MPLS Fundamentals - Chapter 4 - Label Distribution Protocol

• Even if you have mpls enable on all the router/interfaces. If you do not have a IGP running among the routers they would not form ldp neighbor ship. Even if the interfaces are directly connected interfaces & you are able to ping them ldp will not form a neighbor relationship until there is a IGP running.
• On some routers by default tdp is used a the label distribution protocol while on other's ldp is the default, if there is a mismatch in the protocol on 2 router's trying to form a ldp neighbor ship, it would not work.
• The LFIB - which is the table that forwards labeled packets is fed by the label bindings found in the LIB. The LIB is fed by the label bindings received by LDP, Resource Reservation Protocol (RSVP), MP-BGP, or statically assigned label bindings.
• LDP has four major functions:

1.       The discovery of LSRs that are running LDP

2.       Session establishment and maintenance

3.       Advertising of label mappings

4.       Housekeeping by means of notification

• LDP Hello messages are UDP messages that are sent on the links to the 'all routers on this subnet' multicast IP address - in other words, to the 224.0.0.2 group IP multicast address. The UDP port used for LDP is 646.
• The Hello message contains a Hold time. If no Hello message is received from that LSR before the Hold time expires, the LSR removes that LSR from the list of discovered LDP neighbors.
• If the two LDP peers have different LDP Hold times configured, the smaller of the two values is used as the Hold time for that LDP discovery source. Cisco IOS might overwrite the configured LDP Hello interval. It will choose a smaller LDP Hello interval than configured so that it can send at least three LDP Hellos before the Hold time expires.
• This LDP ID is a 6-byte field that consists of 4 bytes identifying the LSR uniquely and 2 bytes identifying the label space that the LSR is using. If the last two bytes are 0, the label space is the platform-wide or per platform label space. If they are non-zero, a per-interface label space is used. If that is the case, multiple LDP IDs are used, where the first 4 bytes are the same value, but the last two bytes indicate a different label space. Per-interface label space is used for LC-ATM links.
• The first 4 bytes of the LDP ID are an IP address taken from an operational interface on the router. If loopback interfaces exist, the highest IP address of the loopback interfaces is taken for the LDP ID or LDP router ID. If no loopback interfaces exist, the highest IP address of an interface is taken.
• In Cisco IOS, the MPLS LDP router ID needs to be present in the routing table of the LDP neighboring routers. If it is not, the LDP session is not formed.
• If two LSRs have discovered each other by means of the LDP Hellos, they attempt to establish an LDP session between them. One LSR tries to open a TCP connection to TCP port 646 to the other LSR.
• The command to change the LDP session keepalive timer is 'mpls ldp holdtime seconds'.You can configure the value of the Hold time to be between 15 and 2,147,483 seconds, with a default of 180 seconds.
• When a router has multiple links toward another LDP router, the same transport address must be advertised on all the parallel links that use the same label space.
• When a router has multiple links toward another LDP router and a different transport address is advertised on those links, the TCP session is still formed, but there is a missing link from the LDP "discovery sources" on the other router. In the previous example, the LDP session is formed, but Ethernet 0/1/3 or Ethernet 0/1/4 is missing from the LDP discovery sources in the output of router london. As such, the traffic from router london toward router new-york is not load-balanced but uses only one outgoing Ethernet link.
• Interfaces belong to the per-platform label space when they are frame-mode interfaces. Interfaces that are not frame-mode interfaces such as LC-ATM interfaces have a per-interface label space.
• With per-interface label space, each label binding has relevance only to that interface. Therefore, for each interface that has a per-interface label space, one LDP session must exist between the pair of routers.
• One example in which the two LDP peers might disagree on the parameters and not form an LDP session is the case of LC-ATM, where the two peers are using different ranges of VPI/VCI values for the labels.
• After the LDP session has been set up, it is maintained by either the receipt of LDP packets or a periodic keepalive message. Each time the LDP peer receives an LDP packet or a keepalive message, the keepalive timer is reset for that peer.
• The downstream LSR is found by looking up the next hop for that prefix in the routing table. Only the remote binding associated with that next-hop LSR should be used to populate the LFIB. This means that only one label from all the advertised label bindings from all the LDP neighbors of this LSR should be used as outgoing label in the LFIB for that prefix. The problem is that the label bindings are advertised as (LDP Identifier, label) without the IP addresses of the interfaces. This means that to find the outgoing label for a particular prefix, you must map to the LDP Identifier the IP address of the interface—pointing back to this LSR—on the downstream LSR. You can only do this if each LDP peer advertises all its IP addresses. These IP addresses are advertised by the LDP peer with Address messages and withdrawn with Withdraw Address messages. You can find these addresses when you are looking at the LDP peer. They are called the bound addresses for the LDP peer.
• The concept of split horizon does not exist; an LDP peer assigns its own local label to a prefix and advertises that back to the other LDP peer, even though that other LDP peer owns the prefix (it is a connected prefix) or that other LDP peer is the downstream LSR.
• Examples in which the targeted LDP session is needed are AToM networks and TE tunnels in an MPLS VPN network.
• If one LSR has MD5 configured for LDP and the other not, the following message is logged: %TCP-6-BADAUTH: No MD5 digest from 10.200.254.4(11092) to 10.200.254.3(646)
• If both LDP peers have a password configured for MD5 but the passwords do not match, the following message is logged: %TCP-6-BADAUTH: Invalid MD5 digest from 10.200.254.4(11093) to 10.200.254.3(646)
• You do not have to clear the LDP neighbor to which you apply the mpls ldp advertise-labels command for it to take effect.
• "Interface config" indicates that LDP is enabled through the interface mpls ip command. "IGP config" indicates that LDP is enabled through the router mpls ldp autoconfig command.
• With MPLS VPN, AToM, Virtual Private LAN Switching (VPLS), or IPv6 over MPLS, the packets must not become unlabeled in the MPLS network. If they do become unlabeled, the LSR does not have the intelligence to forward the packets anymore and drops them.
• The solution is MPLS LDP-IGP Synchronization. This feature ensures that the link is not used to forward (unlabeled) traffic when the LDP session across the link is down. Rather, the traffic is forwarded out another link where the LDP session is still established.
• At the time of writing this book, the only IGP that is supported with MPLS LDP-IGP Synchronization is OSPF.
• The problem that LDP-IGP Synchronization solves cannot happen with BGP and label distribution. Because BGP takes care of the binding advertisement and the control plane for IP routing, the before-mentioned problem cannot happen. Although it is possible for the IGP adjacency to be up while LDP is down on a link, BGP is either up or down, meaning that the installation of the IP prefix in the routing table by BGP is linked to the advertisement of the label binding for that prefix by BGP.
• OSPF does not form an adjacency across a link if the LDP session is not established first across that link. (OSPF does not send out Hellos on the link.)
• By default, if synchronization is not achieved, the IGP waits indefinitely to bring up the adjacency. You can change this with the global command mpls ldp igp sync holddown msecs, which instructs the IGP to wait only for the configured time. After the synchronization Holddown timer expires, the IGP forms an adjacency across the link. As long as the IGP adjacency is up, while the LDP session is not synchronized, the IGP advertises the link with maximum metric.
• When the LDP session between two directly connected LSRs is protected, a targeted LDP session is built between the two LSRs.
• Finally, a useful LDP feature is LDP Graceful Restart. It specifies a mechanism for LDP peers to preserve the MPLS forwarding state when the LDP session goes down. As such, traffic can continue to be forwarded without interruption, even when the LDP session restarts.