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What is meant by the sentence -

Loopback address is the highest IP address and this means that it is used by OSPF router to advertise the routing information.

For example -

f 0/0 -- 192.168.45.4
f0/1 -- 192.168.34.4 
loopback 4.4.4.4

In this scenario, how can we say that the the loop back address - 4.4.4.4 is the highest IP address?

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 7 '17 at 17:33
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It's not. You should really specifically set the OSPF router ID, but, if you don't, it will use highest IPv4 address of any loopback. If no loopbacks exist, it will use the highest IPv4 address of any active interface.

It prefers loopbacks since they never become disabled unless you manually disable them, so the router ID is less likely to change between router reloads. An OSPF router ID will not change until the router is reloaded, or the OSPF process is restarted.

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