Huawei S6700 Series Configuration Manual page 260

S6700 Series Ethernet Switches
Configuration Guide - IP Routing
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BFD for IS-IS
The S6700 supports BFD for IS-IS to detect IS-IS neighbor relationships. BFD can fast detect
the faults on links between IS-IS neighbors and reports them to IS-IS. Fast convergence of IS-
IS is then implemented.
Issue 01 (2012-03-15)
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 S6700s, 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 switch 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.
An IS-IS network running normally is stable. The network seldom changes frequently, and
an IS-IS router does not calculate routes frequently. Therefore, you can set a short interval
(in milliseconds) for triggering the route calculation for the first time. If the network
topology changes frequently, the value of the intelligent timer increases with the calculation
times, and the interval for route calculation becomes longer. This prevents excessive CPU
resource consumption.
The LSP generation intelligent timer is similar to the SPF intelligent timer. In IS-IS, when
the LSP generation timer expires, the system regenerates its own LSP according to the
current topology. In the original implementation mechanism, a timer with a fixed value is
used, which, however, cannot meet the requirements on fast convergence and low CPU
usage. Therefore, the LSP generation timer is designed as an intelligent timer so that it can
respond quickly to some emergent events (such as interface alternation between Up an
Down) to speed up network convergence. In addition, when the network changes
frequently, the value of the intelligent timer becomes greater automatically to prevent
excessive CPU resource consumption.
NOTE
Determine whether to configure intelligent timers based on actual network situations and
specifications of deployed routers.
NOTE
BFD detects only one-hop links between IS-IS neighbors. This is because IS-IS establishes only one-hop
neighbors.
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6 IS-IS Configuration
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