Hello,

I've been reading about routers and how they work.

SO, all the routers are continually updating each other by sending each other their entire routing table.

A packet is transmitted along to the closest/fastest/cheapest next router along the line in order to get to it's destination using the metric or hop count system.

In all the tutorials I've read this looks fine, all the examples are on small networks with 4 or 5 routers in the chain.All the routers also contain all the information required for the networks.The networks are converged, thats the correct term i believe.

OK but what happens if a packet hits a router and that router has no entry for the destination IP in the routing table?

What happens?The packet is returned to the last router?Forwarded again to another router,the default gateway?Dropped?Try again later???

It doesn't make sense to me yet,for a network to work it has to be converged,but that can't be the case in real life on the internet can it?

Thanks,

Babaton

I would like email notifications of responses to this thread because I don't understand it either.
I assume that unless your router can connect to one of the DNS servers out there you are out of luck.
Jeempc

but the DNS server just gets you the IP address.

So once you have the IP address how does the packet get to the otherside of the world?

I suppose the router could broadcast a request to other routers to see who has the info and on and on until it finds a router with the info.A bit like a dns update.

I've just had a thought, does subnetting come into this, can a router work out where to send a packet based on the network number?

hey
if the router has no entry to route the packet it will send to the sender (host or another router) an ICMP message Type 3 with the error code either 0 or 1 and will DROP the packet.

ICMP Message Type 3 means Destination Unreachable.
Error Code 0 means Net Unreachable.
Error Code 1 means Host Unreachable.

another thing i notice in ur post is

Thanks Migo,

That makes sense, but then what does the sender do?

drop the packet also or try again and send to another router?
If the sender does try again with a different router how does it select which one to go for?

Then again if they all kept forwarding to their default routers, the packets could travel around for ages and still not get anywhere so dropping the packet does make more sense.

If it does drop the packet then the only packets to reach their destinations are the ones that onlt encounter routers which hold entries for their destination IPs.

It seems incredible that a packet can get across the net and not encounter a router that doesn't have an entry for the destination IP.

sorry to labour the point but all the tutorials I've read don't seem to say much about what happens when there is no entry in the table.

Cheers.

Thanks Migo,

It's all starting to come clear.I've been greatly underestimating the size of some of these routing tables.

Also, there must be alot of these BGB routers, I guess there must be a lot of them.At least at every ISP.

I also assume a router that size would have a lot of network connections.

I've been thinking about a CCNA, but wasn't sure how much I needed to know as an entry requirement.

I'll look into it.

Thanks again.