In a network where devices run the Intermediate System-to-Intermediate System (IS-IS) protocol, convergence is achieved when
a consistent view of the topology is distributed to all devices in the network. When a network event causes a topology change,
a number of steps must occur in order for convergence to occur. The device that initially detects the topology change (for
example, an interface state change) must inform other devices of the topology change by flooding updated routing information
(in the form of link-state protocol data units [PDUs]) to other devices. All devices, including the device that detected the
topology change, must utilize the updated topology information to recompute shortest paths (run a shortest path first [SPF]),
providing the updated output of the SPF calculation to the device’s routing information base (RIB), which eventually causes
the updated routing information to be used to forward packets. Until all devices have performed these basic steps, some destinations
might be temporarily unreachable. Faster convergence benefits the network performance by minimizing the period of time during
which stale topology information—the previous routing information that will be obsoleted by the updated routing information—is
used to forward packets.
After performing an SPF, IS-IS must install updated routes in the RIB. If the number of prefixes advertised by IS-IS is large,
the time between the installation of the first prefix and the last prefix is significant. Priority-driven IP prefix RIB installation
allows a subset of the prefixes advertised by IS-IS to be designated as having a higher priority. Updates to the paths to
these prefixes are installed before updates to prefixes that do not have this designation. Priority-driven IP prefixes reduce
the convergence time for the important IS-IS IP prefixes and results in faster updating for routes that are dependent on these
prefixes. Faster updates shortens the time during which stale information is used for forwarding packets to these destinations.
Prefixes are characterized as having one of three levels of importance:
High-priority prefixes—prefixes that are tagged with a tag designated for fast convergence.
Medium-priority prefixes—any /32 prefixes that are not designated as high-priority prefixes.
Low-priority prefixes—all other prefixes.
When IS-IS updates the RIB, prefixes are updated in the order based on the associated level of importance.
When you assign a high-priority tag to some IS-IS IP prefixes, those prefixes with the higher priority are updated in the
routing tables before prefixes with lower priority. In some networks, the high-priority prefixes are the provider edge (PE)
loopback addresses. The convergence time is reduced for the important IS-IS IP prefixes and results in reduced convergence
time for the update processes that occur in the global RIB and Cisco Express Forwarding.