IS-IS, fast convergence is achieved through the following
mechanisms:
(1)
Incremental SPF (ISPF)
(2)
Partial route calculation (RPC)
(3) LSP
fast flooding
(4) Intelligent timer (including the SPF intelligent timer
and intelligent timer for generating
LSPs)
In addition, fast fault detection can be achieved through
BFD for IS-IS, and optimizing the IS-IS network can achieve fast convergence.
Configurations of IS-IS Fast Convergence:
isis 1
is-level level-2
cost-style wide
timer lsp-generation
1 50 50 level-2 //Set the interval for
generating the same LSP fragment.
flash-flood 15
level-2 //Configure LSP fast
flooding.
bfd all-interfaces
enable //Enable BFD for IS-IS to
configure BFD for all IS-IS interfaces.
network-entity
22.0000.0000.0001.00
timer spf 1 50 50
//Set the interval for SPF calculation.
traffic-eng level-2 //Configure IS-IS extension to
enable TE on the entire network.
#
interface GigabitEthernet1/0/0
undo shutdown
ip address 22.1.3.1 255.255.255.0
isis enable 1
isis bfd enable
//Enable BFD for IS-IS for a single interface.
interface GigabitEthernet1/0/1
undo shutdown
ip address 22.1.2.1 255.255.255.0
isis circuit-type p2p
//Change the type of a broadcast interface to P2P to avoid DIS election
and thus speed up route convergence.
isis enable 1
isis bfd enable
Notes: The ISPF algorithm and RPC algorithm are the
default algorithms of IS-IS and are enabled by default. Therefore, they do not
need to be configured.
The timer lsp-generation command is used to set the interval
for generating the same LSP fragment.
The parameter init-interval specifies the initial interval
for generating the same LSP fragment. The parameter incr-interval specifies the
incremental interval for generating the same LSP fragment. One incr-interval is
added each time the network topology changes.
When SPF calculation is performed for the first time, the
interval is init-interval. Each time the routes change, the interval is added
by incr-interval until the interval reaches max-interval. After the interval
reaches max-interval for three times, the interval is reduced to init-interval.
In IS-IS, a device recalculates the shortest path when the
LSDB changes. Frequent route calculations consume a lot of resources and thus
degrade the system performance. Delaying SPF calculation can improve efficiency
in route calculation and reduce the consumption of system resources. If the
delay in route calculation is too long, route convergence becomes slow.
To speed up route convergence without affecting the
efficiency of routers, you can use an intelligent timer in SPF calculation.
This timer automatically adjusts the interval according to the frequency of
changes in the LSDB.
Parameters of IS-IS Fast Convergence:
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