Troubleshooting guide

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ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 8 Troubleshooting Tag and MPLS Switching Connections
MPLS Overview
From an historical and functional standpoint, LDP is a superset of Cisco's prestandard Tag Distribution
Protocol (TDP), which also supports MPLS forwarding along normally routed paths. For those features
supported by both LDP and TDP the pattern of protocol exchanges between network routing platforms
is identical. The differences between LDP and TDP for those features supported by both protocols are
largely embedded in their respective implementation details, such as the encoding of protocol messages.
Table 8-1 provides a conversion from the tag switching designations to the equivalent MPLS
designations.
How MPLS Works
As a packet traverses the network in conventional Layer 3 forwarding mechanisms, each router extracts
from the Layer 3 header all the information needed to forward the packet. This information is then used
as an index for a routing table lookup to determine the next hop for the packet.
In the most common case, the only needed field in the header is the destination address. But, in some
cases, other fields might also be needed. This means at each router, as the packet traverses the network,
all the header analysis must be independently completed. Additionally, a complicated table lookup must
be done at each router.
In label switching, the analysis of the Layer 3 header is done only once, when the packet enters the
network at the ingress LSR (label switching router). This LSR reads the Layer 3 header and inserts a
small fixed-format label in front of each data packet. For ATM MPLS connections the label used is the
VPI/VCI of the virtual circuit.
Many different headers can map to the same label, as long as those headers always result in the same
choice of the next hop. A label represents an FEC (forwarding equivalence class) or a set of packets
which might be of different types, but are the same with respect to the forwarding function.
Table 8-1 Equivalency Table for Tag Switching and MPLS Terms
Old Tag Switching Terminology New MPLS IETF Terminology
Tag Switching MPLS, Multiprotocol Label Switching
Tag (short for Tag Switching) MPLS
Tag (item or packet) Label
TDP (Tag Distribution Protocol) LDP (Label Distribution Protocol)
Cisco TDP and LDP (MPLS Label Distribution Protocol)
are nearly identical in function, but use incompatible
message formats and some different procedures.
Tag Switched Label Switched
TFIB (Tag Forwarding Information Base) LFIB (Label Forwarding Information Base)
TSR (Tag Switching Router) LSR (Label Switching Router)
TSC (Tag Switch Controller) LSC (Label Switch Controller)
ATM-TSR (ATM Tag Switch Router) ATM-LSR (ATM Label Switch Router, such as the
Cisco BPX 8650 switch)
TVC (Tag VC, Tag Virtual Circuit) LVC (Label VC, Label Virtual Circuit)
TSP (Tag Switch Path) LSP (Label Switch Path)
XTag ATM (extended Tag ATM port) XmplsATM (extended MPLS ATM port)