Your customer asked you to redesign there is-IS network to reduce to a minimum the number of adjacencies because the network has several routers running L1/L2 mode on the sme Ethernet segment. Which a

Unlike OSPF's DR/BDR model, IS-IS DIS election does not reduce adjacency count - all routers on the segment still form full adjacencies with every other router regardless of DIS assignment.

Adjusting the interface priority of a backup DIS influences only which router wins the DIS election; it has no effect on the number of adjacencies formed between routers on the segment.

Splitting routers between L1-only and L2-only creates two separate topologies on the segment but does not meaningfully reduce the total adjacency count because each group still forms a full mesh within its own level.

Routers operating in L1/L2 mode on a shared Ethernet segment establish both an L1 adjacency and an L2 adjacency with every peer, effectively multiplying the total adjacency count by two. Converting all routers to a single level (either all L1 or all L2) ensures each router pair maintains only one adjacency, achieving the minimum possible adjacency count for the segment.