Configuring IP Router Parameters
24 Router Configuration Guide
QPPB and Multiple Paths to a Destination
When ECMP is enabled some routes may have multiple equal-cost next-hops in the
forwarding table. When an IP packet matches such a route the next-hop selection is typically
based on a hash algorithm that tries to load balance traffic across all the next-hops while
keeping all packets of a given flow on the same path. The QPPB configuration model
described in Associating an FC and Priority with a Route allows different QoS information
to be associated with the different ECMP next-hops of a route. The forwarding-class and
priority of a packet matching an ECMP route is based on the particular next-hop used to
forward the packet.
When Edge PIC [1] is enabled some BGP routes may have a backup next-hop in the
forwarding table in addition to the one or more primary next-hops representing the equal-cost
best paths allowed by the ECMP/multipath configuration. When an IP packet matches such
a route a reachable primary next-hop is selected (based on the hash result) but if all the
primary next-hops are unreachable then the backup next-hop is used. The QPPB
configuration model described in Associating an FC and Priority with a Route allows the
forwarding-class and priority associated with the backup path to be different from the QoS
characteristics of the equal-cost best paths. The forwarding class and priority of a packet
forwarded on the backup path is based on the fc and priority of the backup route.
QPPB and Policy-Based Routing
When an IPv4 or IPv6 packet with destination address X arrives on an interface with both
QPPB and policy-based-routing enabled:
• There is no QPPB classification if the IP filter action redirects the packet to a directly
connected interface, even if X is matched by a route with a forwarding-class and
priority
• QPPB classification is based on the forwarding-class and priority of the route
matching IP address Y if the IP filter action redirects the packet to the indirect next-
hop IP address Y, even if X is matched by a route with a forwarding-class and priority
QPPB and GRT Lookup
Source-address based QPPB is not supported on any SAP or spoke SDP interface of a VPRN
configured with the grt-lookup command.
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