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Creating Loopback interfaces in existing OSPF network

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?

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Hall of Fame Super Blue

Re: Creating Loopback interfaces in existing OSPF network

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

New Member

Re: Creating Loopback interfaces in existing OSPF network

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.

.

Cisco Employee

Re: Creating Loopback interfaces in existing OSPF network

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

Hall of Fame Super Blue

Re: Creating Loopback interfaces in existing OSPF network

Yep, agree with Peter 100% here.

Jon

New Member

Re: Creating Loopback interfaces in existing OSPF network

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

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