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Forwarding Equivalence Class
A Forwarding Equivalence Class section defines a set of packets that would be forwarded on the same path by the
MPLS network. To define FEC, IPv4 routes are advertised by two common methods:
Host Address.The LSR uses the address of the destination host to create this FEC: all packets going to this
destination will take the same LSP.
Prefix. The LSR uses the destination prefix to create this FEC: all packets take the LSP corresponding to the longest
matching prefix.
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Label Distribution Modes
LDP implementation supports two label distribution modes:
Downstream Unsolicited.In this mode, next hop LSRs distribute labels to peers without waiting for a label request.
Downstream on Demand.In this mode, an LSR distributes a label to a peer only if there is a pending label request
from the peer.
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Label Retention Mode
LDP supports two label retention modes:
Liberal Retention Mode.In this mode, the LSR retains all labels received from all sources. This mode helps in fast
LSP setup, in case of a change in next hop.
Conservative Retention Mode.In this mode, the LSR retains only the labels received from peers that are the next
hop for a given FEC. This mode is used by LSRs that have a constraint on the number of labels that it can retain at any
given time.
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LSP Control
LSPs can be set up in the following ways:
Ordered Control.In this mode, an LSR distributes a label for an FEC to its peer only if it has a corresponding label
from its next hop, or if it is the egress node.
Independent Control.In this mode, an LSR may distribute a label to its peers without waiting for a corresponding label
from its next hop.
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Loop Detection
Loop detection can be enabled to detect routing loops in LSPs. There are two methods supported for the loop detection
mechanism:
Hop Count. During setup of an LSP, the LSP passes hop count with the LSP setup messages. This hop count is
incremented by each node router participating in LSP establishment. If the hop count exceeds the maximum configured
value, the LSP setup process is stopped, and a notification message is passed back to the message originator.
Path Vector.A path vector contains a list of LSR identifiers. This is passed as a part of LSP setup messages. Each
LSR participating in the LSP establishment adds its own LSR identifier to the path vector. If an LSR finds its own
identifier in the path vector, it drops the message, and sends a message back to the originator.
Using these messages ensures that a loop is detected while establishing a label switched path, before any data is
passed over that LSP.
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FTN Entry.FTN represents FEC-to-NHLFE, a logical table that implements the MPLS architecture defined in RFC
3031: Each FTN entry is used to map incoming traffic to an MPLS LSP. This information is assigned to a NHLFE at the
edge of the MPLS cloud. NHLFE represents the Next hop label forwarding, which specifies the MPLS properties for
egressing a packet onto LSP. MPLS properties include label, nexthop IP address and outgoing interface
FTN is an entity that is present only on LSP ingress; it is used for PUSHing label information to native packets and
tunneling them via LSP. However, in MPLS-TP, incoming packets are not directly mapped to MPLS-TP LSP. Instead,
services such as PW map, indirectly route map traffic to the LSP.
For MPLS-TP, FTN is only used to specify NHLFE information for the MPLS-TP LSP at the LSP ingress. FTN
implements the forward-component of an LSP on ingress and the reverse component of LSP on egress (for
bidirectional LSPs)
ILM Entryand Reverse ILM Entry.ILM represents the Incoming Label Map. ILM is a logical table that is indexed by
the incoming interface and label. An ILM entry specifies behavior for processing labeled packets that arrive on MPLS
core and egress nodes.
• On core nodes, the behavior is to swap the labels.
• On egress nodes, the behavior is to POP the label and process the native packet.
• For bidirectional LSPs, ILM entries performing label-POP implement the forward component of LSPs on the egress
and the reverse components of LSPs on ingress.
• ILM entries performing label-SWAP implement the forward and reverse components of LSPs on transit/core nodes.
The table below lists the tunnel Modes for MPLS-TP.
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MPLS Tables
The following subsection describes the supported tabled used by MPLS-NSF.
FTN Table
The FTN table maps each FEC to a set of NHLFE objects. This table is used when forwarding packets that arrive
unlabeled, but need to be labeled before being forwarded.
ILM Table
The ILM table maps incoming label to a set of NHLFE objects. In ZebOS, the RIB entry structure is located in the
ilm_entry file.
NHLFE table
The NHLFE （Next Hop Label Forwarding Entry）table is used when forwarding a labeled packet. This table contains the following information:
• Nexthop of a packet
• Operation required to perform on the label stack for a packet, including one of the following operations:
• Replace the label at the top of the label stack with a specified new label (swap)
• Pop the label stack
• Replace the label at the top of the label stack with a specified new label. Then push one or more specified new
label onto the label stack
