Riverstone Networks WICT1-12 - Manual (Page 317)

Riverstone Networks RS Switch Router User Guide Release 8.0 17-9
MPLS Configuration MPLS Architecture Overview
Incoming Label Map (ILM)
The ILM contains mappings of labels to output channels and ports. Each entry in the ILM provides an index into the
OTT. When an MPLS labeled packet arrives at the RS, the router uses the top label to perform a lookup in the ILM
table. From the ILM table entry, the RS determines the proper channel and port on which to forward the packet onto
the LSP. (If there is no matching ILM table entry for a label value in an incoming packet, the packet is dropped.)
For static LSPs, the ILM entry is created with the label value specified with the
mpls create l2-static-path
command. For dynamic LSPs, the ILM entry is created with the label value negotiated via RSVP or LDP.
Use the
mpls show hw-ilm-tbl
command to display entries in the hardware ILM by port and index number. ILM
information is also maintained in the software; use the
mpls show ilm-table
command to display ILM information
by interface name or IP address or for all interfaces.
Context Address Memory (CAM)
The CAM is used to map L2 packets to FECs. The RS performs a CAM lookup only if it is the ingress LSR for an LSP.
An entry in the CAM table contains the destination and source MAC address, ethertype, VLAN, port of entry (POE),
and 802.1q priority. If a CAM entry exists for an L2 packet, the RS retrieves the appropriate OTT index. The OTT
entry identified by the OTT index provides the label to be pushed onto the label stack of the MPLS packet. Use the
mpls show hw-cam-tbl
command to display entries in the CAM by port and index number.
Table Lookups at Ingress LSRs
On ingress LSRs, incoming packets must be classified into FECs in order to put MPLS labels on the packets. The RS
performs OTT lookups to transform packets into MPLS labeled packets. For MPLS labeling of routed packets, the
OTT lookup is based on the L3 table entry. For MPLS labeling of bridged packets, a CAM lookup is first performed
using the L2 header information. If there is a matching CAM entry, there is an FEC for these bridged packets and the
corresponding OTT index is returned. The OTT entry identified by the OTT index provides the label to be pushed onto
the label stack of the MPLS packet.
Table Lookups at Transit LSRs
On transit LSRs, MPLS packets are switched from the input port to the output port of the LSR using only the
information in the top label of the MPLS packet. When a labeled packet arrives at the transit LSR, the router looks up
the top label in the ILM table to determine the exit port on the LSR for the packet and the corresponding OTT entry.
Before the packet leaves the LSR, the top label must be replaced with another label, since labels only have significance
between two LSRs that are connected together. The OTT entry determines the new label for the top of the stack. Before
the MPLS packet leaves the LSR, the MPLS time to live (TTL) for the top label is decremented, the top label is
discarded, and the new label value from the OTT entry is put in its place.
Table Lookups at Egress LSRs
On egress LSRs, the processing of MPLS packets is similar to that done on transit LSRs. The LSR looks up the top
label in the ILM table. For any of the following conditions, the MPLS packet is decapsulated and forwarded as a routed
or bridged packet:
• the ILM entry indicates that this is the end of an L2 tunnel