router-id/priority change

[miracle_david@yahoo.com]

Hi all,

In what cases I should need to change router-id or priority in OSPF topology?

Thanks in advance

[Georg Pauwen]

Hello,

you would typically change these values if you want to influence which router becomes the DR. OSPF routers are, by default,  assigned a DR priority of 1. The router ID is the tiebreaker, with the highest RID being preferred (e.g. 2.2.2.2/32 is higher than 1.1.1.1/32).

[Giuseppe Larosa · Accepted solution]

Hello miracle_david@yahoo.com ,

the OSPF priority can be changed in each interface, the OSPF RID is a global parameter.

Changing the OSPF priority can be used to influence the election of a device as OSPF DR or BDR on a LAN segment.

When multiple LAN segments exists between a set of routers without OSPF priority manipulation the same router would end to be elected as DR in all LAN segments, the one with the highest RID.

In order to divide the work load on multiple routers OSPF priority manipulation is recommended so that different routers are DR/BDR on different LAN segments.

Actually, the OSPF DR election is not deterministic, because the first router connected to a LAN segment wait for a time called wait timer 40 seconds before claiming to be the DR for the segment in absence of other OSPF routers.

Unnecessary DR/BDR elections can be avoided on end user facing Vlans by using the passive interface command at router ospf level.

Many years ago it was recommended to avoid to have ABR routers to be also DR with the same idea to distribute the computing load on multiple nodes. ABR nodes connect to multiple areas build  a different router LSA for each area they are member of and they are in charge to create/regenerate LSA type 3. They create/send into area 0 LSA type 3 representing IP prefixes in the non backbone area and they re-generate LSA type 3 received in area 0 to be sent in the non backbone area(s) they connect to.

So the ABR workload in terms of memory and CPU is greater.

Hope to help

Giuseppe