Introduction to LDP Sessions

Directly Connected MPLS LDP Sessions

Train and Release

LDP/TDP Support

 

 

12.2S, SB, and SR Trains

• LDP is enabled by default.

 

 

• Cisco IOS Release 12.2(25)S and earlier releases: TDP is

 

supported for LDP features.

 

• Cisco IOS Releases 12.2(27)SBA, 12.2(27)SRA,

 

12.2(27)SRB and later releases: TDP is not supported for

 

LDP features.

 

 

12.T/Mainline Trains

• Cisco IOS Release 12.3(14)T and earlier releases: TDP is

 

 

enabled by default.

 

• Cisco IOS Releases 12.4 and 12.4T and later releases:

 

LDP is enabled by default.

 

• Cisco IOS Release 12.3(11)T and earlier releases: TDP is

 

supported for LDP features.

 

• Cisco IOS Release 12.3(14)T and later releases: TDP is

 

not support ed for LDP features.

 

 

Introduction to LDP Sessions

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 directly connected LDP sessions and nondirectly connected LDP sessions.

•Directly Connected MPLS LDP Sessions, page 3

•Nondirectly Connected MPLS LDP Sessions, page 4

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 User Datagram Protocol (UDP) packets to all the routers on the subnet (multicast). 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.

For information about creating LDP sessions, see the Enabling Directly Connected LDP Sessions, page 6.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

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Cisco Systems 12.4 manual Introduction to LDP Sessions, Train and Release LDP/TDP Support
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12.4 specifications

Cisco Systems has consistently been at the forefront of networking technology, and one of its notable software releases is IOS version 12.4. This version introduced significant enhancements and features that continue to influence networking practices. IOS 12.4 was specifically designed to accommodate the growing demands of network reliability, scalability, and advanced functionalities.

One of the primary characteristics of IOS 12.4 is its enhanced security features. The version integrates advanced security protocols, including improvements in IPsec, which allows for secure communication across potentially insecure networks. Additionally, it supports firewall technologies and access control lists (ACLs), ensuring that organizations can implement stringent security measures tailored to their traffic requirements.

Another defining feature of IOS 12.4 is its support for IPv6. As the internet continued to grow, the need for expanded address space became critical. With IOS 12.4, Cisco provided robust capabilities for transitioning from IPv4 to IPv6, ensuring that network managers could adopt the newer standard without sacrificing performance or reliability. This included support for routing protocols and other networking functions that were essential in an IPv6 environment.

Performance improvements were also a key aspect of IOS 12.4. The release optimized routing protocols, including Enhanced Interior Gateway Routing Protocol (EIGRP) and Open Shortest Path First (OSPF), to enhance convergence times and reduce latency. This effectively contributed to improved network efficiency and uptime.

Cisco also included advanced Quality of Service (QoS) capabilities in IOS 12.4, allowing organizations to prioritize critical traffic. Features such as class-based weighted fair queuing and low-latency queuing became invaluable for organizations requiring seamless voice and video communications over IP networks. This focus on QoS demonstrated Cisco's understanding of the growing importance of multimedia applications in modern business environments.

With a set of stable and scalable routing features, IOS 12.4 supports a variety of platforms, enabling businesses to deploy it across different networking hardware to suit their needs. The modularity of this IOS version makes it flexible for various applications, from small business networks to large enterprise systems.

In summary, Cisco Systems' IOS 12.4 brought forth a wealth of features aimed at enhancing security, performance, and flexibility. Through improved routing capabilities, strong IPv6 support, and advanced QoS features, this version laid the foundation for many of the networking principles that organizations still utilize today.
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