LSP Flooding

LSP Flooding
In order to construct the router’s link-state databases, LSP flooding is utilized. In order to create
the Level 1 link-state database, Level 1 LSPs are flooded throughout the Level 1 area. Flooding
Level 2 LSPs over all Level 2 adjacencies creates a Level 2 link-state database. The creation of
these link-state databases would not be possible without the use of SNPs.
All routers on a broadcast multi-access network will receive multicast LSPs from their
neighbors. The designated IS router for Level 1 will multicast a CSNP, which contains information
about all Level 1 LSPs in its link-state database, to the multicast address AllL1ISs.
A Level 2 DIS will do the same except it will multicast the CSNP to the AllL2ISs multicast
address. The default time the CSNP will be multicast is 10 seconds for Cisco devices.
After the DIS has multicast the CSNP, all of the routers on that broadcast multi-access
network will compare the CSNP to all the LSPs stored in their link-state database. If a router
detects that it has an LSP that is missing from the CNSP or if the router has an LSP that is newer
than the CSNP, the router will multicast the LSP to all of its neighbors. It’s feasible that the
neighbors could detect the missing LSP in the CSNP but do nothing, because they already
received the missing LSP from their neighbor. Remember that all routers—even so-called
DROthers in OSPF parlance—form adjacencies with all other routers. Recall that there are
no DROther-to-DROther adjacencies in OSPF.
A PSNP will be multicast by a router if the router notices that an LSP contained in the CSNP is
missing from its link-state database. The DIS will then send the LSP to the router that requested it.
LSP flooding on point-to-point networks functions differently from LSP flooding on
broadcast multi-access networks. A router sends an LSP to its neighbor on the point-topoint
network. The router then waits for a PSNP to be sent from the neighbor acknowledging
the receipt of the LSP. If the router doesn’t receive the PSNP in a specified period of time,
it will retransmit the LSP to the neighbor. The default time period the router will wait is five
seconds for Cisco devices.
Once a router receives all of the LSPs, it runs the SPF algorithm to select the routes to
populate its routing table.