Configuring IP Router Parameters
18 Router Configuration Guide
While SAP ingress and network QoS policies can achieve the same end result as QPPB (for
example, by assigning a packet arriving on a particular IP interface to a specific forwarding-
class and priority/profile based on the source IP address or destination IP address of the
packet) the effort involved in creating the QoS policies, keeping them up-to-date, and
applying them across many nodes is much greater than with QPPB. In a typical application
of QPPB, a BGP route is advertised with a BGP community attribute that conveys a particular
QoS. Routers that receive the advertisement accept the route into their routing table and set
the forwarding-class and priority of the route from the community attribute.
QPPB Applications
There are two typical applications of QPPB:
1. Coordination of QoS policies between different administrative domains.
2. Traffic differentiation within a single domain, based on route characteristics.
Inter-AS Coordination of QoS Policies
The operator of an administrative domain A can use QPPB to signal to a peer administrative
domain B that traffic sent to certain prefixes advertised by domain A should receive a
particular QoS treatment in domain B. More specifically, an ASBR of domain A can advertise
a prefix XYZ to domain B and include a BGP community attribute with the route. The
community value implies a particular QoS treatment, as agreed by the two domains (in their
peering agreement or service level agreement, for example). When the ASBR and other
routers in domain B accept and install the route for XYZ into their routing table, they apply
a QoS policy on selected interfaces that classifies traffic towards network XYZ into the QoS
class implied by the BGP community value.
QPPB may also be used to request that traffic sourced from certain networks receive
appropriate QoS handling in downstream nodes that may span different administrative
domains. This can be achieved by advertising the source prefix with a BGP community, as
discussed above. However, in this case other approaches are equally valid, such as marking
the DSCP or other CoS fields based on source IP address so that downstream domains can
take action based on a common understanding of the QoS treatment implied by different
DSCP values.
In the above examples, coordination of QoS policies using QPPB could be between a
business customer and its IP VPN service provider, or between one service provider and
another.
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