router1 has a route to 10..1.1.0/24 point to router2 as the next hop
u make the null0 config like
ip route 10.1.1.0 255.255.255.0 null0
in this case a packet going to 10.1.1.0/24 from router1 will be send to router2 router2 will take this packet ( not drop it) then send it to the route with null0 then the route will not actually be forward to that network and then u block it but with out any deny or ACL
ok with eigrp summary route the summarized route in the routing table will looks like :
D 18.104.22.168/16 is a summary, 00:00:22, Null0
lets say this is router1
Notice that the summary route is sourced from Null0, and not an actual interface. That is because this route is used for advertisement purposes and does not represent a path that router1 can take to reach that network. On router1, this route has an administrative distance of 5
so any router connected to router one can send packect going to any address within the summary address range and router1 will have route for the more spesific route when come to it like in our example 22.214.171.124/24 through interface 1 and 126.96.36.199/24 through interface 2 and so on, but the nieghbor routers will see only the summary route and router1 as a next-hop
If all addresses have more specific routes on the summary router then the null0 is not really needed and will have no effect. It is used in case part of the network being summarized does not exist.
For example say I have
10.10.2.0/24 and 10.10.3.0/24 but I summarize it to 10.10.2.0/23. This will generate a 10.10.2.0/23 null0 route in the summary router but since I have routes to both the /24 these will override the null0 since they are more specific.
Now lets say I do the same summary but 10.10.3.0/24 is down and therefore not in the routing table. A packet sent to 10.10.3.x will now use the 10.10.2.0/23 null0 route and the packet will be dropped. This is mostly used to prevent routing loops especially when you have default routes configured.
"Notice that the summary route is sourced from Null0, and not an actual interface. That is because this route is used for advertisement purposes and does not represent a path that router1 can take to reach that network. On router1, this route has an administrative distance of 5 "
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