Content

| 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:

- Adjacency came up or went down
- Interface up/down
- Redistributed IP routes change
- Inter-area IP routes change
- An interface is assigned a new metric
- Most other configuration changes

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. and The ACK is communicated with a PSNP.

> Note: Point-to-point is sometimes abbreviated p2p.

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. Therefore, it sends a PSNP to the DIS (R2) to request the missing LSP. The DIS reissues that LSP, and R2 acknowledges it with a PSNP.