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Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
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Chapter 6 QoS with MPLS in an AVVID-Enabled Network
Considerations for MPLS VPN QoS
MPLS VPN QoS
MPLS provides for differentiated service QoS functionality, including:
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LLQ/CBWFQ, which uses a class-map to classify traffic, and then applies a policy map to those
classifications to allocate bandwidth for each class of service. The policy map is then applied to the
MPLS interface.
·
Committed access rate (CAR), which can mark packets upon their arrival and perform recursive
actions on packets, including changing the marking or dropping packets that are out-of
conformance.
·
Class-based policing, which is similar to CAR, but does not support recursive actions.
·
Class-based marking, which sets the MPSL EXP bits instead of accepting the default mapping of IP
Precedence into the EXP bits.
These functions are configured using the modular QoS CLI within Cisco IOS.
For frame-mode MPLS, QoS relies on the use of the MPLS label EXP field. For cell-mode MPLS, which
cannot leverage the EXP field, QoS is achieved by using up to four labels for four individual classes of
service.
Considerations for MPLS VPN QoS
When considering MPLS VPN QoS, you must consider the following factors:
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Convergence
A very important factor to consider in the design of an AVVID network is the convergence of the
network in a time of failure. Specifically, you must consider what happens at the time of failure and what
happens when the link or device is restored. The behavior of the network is quite different in these two
scenarios.
After a Link Failure
In the event of a link failure:
·
MPLS convergence in frame-mode MPLS does not affect the overall convergence time.
·
MPLS convergence occurs immediately after the routing protocol convergence and is based on
labels already stored in the label information base.