3Com 520 User Manual
Page 64
A-10
B
R
I
D
G
I
N
G
A
N
D
R
O
U
T
I
N
G
Different protocols use differing network
characteristics or
metrics
when making routing
decisions. The metric employed by RIP is a
Hop
Count
. A hop count is defined by the number of
routing nodes there are between the source and
destination units. In our example, there are two
hops between LAN1 and LAN 2 going via Routers A
and B. If traffic was directed via Routers A, D, and
then B, this would be three hops.
The algorithm will automatically select to forward the
data packet via Router A, as this route contains the
least number of hop counts which makes it the
preferred direct route.
Every thirty seconds, each IP router will advertise via
RIP datagrams, to all other routers on the
internetwork, how many hops it takes to reach all
connected logical networks, based on the routers
network position and the state of its physical links.
In an ISDN environment, making ISDN calls every
thirty seconds to pass on and receive RIP updates
would be costly. When initially setting up the
network, the OfficeConnect Remote is manually
connected over the ISDN for a three minute period,
in order to learn the topology of the rest of the
network. Once this has been performed the
OfficeConnect Remote will only make ISDN calls to
transmit data packets. While this data transfer is in
progress, RIP updates will be
piggybacked
on to the
call, updating both parts of the network with the
latest RIP information.
It is also possible to assign what are known as
static
routes
, which are manually entered fixed routes. The
network manager may be aware of specific traffic
patterns, or needs to enforce a particular routing
policy. Static routes provide an option to force traffic
through the network in a particular way. The
disadvantage with this approach is that routing
protocols dynamically update all the routers on the
network, with the current network topology,
enabling backup routes to be deployed. In a static
route situation, if the WAN links in that routing
definition are down, then traffic cannot be passed.
Implementing a static route prohibits the router from
being able to offer alternative data paths.
Rc.bk : RCAPPA.FRM Page 10 Thursday, July 10, 1997 9:53 AM