HP 200 Series Services And Applications page 125

Conditional Static Route
A static route can be configured as conditional on the status of a circuit
group other than the one that directly accesses the next hop router. For
example, consider figure 7.
The frame relay subnet 11.1.8.x provides the primary routes.
The ISDN (V.25 bis) subnet 12.1.8.x provides the backup for the
frame relay subnet.
RIP is run over the WAN network, enabling all routers to learn about
the 10.x.x.x networks attached to other routers (when all WAN links
are up).
Routers A, B, and C have static routes (using the ISDN or dial-up
network) to all the non-locally-attached 10.x.x.x networks. The rout-
ing cost for these static routes is configured to be higher than for the
WAN routes, which means that a static route to a particular router
will not be used unless the WAN/RIP route to that router goes down.
On the ISDN/dial-up static routes to the 10.x.x.x networks, the
next-hop router for routers A, B, and C is router D. Router D has
ISDN/dial-up static routes to all of the 10.x.x.x networks, but the
next-hop for these static routes is the router that is directly
connected to each particular 10.x.x.x network. For example, the
next hop for router D to network 10.1.8.x through the ISDN/dial-up
static route is 12.1.8.1.
The configuration in figure 7 solves any single-point WAN failure to routers
A, B, or C. For example, if the WAN link to router B breaks, traffic between
B and all other routers will be conducted through the ISDN/dial-up static
routes to D. (D will advertise the new route to B over its frame relay connec-
tion.) If, however, the WAN line to D breaks, D will either constantly be
calling A, B, or C, or they will be trying to call D when they have data for the
10.4.8.x network accessed through D. Because there would not be enough
ports on D to handle all of the calls that could occur simultaneously, the
above layout is inadequate.
The solution is to implement conditional static routing that will replace the
existing static routing in the event of a break in the WAN connection to
router D. That is, if the circuit group on the WAN route to D breaks, the exist-
ing (ISDN) static routes connecting D to A, B, and C are bypassed for a new
set of static routes configured on D, with C as the next-hop router. The result
is to shift the next-hop router status from D, which has only one static route
for handling all incoming and outgoing traffic, to C, which has both a WAN
and an ISDN route available.
Internet Protocol Routing Service
Static Routing
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