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7.3 | ![]() |
IS-IS Operation | |
7.3.10 | ![]() |
LSP flooding and synchronization (continued) |
New LSPs are flooded when there is a
change in the topology. These changes are triggered by the following:
When a new LSP is received, it is installed in the LSP database and marked for flooding. It is sent to all neighbors. Neighbors in turn flood the LSP further. Only new LSPs are flooded. Old LSPs are simply acknowledged. This is because 'state' is already maintained for this LSP, infinite looping of LSPs is avoided. On a point-to-point link, once an adjacency is established, both ISs
send a CSNP packet. Missing LSPs are sent by both ISs if they are not
present in the received CSNP. Missing LSPs may be requested with a PSNP.
On a LAN, there is a designated
intermediate system (DIS). The DIS has two tasks, which are creating and updating the pseudonode LSP and flooding LSPs over the LAN.
Recall that a DIS is elected for each LAN based on priority, with
highest SNPA (MAC) address breaking the tie. DIS election is
deterministic.
The DIS periodically,
every ten seconds, sends CSNPs listing the LSPs it
holds in its link-state database. This is a multicast to all IS-IS
routers on the LAN. R1 compares this list of LSPs with its topology
table and realizes it is missing one LSP.
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