Initialization message?

It sends a Notification message Signup and view all the answers

What is the purpose of the Keepalive message in LDP session establishment?

To verify the LDP session Signup and view all the answers

What is the role of the LSR with the smaller transport address in LDP session establishment?

It waits for a TCP connection Signup and view all the answers

What is negotiated in the Initialization message?

All of the above Signup and view all the answers

What is the purpose of targeted Hello messages in MPLS LDP sessions?

To establish nondirectly connected MPLS LDP sessions Signup and view all the answers

What happens to the LDP session when the links between neighbor LSRs go down?

The targeted Hellos maintain the session Signup and view all the answers

How do Router 1 and Router 2 exchange targeted Hello messages?

Both routers send targeted Hello messages to each other Signup and view all the answers

What is the default behavior of an LSR regarding targeted Hello messages?

It ignores requests for targeted Hello messages Signup and view all the answers

What is the purpose of the mplsldp discovery targeted-hello accept command?

To respond to requests for targeted Hello messages Signup and view all the answers

Which router mandates the protocol used for a targeted session?

The active LSR Signup and view all the answers

What is LDP session protection?

An enhancement to the basic peer discovery mechanism Signup and view all the answers

What happens when a link directly connecting the LSRs comes back up?

The LSRs can immediately reinstall labels for forwarding use Signup and view all the answers

What does the transit LSR do with the mapping in the Label Mapping message?

It adds the mapping to the label forwarding table and sends a Label Mapping message with a specified FEC to its upstream LSR Signup and view all the answers

What is the purpose of a proxy egress LSP?

To extend an LSP to an MPLS-incapable router Signup and view all the answers

What happens when penultimate hop popping (PHP) is enabled?

The penultimate LSR functions as a special proxy egress Signup and view all the answers

What type of LDP session is established between routers that are not directly connected?

An MPLS LDP targeted session Signup and view all the answers

What is the purpose of a targeted Hello message?

To initiate an LDP session with a nondirectly connected LSR Signup and view all the answers

What happens to the remote adjacency if the local adjacency does not recover after the session hold time elapses?

The remote adjacency is torn down Signup and view all the answers

What is required to establish an MPLS traffic engineering tunnel interface?

A label distribution session between the tunnel headend and the tailend routers Signup and view all the answers

What is the purpose of LDP?

To create LSPs through a network Signup and view all the answers

What type of message is sent by the LSR to its nondirectly connected neighbor?

A targeted Hello message as a UDP packet Signup and view all the answers

What type of peer does LDP maintain the presence of?

Neighbor peer Signup and view all the answers

What is required for MPLS routers to distribute labels among routers?

LDP or other label distribution protocols Signup and view all the answers

What is the term for the process of establishing an LDP session between nondirectly connected LSRs?

Extended discovery Signup and view all the answers

What happens to the LDP session if the session hold time is not specified?

The LDP session is maintained permanently Signup and view all the answers

What is the role of Link Hello messages in LDP?

To establish a local adjacency Signup and view all the answers

What is mapped directly in LDP?

Network layer routing information to data link layer switched paths Signup and view all the answers

What is the purpose of adjacencies in LDP?

To maintain the presence of a peer Signup and view all the answers

What happens when a direct link for a local LDP session fails?

The LDP adjacency is torn down, and the session and labels are deleted Signup and view all the answers

What is the main purpose of LDP session protection?

To minimize packet loss and speed up LDP LSP convergence Signup and view all the answers

What type of messages are exchanged to establish a local LDP session?

Link Hello messages Signup and view all the answers

What happens to the LDP session when the direct link recovers?

The LDP session is maintained Signup and view all the answers

What is formed between the remote LDP session and local LDP session?

A backup relationship Signup and view all the answers

What happens to the LDP LSP traffic when the direct link fails?

The traffic is dropped Signup and view all the answers

What is the benefit of LDP session protection in terms of LSP convergence?

It minimizes the time required for LDP session convergence Signup and view all the answers

What is an additional feature that can be configured to enhance LDP session protection?

Session hold time Signup and view all the answers

What is the result of a forced change in the LDP router ID?

Existing LDP sessions are torn down Signup and view all the answers

What type of label space is used by LC-ATM interfaces?

Interface-specific label space Signup and view all the answers

What is the purpose of the extended discovery mechanism?

To discover LSR peers that are not directly connected Signup and view all the answers

What is the result of an LSR receiving a Link Hello message on a specified interface?

A potential LDP peer is connected to the same interface Signup and view all the answers

What is associated with a destination prefix in an LDP label binding?

A label Signup and view all the answers

What is the encapsulation method used for Link Hello messages?

UDP packets Signup and view all the answers

What is the purpose of the basic discovery mechanism?

To discover directly connected LSR peers Signup and view all the answers

What is the result of a teardown of existing LDP sessions?

LDP label bindings are released Signup and view all the answers

What is the purpose of establishing a proxy egress LSP?

To extend an LSP to an MPLS-incapable router Signup and view all the answers

What type of LDP session is established between routers that are not directly connected?

Targeted LDP session Signup and view all the answers

What is the term for the process of establishing an LDP session between nondirectly connected LSRs?

Extended discovery Signup and view all the answers

What happens to the LDP session when the links between neighbor LSRs go down?

The targeted Hellos maintain the session, allowing the LSRs to retain labels learned from each other. Signup and view all the answers

What is the purpose of LDP session protection?

To maintain a session between LDP peers when the basic peer discovery mechanism fails. Signup and view all the answers

What is required to establish an MPLS traffic engineering tunnel interface?

Establishment of a label distribution session Signup and view all the answers

What is the purpose of a targeted Hello message in MPLS LDP?

To establish a targeted LDP session Signup and view all the answers

What is negotiated in the Initialization message?

The label distribution methodology. Signup and view all the answers

What is the role of the active LSR in a targeted session?

To mandate the protocol used for a targeted session. Signup and view all the answers

What is sent by a nondirectly connected LSR to its neighbor?

A Targeted Hello message Signup and view all the answers

What is the benefit of LDP session protection in terms of LSP convergence?

Faster convergence. Signup and view all the answers

What is the purpose of LDP session protection?

To protect the LDP session from link failures Signup and view all the answers

What is the result of penultimate hop popping (PHP) being enabled?

A penultimate LSR functions as a special proxy egress Signup and view all the answers

What is the term for the process of establishing an LDP session between nondirectly connected LSRs?

Extended discovery. Signup and view all the answers

What is required to establish an MPLS traffic engineering tunnel interface?

An LDP session between nondirectly connected LSRs. Signup and view all the answers

What is the purpose of the mplsldp discovery targeted-hello accept command?

To configure an LSR to respond to targeted Hello messages. Signup and view all the answers

What is the primary function of Targeted Hello messages in LDP session establishment?

To discover potential LDP peers Signup and view all the answers

What is the purpose of the Initialization message in LDP session establishment?

To negotiate LDP session parameters Signup and view all the answers

What happens when the LSR with the larger transport address initiates a TCP connection?

The LSR with the larger transport address initiates an LDP session Signup and view all the answers

What is the result if LSRB rejects some parameters in the Initialization message?

LSRB sends a Notification message to LSRA Signup and view all the answers

What is the purpose of the transport address in LDP session establishment?

To determine the active peer Signup and view all the answers

What is exchanged between LSRs to establish an LDP session?

Hello messages and Initialization messages Signup and view all the answers

What is the role of the LSR with the smaller transport address in LDP session establishment?

It serves as the passive peer Signup and view all the answers

What is the result of successful LDP session establishment?

The LSRs establish a TCP connection Signup and view all the answers

What happens to the LDP adjacency when a direct link for a local LDP session fails?

The adjacency is torn down, and the session and labels are deleted Signup and view all the answers

What is the benefit of LDP session protection in terms of LSP convergence?

It minimizes the time required for LDP session convergence Signup and view all the answers

What type of messages are exchanged to establish a remote LDP session?

Targeted Hello messages Signup and view all the answers

What happens to the LDP LSP traffic when the direct link fails?

It is dropped Signup and view all the answers

What is the purpose of configuring a session hold time?

To enhance LDP session protection Signup and view all the answers

What is formed between the remote LDP session and local LDP session in LDP session protection?

A backup relationship Signup and view all the answers

What happens to the LDP session when the direct link recovers?

The LDP session is maintained by the remote adjacency Signup and view all the answers

What is the main purpose of LDP session protection?

To maintain an LDP session, eliminating the need to reestablish an LDP session or re-distribute labels Signup and view all the answers

What is the primary purpose of LDP Discovery messages?

To notify or maintain the presence of an LSR on an MPLS network Signup and view all the answers

What type of adjacency is established by exchanging Target Hello messages between two LSRs?

Remote adjacency Signup and view all the answers

What is the format of an LDP identifier?

A 4-byte LSR ID and a 2-byte label space Signup and view all the answers

Which protocol is used to transmit LDP Session, Advertisement, and Notification messages?

Transmission Control Protocol (TCP) Signup and view all the answers

What type of peer can be maintained by multiple neighbors?

Distant coexistence peer Signup and view all the answers

What is the purpose of LDP adjacencies?

To maintain the presence of a peer Signup and view all the answers

What is a label space in LDP?

A range of labels allocated between LDP peers Signup and view all the answers

What is required to establish an LDP session?

Only a peer Signup and view all the answers

What does a router do if the mplsldp router-id command is not executed?

Selects the largest loopback address as the LDP router ID Signup and view all the answers

What is the effect of issuing the mplsldp router-id command with the force keyword?

The LDP router ID changes to the IP address of the specified interface and tears down existing LDP sessions Signup and view all the answers

What happens to the LDP router ID if the specified interface is down when the mplsldp router-id interface force command is issued?

The LDP router ID changes to the IP address of the interface when it transitions to up Signup and view all the answers

Why might the normal method for determining the LDP router ID be undesirable?

Because it selects an IP address that the routing protocol cannot advertise to a neighboring router Signup and view all the answers

What is the purpose of the mplsldp router-id command?

To specify the IP address of an interface as the LDP router ID Signup and view all the answers

What happens to MPLS forwarding activity when the LDP router ID is forced to change?

It is interrupted Signup and view all the answers

What is a consequence of a forced change in the LDP router ID?

Teardown of existing LDP sessions Signup and view all the answers

What is the default method for determining the LDP router ID if the mplsldp router-id command is not executed?

Selecting the largest IP address pertaining to an operational interface Signup and view all the answers

What happens to the LDP session when the links between the neighbor LSRs go down?

The targeted Hellos maintain the session, allowing the LSRs to retain labels learned from each other. Signup and view all the answers

What is the difference between an active and a passive LSR in targeted Hello message exchange?

The active LSR initiates targeted Hello messages, while the passive LSR responds to them. Signup and view all the answers

What is the purpose of LDP session protection?

To enhance the basic peer discovery mechanism. Signup and view all the answers

What happens when a link directly connecting the LSRs comes back up?

The LSRs reinstall labels for forwarding use without having to reestablish their LDP session and exchange labels. Signup and view all the answers

What is necessary for an LSR to respond to requests for targeted Hello messages?

The mplsldp discovery targeted-hello accept command must be issued. Signup and view all the answers

What is the role of the active LSR in a targeted session?

It mandates the protocol used for a targeted session. Signup and view all the answers

What is the result of an LSR receiving a targeted Hello message with a response request?

The LSR sends a targeted Hello message in response if its configuration permits. Signup and view all the answers

What is the default behavior of an LSR regarding targeted Hello messages?

It ignores requests from other LSRs that send targeted Hello messages. Signup and view all the answers

What happens to the LDP session when the direct link between two nodes fails?

The LDP session is torn down and the session and labels are deleted. Signup and view all the answers

What is the purpose of LDP session protection?

To maintain an LDP session and eliminate the need to reestablish the session or re-distribute labels Signup and view all the answers

What happens to the remote adjacency if the local adjacency does not recover after the session hold time elapses?

The remote adjacency is torn down. Signup and view all the answers

What is formed between the remote LDP session and local LDP session in LDP session protection?

A backup relationship Signup and view all the answers

What is the benefit of LDP session protection in terms of LSP convergence?

It minimizes the time required for LDP session convergence. Signup and view all the answers

What happens to the LDP LSP traffic when the direct link fails?

The traffic is dropped. Signup and view all the answers

What is an additional feature that can be configured to enhance LDP session protection?

Session hold time Signup and view all the answers

What happens when the direct link recovers?

The LDP session is reestablished and distributes labels. Signup and view all the answers

What is the outcome when the session hold time elapses and the local adjacency does not recover?

The remote adjacency is torn down, and the LDP session is also torn down. Signup and view all the answers

What is the primary function of LDP in an MPLS network?

To map network layer routing information to data link layer switched paths Signup and view all the answers

What type of adjacencies does LDP maintain the presence of?

Peer and neighbor adjacencies Signup and view all the answers

What is the outcome when the session hold time is not specified?

The remote adjacency is maintained permanently Signup and view all the answers

What is required for MPLS routers to distribute labels among routers?

LDP or other label distribution protocols Signup and view all the answers

What is the primary function of Link Hello messages in LDP?

To establish adjacencies between LSRs Signup and view all the answers

What is the result of a local adjacency being torn down?

The remote adjacency continues to maintain the LDP session Signup and view all the answers

What is the purpose of LDP in establishing LSPs?

To map network layer routing information to data link layer switched paths Signup and view all the answers

What is the purpose of a Targeted Hello message in LDP?

To advertise labels to neighbors Signup and view all the answers

What is the role of the Keepalive message in LDP session establishment?

To maintain the LDP session Signup and view all the answers

What is required for MPLS routers to distribute labels among routers?

Label distribution protocol Signup and view all the answers

What type of LDP session is established between routers that are not directly connected?

Targeted session Signup and view all the answers

What is the purpose of LDP session protection?

To provide backup paths for LSPs Signup and view all the answers

What is associated with a destination prefix in an LDP label binding?

A forwarding equivalence class Signup and view all the answers

What is the benefit of LDP session protection in terms of LSP convergence?

Faster convergence time Signup and view all the answers

What is the term for the process of establishing an LDP session between nondirectly connected LSRs?

Extended discovery Signup and view all the answers

What is the primary function of LDP session protection?

To maintain an LDP session over a failed link Signup and view all the answers

What happens to the remote adjacency if the local adjacency does not recover after the session hold time elapses?

The remote adjacency is torn down Signup and view all the answers

What is the benefit of LDP session protection in terms of LSP convergence?

It minimizes packet loss Signup and view all the answers

What type of messages are exchanged to establish a remote LDP session?

Targeted Hello messages Signup and view all the answers

What happens to the LDP LSP traffic when the direct link fails?

It is dropped Signup and view all the answers

What is an additional feature that can be configured to enhance LDP session protection?

Session hold time Signup and view all the answers

What is formed between the remote LDP session and local LDP session?

A backup relationship Signup and view all the answers

What is the purpose of the extended discovery mechanism?

To establish a remote LDP session Signup and view all the answers

What is the primary purpose of extended discovery in MPLS LDP?

To establish a label distribution session between nondirectly connected LSRs Signup and view all the answers

What is required to establish an MPLS traffic engineering tunnel interface?

An LDP session between the tunnel headend and tailend routers Signup and view all the answers

What type of LDP session is established between routers that are not directly connected?

Targeted Session Signup and view all the answers

What is the benefit of LDP session protection in terms of LSP convergence?

Faster LSP convergence Signup and view all the answers

What is the purpose of a proxy egress LSP?

To extend an LSP to a non-MPLS node Signup and view all the answers

What is the term for the process of establishing an LDP session between nondirectly connected LSRs?

Extended Discovery Signup and view all the answers

What is the role of the transit LSR in label distribution?

It adds the mapping to its label forwarding table and sends a Label Mapping message with a specified FEC to its upstream LSR Signup and view all the answers

What is sent by the LSR to its nondirectly connected neighbor in extended discovery?

A Targeted Hello message Signup and view all the answers

What is the primary benefit of LDP session protection in terms of LSP convergence?

Faster LSP convergence Signup and view all the answers

Which mechanism is used by LDP session protection to maintain a session between LDP peers when the basic peer discovery mechanism fails?

Extended discovery mechanism Signup and view all the answers

What is the purpose of targeted Hello messages in MPLS LDP sessions?

To establish a nondirectly connected LDP session Signup and view all the answers

What is the result of configuring an LSR to respond to requests for targeted Hello messages?

The LSR becomes active Signup and view all the answers

What is the role of the active LSR in a targeted LDP session?

It mandates the protocol used for the targeted session Signup and view all the answers

What happens to the LDP session when the links between neighbor LSRs go down?

The LDP session is maintained using targeted Hello messages Signup and view all the answers

What is required to establish an MPLS traffic engineering tunnel interface?

RSVP-TE signaling Signup and view all the answers

What is the purpose of the extended discovery mechanism in LDP session protection?

To maintain an LDP session when the basic peer discovery mechanism fails Signup and view all the answers

What is the purpose of Extended Discovery in LDP?

To discover targeted neighbors Signup and view all the answers

What is the benefit of LDP session protection in terms of LSP convergence?

Faster LSP convergence Signup and view all the answers

What is required to establish an MPLS traffic engineering tunnel interface?

LDP or another label distribution protocol Signup and view all the answers

What is the term for the process of establishing an LDP session between non-directly connected LSRs?

Targeted Session Establishment Signup and view all the answers

What happens to the LDP session if the session hold time is not specified?

The LDP session is maintained permanently Signup and view all the answers

What is the purpose of adjacencies in LDP?

To maintain the presence of a peer Signup and view all the answers

What is mapped directly in LDP?

Network layer routing information to data link layer switched paths Signup and view all the answers

What is required for MPLS routers to distribute labels among routers?

LDP or another label distribution protocol Signup and view all the answers

Study Notes

LDP Session Establishment

Two LSRs exchange Hello messages to establish an LDP session.
The LSR with the larger transport address serves as the active peer and initiates a TCP connection.
After the TCP connection is established, the active peer sends an Initialization message to negotiate parameters used to establish an LDP session.

LDP Session Establishment Process

Upon receipt of the Initialization message, the passive peer replies to the active peer in either of the following situations: • If the passive peer rejects some parameters, it sends a Notification message to terminate LDP session establishment. • If the passive peer accepts all parameters, it sends an Initialization message and a Keepalive message to the active peer.

Targeted Hello Messages

Targeted Hello messages are encapsulated in UDP packets and carry unicast destination addresses and are sent using LDP port 646.
A Targeted Hello message carries an LDP identifier and other information, such as the hello-hold time and transport address.
If an LSR receives a Targeted Hello message, the LSR has a potential LDP peer.

Proxy Egress LSP

A proxy egress extends an LSP to a non-LDP node.
The extended LSP is called a proxy egress LSP.
A penultimate LSR functions as a special proxy egress when penultimate hop popping (PHP) is enabled.

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, which is called extended discovery.

LDP Session Protection

LDP session protection is an enhancement to the basic peer discovery mechanism.
If the basic peer discovery mechanism fails, LDP session protection uses an extended peer discovery mechanism to maintain a session between LDP peers.
LDP session protection helps maintain an LDP session, eliminating the need to reestablish an LDP session or re-distribute labels.

Session Hold Time

A session hold time can be configured to maintain an LDP session within a specified time period after a local adjacency established using Link Hello messages is torn down.
If the local adjacency does not recover after the session hold time elapses, the remote adjacency is torn down, and the LDP session maintained using the remote adjacency is also torn down.

LDP Router ID and Sessions

A change in the LDP router ID tears down existing LDP sessions, releases label bindings, and interrupts MPLS forwarding activity associated with the bindings.

LDP Bindings

An LDP label binding is an association between a destination prefix and a label.
Labels are allocated from a set of possible labels called a label space.
There are two types of label spaces:
- Interface-specific: uses interface resources for labels, and an LDP platform may support zero, one, or more interface-specific label spaces.
- Platform-wide: a single platform-wide label space for use by interfaces that can share the same labels.

LDP Discovery Mechanisms

LDP discovery mechanisms are used by LSRs to discover potential LDP peers.
There are two types of LDP discovery mechanisms:
- Basic discovery mechanism: used to discover directly connected LSR peers on a link.
- Extended discovery mechanism: used to discover LSR peers that are not directly connected to a local LSR.

Basic Discovery Mechanism

An LSR periodically sends Link LDP Hello messages to discover LDP peers and establish local LDP sessions with the peers.
Link Hello messages are encapsulated in UDP packets with a specific multicast destination address and are sent using LDP port 646.
A Link Hello message carries an LDP identifier and other information, such as the hello-hold time and transport address.

Extended Discovery Mechanism

Used to discover LSR peers that are not directly connected to a local LSR.
Establishes nondirectly connected MPLS LDP sessions by enabling the transmission of targeted Hello messages.
Targeted Hello messages are encapsulated in UDP packets and carry unicast destination addresses and are sent using LDP port 646.

LDP Session Protection

LDP session protection is an enhancement to the basic peer discovery mechanism.
Uses an extended peer discovery mechanism to maintain a session between LDP peers if the basic peer discovery mechanism fails.

Process of Establishing an LDP Session

Two LSRs exchange Hello messages to establish an LDP session.
The process involves:
- Exchanging Hello messages carrying transport addresses.
- Establishing a TCP connection using the transport addresses.
- Negotiating parameters used to establish an LDP session, including LDP version, label advertisement mode, Keepalive hold timer value, maximum PDU length, and label space.

LDP Adjacency and Peer Relationships

LDP adjacency maintains a peer relationship between two LSRs
Two types of LDP adjacencies:
- Local adjacency: established by exchanging Link Hello messages between two LSRs
- Remote adjacency: established by exchanging Target Hello messages between two LSRs

LDP Messages

Two LSRs exchange the following messages:
- Discovery message: used to notify or maintain the presence of an LSR on an MPLS network
- Session message: used to establish, maintain, or terminate an LDP session between LDP peers
- Advertisement message: used to create, modify, or delete a mapping between a specific FEC and label
- Notification message: used to provide advisory information or error information

Label Space and LDP Identifier

Label space: defines a range of labels allocated between LDP peers
LDP identifier: identifies a label space used by a specified LSR
LDP identifier consists of 6 bytes: 4-byte LSR ID and 2-byte label space
LDP identifier format: :

LDP Router ID

LDP router ID determines the label space used by an LSR
If the mplsldp router-id command is not executed, the router determines the LDP router ID as follows:
- Examines the IP addresses of all operational interfaces
- Selects the largest loopback address as the LDP router ID if available
- Otherwise, selects the largest IP address pertaining to an operational interface as the LDP router ID
The mplsldp router-id command allows specifying the IP address of an interface as the LDP router ID

LDP Session Establishment and Maintenance

LDP sessions are established between directly connected LSRs
LDP sessions can be maintained using targeted Hello messages between nondirectly connected LSRs
Targeted Hello messages can be sent in several ways:
- One router sends targeted Hello messages carrying a response request to another router
- Both routers send targeted Hello messages to each other
LDP session protection can be implemented to maintain an LDP session and minimize packet loss

LDP Session Protection

LDP session protection is an enhancement to the basic peer discovery mechanism
Uses an extended peer discovery mechanism to maintain a session between LDP peers if the basic peer discovery mechanism fails
Helps maintain an LDP session, eliminating the need to reestablish an LDP session or re-distribute labels

Session Hold Time

Session hold time can be configured to maintain an LDP session within a specified time period
If the local adjacency does not recover after the session hold time elapses, the remote adjacency is torn down, and the LDP session maintained using the remote adjacency is also torn down

Establishing MPLS LDP Sessions

Nondirectly connected MPLS LDP sessions are established by enabling the transmission of targeted Hello messages.
When a link between neighbor LSRs is up, both link and targeted Hellos maintain the LDP session.
If a link between neighbor LSRs goes down, targeted Hellos maintain the session, allowing LSRs to retain labels learned from each other.

Targeted Hello Messages

Targeted Hello messages are encapsulated in UDP packets and carry unicast destination addresses and are sent using LDP port 646.
A Targeted Hello message carries an LDP identifier and other information, such as the hello-hold time and transport address.
If an LSR receives a Targeted Hello message, it has a potential LDP peer.

LDP Session Protection

LDP session protection is an enhancement to the basic peer discovery mechanism.
If the basic peer discovery mechanism fails, LDP session protection uses an extended peer discovery mechanism to maintain a session between LDP peers.
LDP session protection helps maintain an LDP session, eliminating the need to reestablish an LDP session or re-distribute labels.

Process of Establishing an LDP Session

Two LSRs exchange Hello messages to establish an LDP session.
The LSR with the larger transport address serves as the active peer and initiates a TCP connection.
After the TCP connection is successfully established, the active peer sends an Initialization message to negotiate parameters used to establish an LDP session with the passive peer.

Session Hold Time

A session hold time can be configured to maintain an LDP session.
If a local adjacency established using Link Hello messages is torn down, a remote adjacency established using Targeted Hello messages continues to maintain an LDP session within the configured session hold time.

Proxy Egress

A proxy egress extends an LSP to a non-LDP node.
A proxy egress LSP can be established on a network with MPLS-incapable routers or in the Border Gateway Protocol (BGP) route load balancing scenario.

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.

Conclusion

LDP is a set of procedures and messages by which LSRs create LSPs through a network by mapping network layer routing information directly to data link layer switched paths.
LDP maintains the presence of a peer through the adjacencies and the type of peer depends on the type of neighbor that maintains it.

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Description

Learn about the process of establishing an LDP session between two LSRs, including the exchange of Hello messages and Initialization messages. Understand the roles of the active and passive peers in negotiating parameters.