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Now ping is great for testing as an example
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if I ping 10.1.1.2 the ping succeeds
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the reason the first ping failed is due to ARP
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the router needs to ARP for the MAC address of router 2
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but notice if I ping 10.1.2.2 which is the IP address of router 3
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the pings fail because that route is not in the routing table.
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10.1.2.0 is not in the local router's routing table.
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10.1.1.0 is in the local routing table.
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so the router can route appropriately
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now debugs are very useful for testing and troubleshooting
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and in this example, I’m gonna use a command debug ip icmp
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on both router 2 and router 3 just to prove that traffic is arriving
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as expected on the correct routers.
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so ping 10.1.1.2 this IP address
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notice we see the ICMP echo replies sent back to 10.1.1.1
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let's do that so 10.1.2.2 the traffic doesn’t reach this router
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we don’t see any outputs on the screen of router 3
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where’s just to do it again to 10.1.1.2
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we see the debugs because the traffic is being sent from router 1 to router 2
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and is being received by router 2 which in turn is replying back.
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So on router 1, we can use the command debug ip packet
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be careful with this command in a live environment.
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In a live environment, you want to use an access control list
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to limit the traffic that you are debugging in a production
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or live environment, you can kill your router if you run this command.
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But in this live environment, it’s ok to do it.
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so from a testing and troubleshooting point of view
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lets work through logic
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on router 1, we try to ping 10.1.2.2
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we're not getting a reply we run a debug IP ICMP
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on router 3 to see if the traffic is actually arriving at router 3
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and it's not, that’s useful because pings send traffic
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and then expect to respond
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and this would allows to check whether the problem is on the path R1 to R3
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or whether the problem is on the return R3 to R1
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so this is a great command for testing
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so ping 10.1.2.2 notice in the output we're getting unroutable
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the local router is telling us that the source IP address 10.1.1.1
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going to the destination 10.1.2.2 is unroutable
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so it doesn’t know where to forward the traffic.
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another great thing in the output here
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is that we know which interface the router is using for sending packets
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typically a router uses the outgoing interface
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as the source in packets and you can see that here.
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so the source of the ping is 10.1.1.1
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because that’s the outgoing interface to get to a destination
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such as 10.1.2.2
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as an example, if we ping 10.1.1.2 the ping succeed
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we get quite a lot of output
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so scrolling back, there’s the ping, they are the success messages
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we can see that the source IP address is 10.1.1.1
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destination is 10.1.1.2 this IP address
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and this was routed via the Forwarding Information Base or FIB out of F0/0
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so the packet was sent
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and then we received a reply from 10.1.1.2 going to 10.1.1.1
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so here we can see that the packet was forwarded out of the router
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or sent by the router and here we can see a replied back from router 2.
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so a very useful command but once again be careful using it in production
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sh ip route
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shows me that I don’t have a route in the routing table for 10.1.2.0
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and I also don’t have a gateway of last resort
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in other words, I don’t have a default gateway
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or default route configured on this router.
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So the router doesn’t know what to do with the traffic
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and hence when we ping the router tells us that the traffic is unroutable.
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The router doesn’t know where to forward the traffic
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so we need to configure static routes.
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