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OSPF intro: encourage to re-read

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Hans van Kranenburg 2015-06-07 02:49:34 +02:00
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@ -64,7 +64,7 @@ While each router can determine the complete shortest path to any destination in
And why shouldn't we care too much about all of this? The fun part here is that each of the participating routers in the OSPF network knows exactly the same amount of information about the whole network topology, and uses the same way to calculate the shortest routes from itself to all subnets that exist in the network. So, it's not a problem at all that the OSPF process on R2 can only tell the linux kernel to forward packets for `10.34.2.89` to a next hop of `10.1.2.56`, because it can trust on the fact that R5 will always forward them again to `10.0.1.8`, having R6 receive them, which will drop them into the connected subnet to reach the end host in there.
If you're confused now, don't worry. Take a short break and continue with the hands-on part to see it all happen!
If you're confused now, don't worry. Take a short break and continue with the hands-on part to see it all happen! After doing so, re-read the above, which should probably make more sense then.
If you're not confused yet, here's some bonus information to think about: To make it even worse, the IP address of this next hop is not even used when actually forwarding a packet to the next router! It's only being used to determine the layer 2 mac address of the interface of the next hop. :-) An IP packet contains a source and a destination IP address. It does not contain a list of routers that it needs to pass before reaching its destination. It does not even contain the address of the very first next hop that it needs to go to, and the receiving next router has no idea where the packet it just received has been, or which IP address was used to forward it. It only sees the mac address of the sending router.