I was studying OSPF and this came up from a training material,in which the instructor says:

"in turn a loopback interface's higher level of stability and availability results in fewer SPF recalculations. which results in a more stable network overall."

there's no doubt about the higher level of stability and availability over physical interfaces, I mean for the sake of kinda connectivity and to make sure the device/router is up and running, physical interfaces could go bad easily and could make us some problem,but for loopback it's a different story,the only way for loopback int to be unavailable is for it to be manually deleted or for the entire router to go down. but how possibly using or not using loopback could affect SPF calculation??

  • Please give us more context about what the instructor was talking about. Was he talking about ASBRs or Virtual Links?
    – user5025
    May 1 '14 at 15:11

Loopbacks are considered host addresses in OSPF. If there are no interfaces that are UP/UP when OSPF performs its check, it will use the configured loopback. Without the loopback and an UP/UP physical interface, you will instead receive an error: "cannot allocate router id". The SPF calculation in an environment with an available loopback starts at the loopback itself.



ex. Say you are in a situation where you have two interfaces on a router with a loopback and two interfaces on a router without a loopback. The router A, with two interfaces and a loopback set as the router ID will lose one of its two links. In this case, because the loopback is set as the router ID, the protocol will continue running as there is a physical link still up for traffic to traverse.

In router B, you have g1/0/1 set as your router ID and it goes down for whatever reason...you now have no way into that router despite having g1/0/2 open.

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