Least-Fill Bandwidth Rule in CSPF ECMP Selection
Page 142 7450 ESS MPLS Guide
Least-Fill Bandwidth Rule in CSPF ECMP Selection
When multiples equal-cost paths satisfy the constraints of a given RSVP LSP path, CSPF in the
router head-end node will select a path so that LSP bandwidth is balanced across the network
links. In releases prior to R7.0, CSPF used a random number generator to select the path and
returned it to MPLS. In the course of time, this method actually balances the number of LSP paths
over the links in the network; it does not necessarily balance the bandwidth across those links.
The least-fill path selection algorithm identifies the single link in each of the equal cost paths
which has the least available bandwidth in proportion to its maximum reserved bandwidth. It then
selects the path which has the largest value of this figure. The net affect of this algorithm is that
LSP paths will be spread over the network links over time such that percentage link utilization is
balanced. When the least-fill option is enabled on an LSP, during a manual reset CSPF will apply
this method to all path calculations of the LSP, also at the time of the initial configuration.
Inter Area TE LSP (ERO Expansion Method)
Inter area contiguous LSP scheme provides end-to-end TE path. Each transit node in an area can
set up a TE path LSP based on TE information available within its local area.
A PE node initiating an inter area contiguous TE LSP does partial CSPF calculation to include its
local area border router as a loose node.
Area border router on receiving a PATH message with loose hop ERO does a partial CSPF
calculation to the next domain border router as loose hop or CSPF to reach the final destination.
Area Border Node FRR Protection for Inter Area LSP
This feature enhances the prior implementation of an inter-area RSVP P2P LSP by making the
ABR selection automatic at the ingress LER. The user will not need to include the ABR as a loose-
hop in the LSP path definition.
CSPF adds a new capability to compute all segments of a multi-segment intra-area or inter-area
LSP path in one operation. In previous releases, MPLS makes a request to CSPF for each segment
separately.
Figure 7 1 illustrates the role of each node in the signaling of an inter-area LSP with automatic
ABR node selection.
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