default gateway in stub networks

binarysoul

In the Todd Lammel book I read, "You can only use default routing on stub networks—those with
only one exit path out of the network."

Then how does a router in a non-stub network forwards a packet to a network that's not in its table? Does it forward it to a router it thinks is a stub network? thanks

SRTMCSE

A router that receives a packet destined for a network not in it's table, discards it, UNLESS there is a default route, which in the event of a non stub network should probably be just a regular static or dynamic route entry.

Tell you what, default routing in a stub network works nicely in production. Was working on a router today in a stub network and we added a network on the remote end and it was so nice thinking "Hmmm have to add a new route....oh wait, no I don't the remote router has the route already."

dtlokee

binarysoul wrote:

In the Todd Lammel book I read, "You can only use default routing on stub networks—those with
only one exit path out of the network."

Then how does a router in a non-stub network forwards a packet to a network that's not in its table? Does it forward it to a router it thinks is a stub network? thanks

The case is you CAN use a default route, not that you MUST use a default route.

A stub network is typically defined as a network (or part of your network) that only has a single exit point. A good example of this is a company that only has a single connection to the Internet. In that case the routers in the company's network will need routes to any internal networks (usually going to be private addresses) and then one default route to the Internet. It wouldn't make sense for the routers to have a complete routing table for all the prefixes on the Internet because the next hop address for all of them will be the same (therefore a default route will work nicely)

Now in this example the routers of the Internet would need to have a path back to your stub network to allow the return traffic. In most cases you will be using IP addresses allocated by your ISP and the ISP has already added them to their routing tables. If you had your own range of public addresses, you'd need to share those routes with your ISP (if they let you) so they could be announced to your ISP and the rest of the Internet.