W4_JTO_Ph2_Datacom_IT-part-3.pdf

JTO Phase II Data Network & IT LDP 3 LDP 3.1 Learning Objective This chapter will make you understand about concepts of Label Distribution Protocol, which is used in MPLS network for purpose of Label Di...

JTO Phase II Data Network & IT LDP 3 LDP 3.1 Learning Objective This chapter will make you understand about concepts of Label Distribution Protocol, which is used in MPLS network for purpose of Label Distribution in order to set up path for data communication. 3.2 INTRODUCTION Label distribution protocol is a set of rules and procedures that one LSR can use to inform another LSR about which label will be used to forward MPLS traffic between and through them. The path set up by these bilateral agreement is called label switched path (LSP). MPLS architecture does not assume a single label distribution protocol. Following Protocols can be used : Label Distribution Protocol LDP Constraint-based Routing with LDP CR-LDP RSVP with TE extensions RSVP-TE BGP-4 3.3 LABEL DISTRIBUTION PROTOCOL 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. LSPs may have their two end points. One at a directly attached LSR and another at a network egress LSR i.e. number of LSRs away. A FEC be defined for each of the LSP. Each EFC contains one or more FEC elements. Each element identifies which set of incoming packets will be mapped to an LSP at the ingress router. LDP defines messages in the label distribution process and procedures for processing the messages. Label switching routers (LSRs) obtain information about incoming labels, next-hop nodes, and outgoing labels for specified FECs based on the local forwarding table. LSRs use the information to establish LSPs. Two LDP peers set up LDP sessions and exchange Label Mapping messages over the session so that they establish an LSP. When an LSR receives a Hello message from a peer, the LSR establishes an LDP adjacency with the peer may exist. An LDP adjacency maintains a peer relationship between the two LSRs. There are 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 maintains the presence of a peer through this adjacencies and the type of peer depends on the type of neighbor that maintains it. A peer can be maintained by multiple neighbors, and if it is maintained by both the local adjacency and the remote adjacency, the peer type is a distant coexistence peer. Only a peer can establish an LDP session. JTO Phase II (DNIT) Version 1.0 Aug 2021 Page 40 of 174 For Restricted Circulation JTO Phase II Data Network & IT LDP 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. LDP transmits Discovery messages using the User Datagram Protocol (UDP) and transmits Session, Advertisement, and Notification messages using the Transmission Control Protocol (TCP). 3.4 Label Space and LDP Identifier  Label space A label space defines a range of labels allocated between LDP peers.  LDP identifier An LDP identifier identifies a label space used by a specified LSR. An LDP identifier consists of 6 bytes including a 4-byte LSR ID and a 2-byte label space. An LDP identifier is in the format of :. LDP Router ID The router determines the LDP router ID as follows, if the mplsldp router-id command is not executed, 1. The router examines the IP addresses of all operational interfaces. 2. If these IP addresses include loopback interface addresses, the router selects the largest loopback address as the LDP router ID. 3. Otherwise, the router selects the largest IP address pertaining to an operational interface as the LDP router ID. The normal (default) method for determining the LDP router ID may result in a router ID that is not usable in certain situations. For example, the router might select an IP address as the LDP router ID that the routing protocol cannot advertise to a neighboring router. The mplsldp router-id command allows you to specify the IP address of an interface as the LDP router ID. Make sure the specified interface is operational so that its IP address can be used as the LDP router ID. When you issue the mplsldp router-id command with the force keyword, the effect of the mplsldp router-id command depends on the current state of the specified interface:  If the interface is up (operational) and if its IP address is not currently the LDP router ID, the LDP router ID changes to the IP address of the interface. This forced change in the LDP router ID tears down any existing LDP sessions, releases label bindings learned via the LDP sessions, and interrupts MPLS forwarding activity associated with the bindings.  If the interface is down (not operational) when the mplsldp router- id interface force command is issued, when the interface transitions to up, the JTO Phase II (DNIT) Version 1.0 Aug 2021 Page 41 of 174 For Restricted Circulation JTO Phase II Data Network & IT LDP LDP router ID changes to the IP address of the interface. This forced change in the LDP router ID tears down any existing LDP sessions, releases label bindings learned via the LDP sessions, 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. The label used in a label binding is allocated from a set of possible labels called a label space. LDP supports two types of label spaces:  Interface-specific—An interface-specific label space uses interface resources for labels. For example, label-controlled ATM (LC-ATM) interfaces use virtual path identifiers/virtual circuit identifiers (VPIs/VCIs) for labels. Depending on its configuration, an LDP platform may support zero, one, or more interface-specific label spaces.  Platform-wide—An LDP platform supports a single platform-wide label space for use by interfaces that can share the same labels. For Cisco platforms, all interface types, except LC-ATM, use the platform-wide label space. 3.5 LDP Discovery Mechanisms An LDP discovery mechanism is used by LSRs to discover potential LDP peers. LDP discovery mechanisms are classified into the following types:  Basic discovery mechanism: used to discover directly connected LSR peers on a link. An LSR periodically sends Link LDP Hello messages to discover LDP peers and establish local LDP sessions with the peers. The 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. If an LSR receives a Link Hello message on a specified interface, a potential LDP peer is connected to the same interface.  Extended discovery mechanism: used to discover the LSR peers that are not directly connected to a local LSR. The 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. 3.6 Process of Establishing an LDP Session Two LSRs exchange Hello messages to establish an LDP session. JTO Phase II (DNIT) Version 1.0 Aug 2021 Page 42 of 174 For Restricted Circulation JTO Phase II Data Network & IT LDP Figure 23: LDP session establishment In above figure, the process of establishing an LDP session is as follows: 1. Two LSRs exchange Hello messages. After receiving the Hello messages carrying transport addresses, the two LSRs use the transport addresses to establish an LDP session. The LSR with the larger transport address serves as the active peer and initiates a TCP connection. LSRA serves as the active peer to initiate a TCP connection and LSRB serves as the passive peer that waits for the TCP connection to initiate. 2. After the TCP connection is successfully established, LSRA sends an Initialization message to negotiate parameters used to establish an LDP session with LSRB. The main parameters include the LDP version, label advertisement mode, Keepalive hold timer value, maximum PDU length, and label space. 3. Upon receipt of the Initialization message, LSRB replies to LSRA in either of the following situations:  If LSRB rejects some parameters, it sends a Notification message to terminate LDP session establishment.  If LSRB accepts all parameters, it sends an Initialization message and a Keepalive message to LSRA. 4. Upon receipt of the Initialization message, LSRA performs operations in either of the following situation:  If LSRA rejects some parameters after receiving the Initialization message, it sends a Notification message to terminate LDP session establishment.  If LSRA accepts all parameters, it sends a Keepalive message to LSRB. After both LSRA and LSRB have accepted each other's Keepalive messages, the LDP session is successfully established. LDP peers send messages, such as Label Mapping messages, over an LDP session to exchange label information with each other to establish an LSP. Relevant standards define the label advertisement, distribution control, and retention modes. JTO Phase II (DNIT) Version 1.0 Aug 2021 Page 43 of 174 For Restricted Circulation JTO Phase II Data Network & IT LDP 3.7 Label Advertisement Modes An LSR on an MPLS network binds a label to a specific FEC and notifies its upstream LSRs of the binding. This process is called label advertisement. The label advertisement modes on upstream and downstream LSRs must be the same. The label distribution control mode defines how an LSR distributes labels. The label retention mode defines how the LSR handles label mapping from non-preferred next hop. Establishment of an LDP LSP The process of establishing an LDP LSP is as follows: 1. If a label edge router (LER) on an MPLS network discovers a new direct route due to a network route change, and the address carried in the new route does not belong to any existing forwarding equivalence class (FEC), the LER creates a FEC for the address. 2. If the egress has available labels for distribution, it distributes a label for the FEC and pro-actively sends a Label Mapping message to its upstream transit LSR. The Label Mapping message contains the assigned label and an FEC bound to the label. 3. The transit LSR adds the mapping in the Label Mapping message to the label forwarding table and sends a Label Mapping message with a specified FEC to its upstream LSR. 4. The ingress LSR also adds the mapping to its label forwarding table. The ingress LSR establishes an LSP and forwards packets along the LSP. 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. 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. For example, on the network shown in figure, LSRA, LSRB, and LSRC, all except LSRD, are in an MPLS domain. An LSP is established along the path LSA -> LSRB -> LSRC. LSRC functions as a proxy egress and extends the LSP to LSRD. The extended LSP is a proxy egress LSP. Figure 24: Proxy Egress 3.8 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 JTO Phase II (DNIT) Version 1.0 Aug 2021 Page 44 of 174 For Restricted Circulation JTO Phase II Data Network & IT LDP 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. 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 mplsldp 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. 3.9 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. After the basic peer discovery mechanism recovers, LDP can use it to rapidly converge routes and reestablish an LSP. If a direct link for a local LDP session fails, the LDP adjacency is torn down, and the session and labels are deleted. After the direct link recovers, the local LDP session is reestablished and distributes labels so that an LSP can be reestablished over the session. Before the LSP is reestablished, however, LDP LSP traffic is dropped. To speed up LDP LSP convergence and minimize packet loss, LDP session protection needs to be implemented. LDP session protection helps maintain an LDP session, eliminating the need to reestablish an LDP session or re-distribute labels. In figure below, LDP session protection is configured on the nodes at both ends of a link. The two nodes exchange Link Hello messages to establish a local LDP session and JTO Phase II (DNIT) Version 1.0 Aug 2021 Page 45 of 174 For Restricted Circulation JTO Phase II Data Network & IT LDP exchange Targeted Hello messages to establish a remote LDP session, forming a backup relationship between the remote LDP session and local LDP session. Figure 25: LDP session Protection In figure above, if the direct link between A and B fails, the adjacency established using Link Hello messages is torn down. Because the indirectly connected link is working properly, the remote adjacency established using Targeted Hello messages remains. Therefore, the LDP session is maintained by the remote adjacency, and the mapping between FECs and labels for the session also remains. After the direct link recovers, the local LDP session can rapidly restore LSP information. There is no need to reestablish the LDP session or re-distribute labels, which minimizes the time required for LDP session convergence. 3.10 Session Hold Time In addition to LDP session protection, a session hold time can be configured. After 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. 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. If the session hold time is not specified, the remote adjacency permanently maintains the LDP session. 3.11 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. It uses different types of messages to establish and maintain and terminate the LSP. For MPLS routers, LDP or other label distribution protocols are required to distribute the labels among the routers to establish the sessions. JTO Phase II (DNIT) Version 1.0 Aug 2021 Page 46 of 174 For Restricted Circulation