Designated router in ISIS

In OSPF, DR “Designated Router” is used in Broadcast networks to eliminate LSAs “Link-State Advertisement”¬† flooding everywhere and less the adjacencies to be maintaned with the DR/BDR only, so that after DR/BDR elections all routers will multicast their LSAs to the destination address “ALLDRs” then the DR will multicast all LSAs to all OSPF routers on “ALLSPF” and this mechanism guarantees an identical LSDB “Link-State DataBase” on all routers in the area.

ISIS routers are known as Intermediate Systems, so that the Designated Router in ISIS is known as DIS “Designated Intermediate System“. Unlike OSPF, the DIS has no backup also the elected router is not guaranteed to remain the DIS and this criteria is well known by preemptive so that if a router with higher priority or with the same priority and higher MAC came alive it will take the lead and be the DIS immediately but the main difference that each router establish adjacencies with all other routers on the segment not only with the DIS.

Now, we want to know how DIS works and if it has the same functions of DR in OSPF, but NO it has a slight difference than OSPF DR in operation.

In ISIS domain the LSP is typically flooded to all adjacent neighbors except the originator, these LSPs are identified by the system ID of the originator and LSP fragment number starting at 0.

Sequence number PDUs (SNPs) are used to acknowledge the receipt of LSPs and to maintain LSDB synchronization. There are two types of SNPs: Complete “CSNP” and Partial “PSNP”, both of them have the same format and the difference is that CSNP carries summaries of all LSPs in the LSBD, while PSNP carries specific LSP’s summaries.

DIS is the only router that sends CSNPs to all adjacent neighbors then the receiver compares the LSPs summaries in its LSDP with the CSNP and if missing one or more LSP or has old versions, the router requests immediately these LSPs by sending PSNP to the DIS. PSNP also has another function is to acknowledge the receipt of LSPs.

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