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I was trying to configure ospf router, I have a router connected to FW in G1/0/10 and mpls router connected g1/0/1, when i typed passive interface default in the ospf router i lost all connectivity and the neighbor between the mpls and core switch, so my question is this command take the internet down?

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A passive interface in OSPF does not send hello packets, so it will never form a neighbor relationship nor exchange routes with another OSPF router.

By setting the passive interface default command, you make all OSPF interfaces passive. The router will lose all its neighbors and with them, routing information.

It is often good practice to use the passive interface default on devices, especially ones with lots of interfaces, then issue the no passive interface command just for interfaces that actually have OSPF neighbors.

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    Thank you for thr reply so that is normal to lose connectivity.
    – RR Cisco
    Dec 22 '21 at 18:17
  • i had to setup no passive interface in the interfaces that created a neighborship with the mpls and with the FW, then when i added passive interface default i lost the connection. then how i will add this command without losing a connectivity? if i put at first step i will lose the connectivity and i will not able to configure the other interface as no passive. I don't have out of band connection to the core switch.
    – RR Cisco
    Dec 23 '21 at 14:49
  • You may not be able to. You can also put passive interface x/y on the ones you don't want neighbors.
    – Ron Trunk
    Dec 23 '21 at 15:00
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By setting an interface to passive, you will never form a neighbor on that interface. Unless you use the no passive interface command on at least one interface, you will have no neighbors and get no routes from any other router. Basically, you could just disable OSPF and get the same result.

If you have many interfaces where there will be no neighbors, then the passive interface default command is useful, but you then need to use the no passive interface <interface> command for those interfaces where you do want neighbors.

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