Content

| 7.3 |  | IS-IS Operation |
| 7.3.9 |  | LSP flooding and synchronization |

The following are the two types of link-state PDUs:

- Non-pseduonode PDUs - represents a router, including all ISs and LANs connected with the router.
- Pseudonode PDUs - represents a LAN, including all ISs attached to that LAN, and are generated by the DIS.

An L1 router creates an L1 LSP, an L2 router creates and L2 LSP, and an L1L2 router creates both an L1 and an L2 LSP.

The DIS creates one pseudonode LSP for L1, one for L2, and one for each LAN. The use of pseudonode LSPs reduces the number of adjacencies on a LAN and therefore reduces the flooding of LSPs on the LAN. and . The DIS will create and flood a new pseudonode LSP when the following happens:

- A new neighbor comes up or goes away
- The refresh interval timer expires

The pseudonode LSP is generated by the DIS. The DIS reports all LAN neighbors, including the DIS, in the pseudonode LSP with a metric of zero. All LAN routers, including the DIS, report connectivity to the pseudonode in their LSPs. This is similar in concept to the network LSA in OSPF.
Each IS will create and flood a non-pseudonode LSP when the following happens:

- A new neighbor comes up or goes away
- New IP prefixes are inserted or removed
- The metric of a link changes
- The refresh interval timer expires

For L1 LSP databases, L1 CSNPs and L1 partial sequence number PDUs (PSNPs) are utilized. For L2 LSP databases, L2 CSNPs and L2 PSNPs are utilized.
A CSNP does the following:

- Describes all LSPs in the LSP database, in range. Contains an address range, LSP ID, sequence number, checksum, and remaining lifetime.
- Used in two cases, periodic multicast by DIS, every ten seconds, and on point-to-point links when the link comes up.
- If LSP database is large, multiple CSNPs are sent.

Each router floods its LSPs to adjacent neighbors. The LSPs are passed along unchanged to other adjacent routers until all the routers in the area have received them. All the L1 LSPs received by one router in an area describe the topology of the area.

The IS-IS link-state database consists of all the LSPs the router has received. Each node in the network maintains an identical link-state database. A change in the topology means a change in one or more of the LSPs. The router that has experienced a link going up or down will resend its LSP to inform the other routers of the change. The LSP sequence number is increased by one to let the other routers know that the new LSP supersedes the older LSP. When a router first originates an LSP, the LSP sequence number is one (1). If the sequence number increases to the maximum of 0xFFFFFFFF, the IS-IS process must shut down. IS-IS must shut down for at least 21 minutes, which is the MaxAge + ZeroAgeLifetime. This allows the old LSPs to age out of all the router databases.

Flooding is the process by which these new LSPs are sent throughout the network to ensure that the databases in all routers remain identical. If the LSP database is not synchronized, routing loops might occur. When a router receives a new LSP, it floods this LSP to its neighbors, except the neighbor that sent the new LSP.

On point-to-point links, the neighbors acknowledge the new LSP with a PSNP, which holds the LSP ID, sequence number, checksum, and remaining lifetime. When the acknowledgment PSNP is received from a neighbor, the originating router stops sending the new LSP to that particular neighbor although it may continue to send the new LSP to other neighbors that have not yet acknowledged it.

On LANs there is no explicit acknowledgment with a PSNP. Missing LSPs are detected when a CSNP is received and the list of LSPs within the CSNP is compared with the LSPs in a the database of that router. If any LSPs are missing or outdated, the router will send a request for these in the form of a PSNP.

If a router receives an LSP that has an older sequence number than the one in its IS-IS database, it sends the newer LSP to the router that sent the old LSP. The router keeps resending it until it receives an acknowledgment PSNP from the originator of the old LSP.

LSPs must be flooded throughout an area for the databases to synchronize and for the SPF tree to be consistent within an area. It is not possible to control which LSPs are flooded by using a distribute list, although it is possible to use a route-map to control which routes are redistributed into IS-IS from another routing protocol.