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johnifanx98 wrote: » If it's for router id, isn't enough to use physical interface's ip address. I understand that physical interface's IP may change, but it's always UNIQUE.
phoeneous wrote: » Thats actually a good fundamental question that I wont give away the answer to
johnifanx98 wrote: » Maybe. But i could not see any relation with my question...
KPWright wrote: » I think there actually is a relationship to your original post. You wanted to know why you should configure a loopback when the interfaces would generate an id for the router just as well. The problem is if you allow ospf to select the highest interface address and that interface goes down (or you take it down) the router will then choose another id. Since it now has a new id, it will have to re-advertise and negotiate all of it's routes with it's neighbors using the new id. When the other interface comes back up, this will all happen again. All of this needless chatter just puts traffic on the network and possible delay in communications. If you configure one or more loopback addresses, ospf will take the highest of these as the id regardless of the interfaces. This won't change unless you re-configure the loopback or the router itself goes down. So your answer is you configure the loopback to limit the amount of router generated traffic on your network.
networker050184 wrote: » That is not true. Even if you shutdown the interface or remove the IP address OSPF will keep the same router id. The only way to have the router change its router id is to maunually set it and clear the process or reload the router.
johnifanx98 wrote: » By comparing loopback addr of a computer, I saw this interesting thing. A computer's loopback is always 127.0.0.1, which is reserved. However, seems like an OSPF's loopback address is in the space of a real network ip addresses. It does not make much sense to allocate real IP addresses to some virtual interfaces... Also, this leaves a question open: how to avoid IP addresses confliction within a certain space, say, area. Just by manual check?
networker050184 wrote: » Why wouldn't it make sense to allocate an address? It gives you a testing point and a consistent address for your other services like SSH, SNMP trap source, syslog source, next hop for BGP etc. that won't change even if you re IP your interfaces. You would avoid address overlap the same way you would with any other interface address. Some people use a spreadsheet or anyone of the paid IP tracking systems available out there to track IP assignments.
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