Connected: An Internet Encyclopedia
16.5. Incremental updates -- summary link advertisements
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16. Calculation Of The Routing Table
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Next: 16.6. Incremental updates -- AS external link advertisements
16.5. Incremental updates -- summary link advertisements
16.5. Incremental updates -- summary link advertisements
When a new summary link advertisement is received, it is not
necessary to recalculate the entire routing table. Call the
destination described by the summary link advertisement N (N's
address is obtained by masking the advertisement's Link State ID
with the network/subnet mask contained in the body of the
advertisement), and let Area A be the area to which the
advertisement belongs. There are then two separate cases:
- Area A is the backbone and/or the router is not an area
border router.
In this case, the following calculations must be performed.
First, if there is presently an inter-area route to the
destination N, N's routing table entry is invalidated,
saving the entry's values for later comparisons. Then the
calculation in Section 16.2 is run again for the single
destination N. In this calculation, all of Area A's summary
link advertisements that describe a route to N are examined.
In addition, if the router is an area border router attached
to one or more transit areas, the calculation in Section
16.3 must be run again for the single destination. If the
results of these calculations have changed the cost/path to
an AS boundary router (as would be the case for a Type 4
summary link advertisement) or to any forwarding addresses,
all AS external link advertisements will have to be
reexamined by rerunning the calculation in Section 16.4.
Otherwise, if N is now newly unreachable, the calculation in
Section 16.4 must be rerun for the single destination N, in
case an alternate external route to N exists.
- Area A is a transit area and the router is an area
border router.
In this case, the following calculations must be performed.
First, if N's routing table entry presently contains one or
more inter-area paths that utilize the transit area Area A,
these paths should be removed. If this removes all paths
from the routing table entry, the entry should be
invalidated. The entry's old values should be saved for
later comparisons. Next the calculation in Section 16.3 must
be run again for the single destination N. If the results of
this calculation have caused the cost to N to increase, the
complete routing table calculation must be rerun starting
with the Dijkstra algorithm specified in Section 16.1.
Otherwise, if the cost/path to an AS boundary router (as
would be the case for a Type 4 summary link advertisement)
or to any forwarding addresses has changed, all AS external
link advertisements will have to be reexamined by rerunning
the calculation in Section 16.4. Otherwise, if N is now
newly unreachable, the calculation in Section 16.4 must be
rerun for the single destination N, in case an alternate
external route to N exists.
Next: 16.6. Incremental updates -- AS external link advertisements
Connected: An Internet Encyclopedia
16.5. Incremental updates -- summary link advertisements