Hi Homin,
For your qestion on Null Interfaces and BGP, Here is a example -
Null Interfaces are very useful when it comes to route summarisation for BGP Speakers, take the following Example -
Say you have a collection of subnets that reside in a BGP speakers routing table that need to be summarised, in this case a static route with the summary address can be configured. The unique thing here is that the characteristic of the static route created will be that it points to the Null Interface.
Now, BGP Speaker will advertisethe static route pointing to the Null Interface, all other (down stream) BGP Speakers will not be affected by the Null Interface, they'll forward the traffic destined to the summary address to the BGP Speaker that sourced the routing advertisment - in this case - the BGP Speaker with the static route using the Null Interface.
So now when packets destined to valid sunbets arrive at the sourcing BGP Speaker, the longest IP match rule will be applied and hence the packet will be forwarded to the correct destination. And the packets that which are not destined for the correct subnet and are not on the BGP routing table will match the summarised static route entry using the Null Interface and because of this the packet will be dropped.
-On a side note- EIGRP, actually inserts a entry into its routing table pointing to a Null Interface when you use manual route summarisation command.
If you really want to get to grips with BGP/Longest match rule/Null Interfaces etc, I'd recommend a fine book by a very fine expert - Basam Halabi / Cisco Press / Internet Routing Architectures.
Effectively all a Null Interface does is that it acts like a 'rubbish bin' for unwanted packets etc.
Hope this helped in your understanding.
