Comparison of the IS-IS Restart Mechanisms 131
To have the neighbor set SRMﬂags for all LSPs (ensuring database synchronization), R2
transmits the CSNP containing the special LSP. The special CSNP is transmitted as a
6. Upon receipt of the CSNP (covering the entire range of LSP IDs) and containing the
special LSP, R1 sets SRMﬂags for all LSPs. In addition, R1 transmits a PSNP requesting
the special LSP from R2.
7. After sending the CSNP containing the special LSP and having received its conﬁrmation
through the PSNP requesting the special LSP, R2 sends a set of normal CSNPs so that R1
does not unnecessarily transmit those LSPs that it already possesses.
8. Upon receiving a normal set of CSNPs, R1 clears the SRMﬂags for those LSPs that R2
already claims to have. For the remaining LSPs with SRMﬂags set, R1 transmits the
9. Following normal procedures, R2 synchronizes its database. Because R2 does not have
any reliable method to determine when its database is complete, it uses heuristics such as
a timer or waits until the LSPs’ rate of arrival falls below a certain threshold. Upon
completion of the database synchronization, R2 runs the SPF algorithm and updates the
FIB tables. To avoid causing a routing ﬂap in other routers, R2 regenerates its own LSPs,
which are the same as those generated before this restart. Therefore, reception of R2’s
LSPs should not trigger SPF calculations on R1 and other routers in the domain.
10. Normal operation resumes.
Comparison of the IS-IS Restart Mechanisms
The advantages of the IETF IS-IS restart approach include scalability (does not require
IS-IS–related state preservation across the restart) and standards-based implementation.
Disadvantages of this approach are that it requires IS-IS protocol extensions and support from
its neighbors (helper nodes). To elaborate on this further, consider a router R1 in a provider’s
network that is peering with an R2 that might be managed by another provider or a customer.
The IETF IS-IS restart mechanism requires that both R1 and R2 support this capability.
However, the customer or the network provider might not always be willing to upgrade, or R2
might not be upgradeable.
An advantage of the Cisco IS-IS restart approach is that it does not require support from
neighbors. Therefore, unlike the IETF IS-IS restart, a Cisco IS-IS restart- and NSF-capable
router handles the case, whereas its neighbors are not IS-IS restart-capable. However, Cisco
IS-IS restart might be less scalable than the IETF IS-IS restart, because some IS-IS-related state
information must be preserved across the restart. Another drawback of this approach is the lack
of a reliable mechanism to determine when database synchronization is complete. Note that the
Cisco IOS architecture supports both IETF and Cisco-speciﬁc IS-IS restart mechanisms.
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