OSPF Router ID

[johnlloyd_13]

hi all,

just a quick and basic question.

i'm going to configure a new OSPF router and will have a point-to-point link with our core router.

my questions are:

is router-id neccessary for this case? meaning, do i need to hard code the router-id IP under OSPF sub-command?

been reading white papers with examples, does the router-id need to be routable? or is it just used to identify the router for DR/BDR (for LAN)?

[Rolf Fischer · Accepted solution]

Hi John,

a router ID is always needed in OSPF to identify a router (and its LSAs) clearly in the LSDB.

It is a 32 bit value in the format of an IPv4 address but it isn't an IP address. You could even assing values like 0.0.0.1, 127.0.0.0, 224.0.0.0 or 255.255.255.255 which wouldn't be accepted as interface addresses. So the RID doesn't have to be routable but it needs to be unique within the OSPF domain. Think of it more like a name.

If you don't configure an explicit RID under the OSPF process, the highest loopback IP address will be used (the loopback interface doesn't have to be OSPF enabled). If no active loopback interface is available, the highest IP address of the active non-loopback interfaces will be used. It's recommended to assign the RID statically or use a loopback interface to avoid problems with state changes (link down) of physical interfaces.

HTH

Rolf

[johnlloyd_13]

hi rolf,

thanks for this info! i knew i read somewhere that router-id doesn't need to routable.