MPLS
Page 22 7210 SAS M, T, X, R6, Mxp MPLS Configura-
tion Guide
Label Switching Routers
LSRs perform the label switching function. LSRs perform different functions based on it’s
position in an LSP. Routers in an LSP do one of the following:
• The router at the beginning of an LSP is the ingress label edge router (ILER). The ingress
router can encapsulate packets with an MPLS header and forward it to the next router
along the path. An LSP can only have one ingress router.
• A Label Switching Router (LSR) can be any intermediate router in the LSP between the
ingress and egress routers. An LSR swaps the incoming label with the outgoing MPLS
label and forwards the MPLS packets it receives to the next router in the MPLS path
(LSP). An LSP can have 0-253 transit routers.
• The router at the end of an LSP is the egress label edge router (ELER). The egress router
strips the MPLS encapsulation which changes it from an MPLS packet to a data packet,
and then forwards the packet to its final destination using information in the forwarding
table. Each LSP can have only one egress router. The ingress and egress routers in an LSP
cannot be the same router.
A router in your network can act as an ingress, egress, or transit router for one or more LSPs,
depending on your network design.
An LSP is confined to one IGP area for LSPs using constrained-path. They cannot cross an
autonomous system (AS) boundary.
Static LSPs can cross AS boundaries. The intermediate hops are manually configured so the LSP
has no dependence on the IGP topology or a local forwarding table.
LSP Types
The following are LSP types:
• Static LSPs — A static LSP specifies a static path. All routers that the LSP traverses must
be configured manually with labels. No signaling such as RSVP or LDP is required.
• Signaled LSP — LSPs are set up using a signaling protocol such as RSVP-TE or LDP.
The 7210 SAS M supports only RSVP-TE for setting up LSPs. The signaling protocol
allows labels to be assigned from an ingress router to the egress router. Signaling is
triggered by the ingress routers. Configuration is required only on the ingress router and is
not required on intermediate routers. Signaling also facilitates path selection.
There are two signaled LSP types:
→ Explicit-path LSPs — MPLS uses RSVP-TE to set up explicit path LSPs. The hops
within the LSP are configured manually. The intermediate hops must be configured as
either strict or loose meaning that the LSP must take either a direct path from the
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