Huawei Quidway NetEngine80E Configuration Manual page 527

HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
IS-IS Preference
If there are redundant links on an IS-IS network, there may be multiple equal-cost routes.
The router allows you to configure preference values for equal-cost IS-IS routes so that only the
route with the highest preference will be used and the others will function as backups.
This facilitates traffic management, improves the network reliability, and avoids configuration
change.
IS-IS Fast Convergence
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Issue 02 (2014-09-30)
Incremental SPF (I-SPF)
I-SPF calculates only changed routes at a time, but not all routes.
ISO-10589 defines Dijkstra as the algorithm to calculate routes. When a node is added to
or removed from a network topology, all routes of all nodes need to be calculated if the
Dijkstra algorithm is adopted. As a result, it takes a long time and occupies excessive
resources, reducing the route convergence speed of the entire network.
I-SPF improves this algorithm. Except for the first time, only changed nodes instead of all
nodes are involved in calculation. The SPT generated at last is the same as that generated
by the Dijkstra algorithm. This decreases the CPU usage and speeds up route convergence.
Partial route calculation (PRC)
Similar to I-SPF, only changed nodes are involved in PRC. PRC, however, does not
calculate the shortest path but updates leaf routes based on the SPT calculated by I-SPF.
In route calculation, a leaf represents a route, and a node represents a router. If the SPT
calculated using I-SPF changes, PRC calculates all the leaves on only the changed node;
if the SPT calculated using I-SPF does not change, PRC calculates only the changed leaf.
For example, if an interface of a node is enabled with IS-IS, the SPT of the entire network
remains unchanged. In this case, PRC updates the routes on only the interface of this node,
reducing the CPU usage.
PRC working with I-SPF further improves the convergence performance of the network.
As an improvement of the original SPF algorithm, RPC and I-SPF replace the original
algorithm.
NOTE
In real world applications of NE80E/40Es, only I-SPF and PRC are used to calculate IS-IS routes.
LSP fast flooding
Based on the RFC, when IS-IS receives LSPs from other routers and the LSPs are more
updated than those in its own LSDB, IS-IS uses a timer to flood out the LSPs in the LSDB
at specified intervals. Therefore, the LSDB synchronization is slow.
LSP fast flooding addresses the problem. When a router configured with this feature
receives one or more LSPs, it floods out the LSPs less than the specified number before
route calculation. This accelerates the LSDB synchronization and speeds up network
convergence to the great extent.
Intelligent timer
Although the route calculation algorithm is improved, the long interval for triggering route
calculation also affects the convergence speed. Using a millisecond timer can shorten the
interval, however, excessive CPU resources will be consumed if the network topology
changes frequently. An SPF intelligent timer can quickly respond to certain emergent events
and also prevent excessive CPU resource consumption.
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