Understanding the network.pdf - Back to Home

The routing entry for the intranet side (same example) would look like this:
Destination Network Mask Gateway
0.0.0.0 0.0.0.0 12.127.247.109
As far as the intranetwork is concerned, it sends all datagrams for which it has no
local route to the interexchange, where they become the ISP's problem. This
functions the same way as a default route on a host, and the entire intranet's
datagrams are sent to the backbone router.
The backbone-to-backbone interexchange point is where the power of dynamic
routing comes through. In most networks, links are point to point. In others,
redundancy is important, so multiple network links are used to connect some of the
segments. In our example, the internetwork backbone is using a partial mesh
topology. With a partial mesh, each of the routers has more than one possible route
path to any network point. A full mesh is where every device on the network has a
redundant path to every other device on the network. Full mesh networks are
extremely cost prohibitive; in most cases, a partial mesh approach can adequately
meet most networking requirements. The problem from a routing point of view is
managing a routing table for a network that is comprised of so many alternative
routes. Although diversity is good to ensure that the datagrams get delivered, it can
cause havoc with routing. With diversity comes a large routing table.
To get an idea of how large a regional network's table could get, look at the possible
backbone paths available for intranets homed on GW1. To get to the intranets
homed on the other backbone routers, the paths available to route packets to the
Internet exchange points are as follows:
GW1 to GW2
Path Number of Hops
gw1, gw2 2
gw1, gw3, gw5, gw2 4
gw1, gw3, gw4, gw2 4
gw1, gw4, gw2 3
gw1, gw3, gw4, gw5, gw2 5
gw1, gw3, gw5, gw2 3
gw1, gw3, gw5, gw4, gw2 5
GW1 to Internet GW Number of Hops
gw1 1
gw1, gw3, gw5 3
gw1, gw4 2