Simple routing diagram

[the_rock]

Hey boys and girls,

I figured would share something my colleague made for a customer, as I see people still "struggling" with this. There are lots of times where we troubleshoot issues, but we ommit to take a step back and realize that problem may lie in something so trivial. Anyway, I attached the screenshot of it, and for those of you who follow chess, as our friend Hans Moke Niemann would say "Chess speaks for itself" : - )

Hope its helpful.

Best,

Andy

 

[Bob_Zimmerman]

It's worth noting interface routes, static routes, dynamic routes, and default gateway aren't separate steps. PBR is special because it can't control traffic originating from the firewall, but everything else is just a few steps:

1. Find the most specific prefix in the routing table for the destination IP. If there is no prefix for the destination IP (meaning no default route and no route covering the destination), return destination network unreachable (ICMP type 3 code 0).
2. Pick the ARP/NDP destination: If the route has no gateway, ARP/NDP for the destination address out the route's interface. If the route has a gateway, ARP/NDP for the gateway address out the route's interface.
3. If no ARP/NDP response, return host unreachable (ICMP type 3 code 1). If you get an ARP/NDP response, frame the packet to that MAC and send it out the route's interface.

When you learn multiple routes for the same prefix, the administrative distance picks which one goes into the routing table (the FIB) in the OS. That's the only differentiation between static routes, dynamic routes, and the default gateway (not actually special, just a static or dynamic route for 0/0).

Interface routes are just routes without a gateway set (I've never seen a route with no gateway learned via dynamic routing, but I don't know offhand if it's absolutely impossible).

[the_rock]

I agree, of course there is way more to it than diagram, but its just super basic example I wanted to share.

Andy