Discussion

14.13 Automatically Allocating Bandwidth
Problem
You want an automatic way to optimize the amount of bandwidth allocated to each
LSP to minimize or eliminate any contention for the available bandwidth on the
shared links.
Solution
MPLS can automatically allocate bandwidth for an LSP and can automatically adjust
the allocation as necessary from time to time. The configuration has two parts. First,
MPLS must gather bandwidth statistics:
[edit protocols mpls]
aviva@R1# set statistics auto-bandwidth
aviva@R1# set statistics file mpls-bandwidth-stats world-readable
Second, configure the LSP to automatically allocate and adjust the bandwidth for an
LSP:
[edit protocols mpls]
aviva@R1# set label-switched-path R1-to-R5 auto-bandwidth minimum-bandwidth 50m
Discussion
In a network with bandwidth constraints, it can be difficult for RSVP to set up LSPs
when insufficient bandwidth is available. If you choose to allocate LSP bandwidth
manually (see Recipe 14.11), it can be challenging to figure out how much bandwidth
to set aside for an individual LSP or for a series of LSPs so that they are always
available to carry your customer’s traffic. MPLS autobandwidth is a JUNOS mechanism
that automatically allocates bandwidth for an LSP. It works by monitoring the
rate of traffic flow through an LSP and periodically resizing the allocated bandwidth
to match the flow rate. In effect, autobandwidth adaptively requests bandwidth reservations
based on actual LSP usage.
When MPLS resizes the bandwidth, it calculates and sets up a new LSP and sets it up
in a make-before-break fashion, then tears down the old LSP after the new one is
established. To avoid double-counting of resources, the LSP is set up as adaptive.
Thus, the new and old LSPs share bandwidth over common links. In this process, the
LSP might be rerouted. If this happens, all traffic already in the existing LSP continues
to the egress router and any new traffic entering the LSP travels along the newly
established path.
You turn on autobandwidth at the ingress router. No configuration is necessary on
the other routers in the LSP. As a first step, enable MPLS statistics collection (with
the set statistics auto-bandwidth command) and create a logfile to track actual
bandwidth usage on the LSP. This recipe creates a file named mpls-bandwidth-stats.
Automatically Allocating Bandwidth | 535
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