The reason for this null interface route is to prevent routing loops.
For example, let's say there are two ASes: AS1 and AS2
AS1 - 10.0.0.0/24;
10.0.1.0/24;
10.0.0.0/23 summary route advertising to AS2
default route pointing to AS2
AS2 - 20.0.0.0/24;
20.0.1.0/24;
20.0.0.0/23 summary route advertising to AS1
default route pointing to AS1
If you try and ping from AS2 to 10.0.1.1, it won't be a problem under normal
circumstances. However, if 10.0.1.0 disappears for whatever reason within
AS1, the following will happen:
- Packet from AS2 goes into AS1
- AS1 doesn't know anything about network 10.0.1.0 but.... AS1 has a
default route to AS2
- Packet gets forwarded back to AS2
- AS2 looks up its routing table and forwards it back to AS1
... and on and on.
With a null interface route in place, it will prevent the loop from
happening. If we take the scenario above, here's what will happen:
- Packet from AS2 goes into AS1
- AS1 doesn't know anything about network 10.0.1.0 but.... it can match the
less specific "null interface route" (e.g. 10.0.0.0/8 --> null0)
- Packet goes into a black hole and error message generated back to
originating machine.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=25962&t=25923
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