Link-State Routing
Initial Sequence Numbers
The sequence-number strategy outlined on the previous page works well,
except in a special situation:
If a router restarts after a failure, what should the
initial sequence number be for the LSPs it begins
sending?
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A restarted router will not remember the LSPs it sent in the past. If
an initial sequence number is incorrectly selected, it may send out
new LSPs which will be ignored.
For example, if a router
has in its database an LSP from router
with sequence number 10, and
,
after a restart, chooses 0 as its initial sequence number,
will ignore new LSPs sent by
.
A Solution
A solution to this problem is called the lollipop sequence
space. The idea of this method is for a router to use, upon
restart, a sequence number that is different from any sequence number
that router could have used in the past.
In this scheme, the sequence numbers used by routers are either
negative, zero, or positive. Upon a router's restart, it begins
assigning the smallest possible negative sequence
number to the LSPs it sends out. If
is the size of the range, this number will be
(and the largest possible sequence number will be
).
Subsequent LSPs are numbered
,
and so on.
When the sequence number becomes positive, the subsequent numbers
"wrap around" in the "circular" part of the sequence space (as we saw
on the previous
page).
The new rule for comparing the "age" of LSPs is:
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