OSPF and ISIS SequenceNumber

Experts, can you help me understand the following.....

OSPF from 'routing TCP/IP vol I pag 455':
"If the present sequence number is MaxSequenceNumber and a new instance
of the LSA must be created, the router must first flush the old LSA from
all databases. This is done by setting the age of the existing LSA no
MaxAge (defined later in this section) and reflooding it over all
adjacencies."

ISIS from 'routing TCP/IP vol I pag 611':
"If the Sequence Number increments to the maximum (0xFFFFFFFF), the
IS-IS process must shut down for at least 21 minutes (MaxAge +
ZeroAgeLifetime) to allow the old LSPs to age out of all databases."

IS-IS implements as-well a procedure to purge LSPs by setting the
RemainingLifetime to 0; so why does it use such a dramatic measure of
shutting down instead of purging the troubled LSP as OSPF does?

Thanks,
bit.

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Received on Sat Apr 10 2010 - 08:13:17 ART