Label Switched Paths; Class Of Service Map - Cisco 10000 Series Configuration Manual

Chapter 20 Configuring Quality of Service for MPLS Traffic

Label Switched Paths

IP packets travel through the core of an MPLS-enabled service provider network by means of multiple,
label-switched paths (LSPs). In ATM networks, label virtual circuits (LVCs) are automatically
established for each IP destination prefix. A standard IP access list (ACL) together with a class of service
(CoS) map and a prefix map are used to specify the number of classes (and LVCs) per IP destination.
For information on a CoS map, see the
If there are multiple equal-cost paths through an ATM network from a P router within the core of the
network to a destination, it is possible that each LVC relating to the same destination could take a
different path through the network, since each LVC could be set up along an alternate equal-cost path.
For example, if four equal-cost paths exist through the network, the first LVC would be set up along the
first path, the second LVC would be set up along the second path, and so on. There is no guarantee,
however, that each LVC would be set up along a parallel path in the network, nor is there any requirement
that each LVC be set up in such a manner.
If there are multiple equal-cost paths through an ATM network from a PE router on the edge of the
network to a destination, LVCs are established for all configured classes of service for each of the
equal-cost paths. The configured load-balancing mechanism determines path selection for data
forwarding.

Class of Service Map

A class of service (CoS) map is a template that maps EXP values to a VC number within an LVC service
group. The Cisco IOS software uses the CoS map to create a binding table that maps EXP values to the
actual VCs. Each LVC has a CoS map and a separate binding table.
You can specify a maximum of four LVCs for each service group.
Based on this map, the binding table will have four VCs named available, standard, premium, and
control. The two least significant bits of the EXP field determine the LVC to which the IP packets will
be directed.
Table 1
EXP Values
0, 4
1, 5
2, 6
3, 7
You can configure a CoS map to limit the number of LVCs created and to redefine the mapping of the
EXP bits.
named available and premium.
OL-7433-09
Default CoS Map
VC Number
0
1
2
3
Table 2 shows a configured CoS map. Based on this map, the binding table will have two VCs
"Class of Service Map" section on page
VC Name
Available
Standard
Premium
Control
Cisco 10000 Series Router Quality of Service Configuration Guide
MPLS CoS Multi-VC Mode
20-13.
Table 1 shows the default CoS map.
20-13