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In this situation, if I were on a network behind router 1 and i sent a packet (like a simple ping or traceroute) to a destination that neither router 1 nor router 2 know, the two routers would send that packet back and forth forever until either one of them went offline or a path to that network were to become known via a routing protocol to one or the other. Vice versa if router 2 receives a packet with a destination address it doesn't have a specific route to it sends it to default, to router 1, assuming that router 1 must know.

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The underlying assumption in a setup like this is that if router 1 receives a packet with a destination address it doesn't have a specific route to, it follows default to router 2 if router 1 does not know how to reach a particular destination, router 2 must know how to reach it. A default route, also called a 'gateway of last resort', is where I send traffic if I don't know a specific route to that destination. Imagine if I had two routers that each had a default route pointing at the other.

This is to prevent a packet from being endlessly passed back and forth between two routers forever if there were a loop somewhere. If it reaches 0, then the router will discard it. Every router that this packet passes through will decrement the TTL by 1. This is an 8-bit field (maximum value of 255) that starts off at some value determined by the host that generated the packet-common values tend to be 255 (the maximum) or 64. There are different implementations of traceroute, but what they all basically rely on is a field in every IP packet called the Time To Live (TTL). To understand why, some background on how traceroute works is needed. I launch a traceroute to a host or IP and see it end at some point before that host or IP), I'd be more concerned, but loss or latency on hops in the middle is not unusual. If I saw loss from that host, or if the traceroute simply did not complete (i.e. 30.2% loss on hop 12? What's that about? Does that mean 30% of my traffic is getting dropped? Short answer, no - the important thing to note here is that the final hop, hop #16, shows 0.0% loss.

In particular, let's look at hops 9-12 in the middle: 9. Keys: Help Display mode Restart statistics Order of fields quitĤ. Here's an output from my computer tracing to (IPs removed): My traceroute Traceroute and tools based on it like Pingplotter and the *nix mtr (MyTraceroute or Matt's Traceroute) are useful tools for determining reachability and the path you take to get to a destination, but some of the feedback should be taken with a grain of salt.

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