Troubleshooting guide

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ATM and Layer 3 Switch Router Troubleshooting Guide
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Chapter 8 Troubleshooting Tag and MPLS Switching Connections
MPLS Overview
The selection of the label to be inserted is not based exclusively on the contents of the Layer 3 packet
header. Forwarding decisions might be based on routing policy.
Once a label is assigned, and added at the front of the Layer 3 packet, it is carried across the network as
part of the packet. The labels are swapped at each LSR and forwarding decisions are made using the
LFIB (label forwarding information base).
The 32-bit MPLS label is located after the Layer 2, header and before the IP header. The MPLS label
contains the following fields:
• Label field (20-bits)—Carries the actual value of the MPLS label.
• CoS field (3-bits)—Can affect the queuing and discard algorithms applied to the packet as it is
transmitted through the network.
• Stack (S) field (1-bit)—Supports a hierarchical label stack.
• TTL (time-to-live) field (8-bits)—Provides conventional IP TTL functionality.
This entire MPLS label is also called a “Shim” header.
Distribution of Label Bindings
Each label switching router (LSR) in the network makes an independent, local decision as to which label
value to use to represent an FEC. This association is known as a label binding. Each LSR informs its
neighbors of the label bindings it has made. This awareness of label bindings by neighboring routers and
switches facilitates the following protocols:
• Tag Distribution Protocol (TDP)—Used to support MPLS forwarding along normally routed paths
• Resource Reservation Protocol (RSVP)—Used to support MPLS traffic engineering
• Border Gateway Protocol (BGP)—Used to support MPLS virtual private networks (VPNs)
The MPLS LDP (label distribution protocol) provides a standard methodology for hop-by-hop, or
dynamic label, distribution in an MPLS network by assigning labels to routes that have been chosen by
the underlying Interior Gateway Protocol (IGP) routing protocols. The resulting labeled paths, called
label switch paths or LSPs, forward label traffic across an MPLS backbone to particular destinations.
These capabilities enable service providers to implement Cisco MPLS-based IP VPNs and IP+ATM
services across multivendor MPLS networks.
LDP allows label switching routers (LSRs) to request, distribute, and release label prefix binding
information to peer routers in a network. LDP enables LSRs to discover potential peers, and to establish
LDP sessions with those peers to order to exchange label binding information.
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, LC-ATM interfaces use 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.
Figure 8-2 shows the summary route propagation between four LSRs in an MPLS network.