Extreme Networks BlackDiamond 6800 MPLS Installation And User Manual page 107

Table 6-1 describes the label bindings in the MPLS forwarding table for LSR A that are
maintained for FECs reachable via LSR A to LSR C, shown in Figure 6-1.
Table 6-1: Label Bindings for LSR A
Destination
10.1.1.1/32
10.0.1.0/24
10.0.2.0/24
10.0.3.0/24
A direct LSP is always preferred over an indirect LSP. When a route table entry is added
or updated, MPLS first checks for the existence of a direct LSP. If a direct LSP exists, the
information is simply added to the route table entry at that time.
Managing indirect LSP entries is more involved. The OSPF Shortest Path First (SPF)
algorithm determines the availability of indirect LSPs through an egress router. The
intra-area SPF algorithm begins with the calculating router as the root of a graph. The
graph is expanded by examining the networks connected to the root and then the
routers connected to those networks. Continuing in this manner, the graph is built as a
series of parent and child nodes. A check is made for a direct LSP as each entry is
added. A check is also made for an indirect LSP that can be inherited from the parent
node. Thus, for each route table entry, the modified SPF algorithm determines whether
a direct LSP is available and whether an indirect LSP is available for use whenever a
direct LSP is not present.
This design allows label mapping changes for direct LSPs to be managed without
requiring an SPF recalculation. An SPF recalculation is performed when advertisements
and withdrawals of label mappings for /32 FECs are received, which is analogous to
the situation where an OSPF link changes state.
The modification to the SPF algorithm described above is important, because it enables
the capabilities provided by LDP or RVSP-TE LSPs to be fully utilized, while
minimizing the resources devoted to label management.
For example, in a network where all the LSRs implement this feature (such as an
all-Extreme MPLS network), labels only need to be advertised for the direct interfaces of
the LSRs. Yet, LSPs can still be used to route traffic destined for non-MPLS domains.
More specifically, LSPs can be used for all routes advertised by OSPF, with the possible
exception of LDP LSPs to routes summarized by OSPF area border routers (ABRs). The
MPLS Module Installation and User Guide
Next Hop Direct LSP Label Indirect LSP Label
10.2.1.1 31
10.2.1.1 32
10.2.1.1 33
10.2.1.1 34
Routing Using LSPs
30
31
31
31
6-3