Routing and WAN connections
BAT54-Rail/F..
Release
7.54
06/08
11.2
IP routing
363
U Which information does the router take from received IP
RIP packets?
When the router receives such IP RIP packets, it incorporates them in its dy-
namic routing table, which looks something like this:
U What do the entries mean?
IP address and network mask identify the destination network, the distance
shows the number of routers between the transmitter and receiver, the last
column shows which router has revealed this route. This leaves the 'Time'.
The dynamic table thus shows how old the relevant route is. The value in this
column acts as a multiplier for the intervals at which the RIP packets arrive.
A '1', therefore, stands for 30 seconds, a '5' for about 2.5 minutes and so on.
New information arriving about a route is, of course, designated as directly
reachable and is given the time setting '1'. The value in this column is auto-
matically incremented when the corresponding amount of time has elapsed.
The distance is set to '16' after 3.5 minutes (route not reachable) and the
route is deleted after 5.5 minutes.
Now if the router receives an IP RIP packet, it must decide whether or not to
incorporate the route contained into its dynamic table. This is done as fol-
lows:
D The route is incorporated if it is not yet listed in the table (as long as there
is enough space in the table).
D The route exists in the table with a time of '5' or '6'. The new route is then
used if it indicates the same or a better distance.
D The route exists in the table with a time of '7' to '10' and thus has the dis-
tance '16'. The new route will always be used.
D The route exists in the table. The new route comes from the same router
which notified this route, but has a worse distance than the previous entry.
If a device notifies the degradation of its own static routing table in this
way (e.g. releasing a connection increases the distance from 1 to 2, see
below), the router will believe this and include the poorer entry in its dy-
namic table.
Note: RIP packets from the WAN will be ignored and will be rejected imme-
diately. RIP packets from the LAN will be evaluated and will not be prop-
agated in the LAN.
IP address IP netmask Time Distance Router
192.168.120.0 255.255.255.0 1 2 192.168.110.1
192.168.130.0 255.255.255.0 5 3 192.168.110.2
192.168.140.0 255.255.255.0 1 5 192.168.110.3
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