Creating Loopback interfaces in existing OSPF network

[tskelly]

In reviewing an existing OSPF network loopback interfaces were not configured on routers.  If loopback interfaces are created while OSPF is running how will OSPF react?  Will I need to reset the OSPF process or will the router need to be rebooted?

[Jon Marshall]

tskelly wrote:
In reviewing an existing OSPF network loopback interfaces were not configured on routers.  If loopback interfaces are created while OSPF is running how will OSPF react?  Will I need to reset the OSPF process or will the router need to be rebooted?

Depends what you want the loopback interface to be used for. If for the OSPF router-id you would need to bump the OSPF process. If you just want to add a loopback then simply configure it and if you want to advertise it via OSPF add it under the "router ospf ".

Bear in mind loobacks are always advertised as /32s in OSPF unless you configure "ip ospf network point-to-point" under the loopback interface.

Jon

[tskelly]

Thanks Jon.  I would be looking to use the loopback address as the RID, so I should be able to reset the OSPF process to get the RID changed.

.

[Peter Paluch]

Hello,

If a particular OSPF RID is to be maintained on a router, I often recommend also using the router-id command in the OSPF configuration to explicitly configure the RID. Using loopbacks is fine but eventually they may get readdresses or new ones added that have a higher IP address, and the OSPF RID may change as the result. The router-id command in the OSPF configuration provides a stable RID without particular regards to live interfaces.

Best regards,

Peter

[Jon Marshall]

Yep, agree with Peter 100% here.

Jon

[hamalawy1]

hi there,

no, OSPF will not react until the router is rebooted.. but always remember the OSPF prefers loopback interfaces over physical interfaces ip address as a router ID.

and it prefers the higher loopback interface ip address as it's router ID.. so, it is more prefered to type the the command router-id  unter the ospf process and clear the ospf process ( which by the way doesn't need to reboot the router). because this command has severeal advantages over letting OSPF choose the higher loopback ip address

1- it overrides the loopback ip address ( if later on someone entered a higher loopback ip address and the router rebooted - BOOM - the network goes wacky!)

2- it is just a tag and not a real ip address ( it has the same format of the ip address but it isn't really, you can put anything you want )

3- you only have to clear the ospf process which mean minimum downtime to your network

hope that helps,

S.Hamalawy