ISIS compares two LSP

A router processes a received LSP as follows:

- Updating the LSDB on a broadcast link

1. When the DIS receives an LSP, it searches the LSDB for the related records. If the DIS does not find the LSP in its LSDB, it adds the LSP to its LSDB and broadcasts the new LSDB.

2. If the sequence number of the received LSP is greater than that of the local LSP, the DIS replaces the local LSP with the received LSP in the LSDB and broadcasts the new LSDB.

3. If the sequence number of the received LSP is less than that of the local LSP, the DIS sends the local LSP in the LSDB to the inbound interface.

4. If the sequence number of the received LSP is equal to that of the local LSP, the DIS compares the Remaining Lifetime of the two LSPs. If Remaining Lifetime of the received LSP is 0, the DIS replaces the LSP with the received LSP, and broadcasts the new LSDB. If the Remaining Lifetime of local LSP is 0, the DIS sends the LSP to the inbound interface.

5. If the sequence number of the received LSP and the local LSP in the LSDB are the same and neither Remaining Lifetime is 0, the DIS compares the checksum of the two LSPs. If the received LSP has a greater checksum than that of the local LSP in the LSDB, the DIS replaces the local LSP in the LSDB with the received LSP and advertises the new LSDB. If the received LSP has a smaller checksum than that of the local LSP in the LSDB, the DIS sends the local LSP in the LSDB to the inbound interface.

6. If the checksums of the received LSP and the local LSP are the same, the LSP is not forwarded.

- Updating the LSDB on a P2P link

1. If the sequence number of the received LSP is greater than that of the local LSP in the LSDB, the router adds the received LSP to its LSDB. The router then sends a PSNP packet to acknowledge the received LSP and sends the LSP to all its neighbors except the neighbor that sends the LSP.

2. If the sequence number of the received LSP is less than that of the local LSP, the router directly sends its LSP to the neighbor and waits for a PSNP from the neighbor as an acknowledgement.

3. If the sequence number of the received LSP is the same as that of the local LSP in the LSDB, the router compares the Remaining Lifetimes of the two LSPs. If Remaining Lifetime of the received LSP is 0, the router adds the LSP to its LSDB. The router then sends a PSNP to acknowledge the received LSP. If Remaining Lifetime of the local LSP is 0, the router directly sends the local LSP to the neighbor and waits for a PSNP from the neighbor.

4. If the sequence number of the received LSP and the local LSP in the LSDB are the same, and neither Remaining Lifetime is 0, the router compares the checksum of the two LSPs. If the received LSP has a greater checksum than that of the local LSP, the router adds the received LSP to its LSDB. The router then sends a PSNP to acknowledge the received LSP. If the received LSP has a smaller checksum than that of the local LSP, the router directly sends the local LSP to the neighbor and waits for a PSNP from the neighbor. At last, the router sends the LSP to all its neighbors except the neighbor that sends the LSP.

5. If the checksums of the received LSP and the local LSP are the same, the LSP is not forwarded.

Here is a link more information for your reference:

https://support.huawei.com/enterprise/en/doc/EDOC1100055485/4110e8a/basic-protocols-of-is-is

Hi @kacpersky,

The scenario that you are mentioning "If Remaining Lifetime of the received LSP is 0, the DIS replaces the LSP with the received LSP, and broadcasts the new LSDB" it's a special LSP called a purged LSP (an LSP with remaining lifetime set to 0) and is always considered newer than a non-purged copy of the same LSP or than the same LSP with a non-zero Remaining Lifetime.

You can refer to ISO 10589 Section 7.3.16 for more information.

ftp://ftp.iks-jena.de/pub/mitarb/lutz/standards/iso/iso10589.pdf

https://tools.ietf.org/html/rfc7987#section-7.3.15.1