LDP Autoconfiguration

LDP is enabled on an interface by configuring the interface command mpls ip. On an LSR, LDP is usually enabled on all the interfaces on which the IGP is enabled. Much easier than configuring mpls ip on every interface separately is enabling LDP Autoconfiguration for the IGP. Every interface on which the IGP is running then has LDP enabled. The OSPF router command to enable LDP Autoconfiguration is this:

mpls ldp autoconfig [area area-id]

As you can see, it can be enabled for just a specific OSPF area. You can also disable it from specific interfaces if you want to. The interface command to disable LDP Autoconfiguration on an interface is as follows:

no mpls ldp igp autoconfig

Look at Example 4-23. "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.

Example 4-23 Configuration Example of LDP Autoconfiguration "1

hostname madrid

router ospf 1

mpls ldp autoconfig area 0 router-id 10.200.254.5 log-adj acency-changes network 10.200.254.0 0.0.0.255 area 0 network 10.200.0.0 0.0.255.255 area 0

madrid#show mpls interfaces detail

Interface Ethernet3/1:

IP labeling enabled (ldp): Interface config IGP config LSP Tunnel labeling enabled BGP labeling not enabled MPLS operational

Interface Serial4/0:

IP labeling enabled (ldp): Interface config IGP config LSP Tunnel labeling enabled BGP labeling not enabled MPLS operational

Example 4-23 Configuration Example of LDP Autoconfiguration (Continued)

madrid#show mpls ldp discovery detail

Local LDP Identifier: 10.200.254.5:0 Discovery Sources: Interfaces:

Ethernet3/1 (ldp): xmit/recv

Enabled: Interface config, IGP config;

Hello interval: 5000 ms; Transport IP addr: 10.200.254.5 LDP Id: 10.200.254.2:0

Src IP addr: 10.200.215.1; Transport IP addr: 10.200.254.2 Hold time: 15 sec; Proposed local/peer: 15/15 sec Reachable via 10.200.254.2/32 Serial4/0 (ldp): xmit/recv

Enabled: Interface config; IGP config;

Hello interval: 5000 ms; Transport IP addr: 10.200.254.5 LDP Id: 10.200.254.4:0

Src IP addr: 10.200.216.2; Transport IP addr: 10.200.254.4 Hold time: 15 sec; Proposed local/peer: 15/15 sec Reachable via 10.200.254.4/32

MPLS LDP-IGP Synchronization

A problem with MPLS networks is that LDP and the IGP of the network are not synchronized. Synchronization means that the packet forwarding out of an interface happens only if both the IGP and LDP agree that this is the outgoing link to be used. A common problem with MPLS networks that are running LDP is that when the LDP session is broken on a link, the IGP still has that link as outgoing; thus, packets are still forwarded out of that link. This happens because the IGP installs the best path in the routing table for any prefix. Therefore, traffic for prefixes with a next hop out of a link where LDP is broken becomes unlabeled. This is not a big problem for networks that are running IPv4-over-MPLS only. At the point where LDP is broken, the packets become unlabeled. The packets are forwarded as IPv4 packets until they become labeled again on the next LSR. However, this is a problem for more than just the IPv4-over-MPLS case. 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.

In the case of MPLS VPN, the packets are IPv4 packets, but they should be forwarded according to a VRF routing table. This table is private for one customer and is present only on the edge LSRs or PE routers. Therefore, when the MPLS VPN packets become unlabeled on the core LSRs—the P routers—they are dropped. The same is true for AToM and IPv6 traffic. The core LSRs cannot forward them unlabeled. One LDP session being down while the IGP adjacency is up between two LSRs can result in major problems because much traffic can be lost. Figure 4-8 illustrates an LDP session being down between two LSRs in the MPLS core and labeled packets being dropped.

Figure 4-8 LDP Session Down Between LSRs

Ingress LSR

IGP Adjacency UP LDP Session DOWN

IGP Adjacency UP LDP Session DOWN

Egress LSR

The same problem can occur when LSRs restart. The IGP can be quicker in establishing the adjacencies than LDP can establish its sessions. This means that the IGP forwarding is already happening before the LFIB has the necessary information to start the correct label forwarding. The packets are incorrectly forwarded (unlabeled) or dropped until the LDP session is established.

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.

NOTE 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.

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