HP 200 Series Services And Applications page 198

Improving Network Availability
Permanent Alternate Paths
Figure 1. Improving Availability with an Alternate Path
Data is routed between any two networks on the lowest-cost path. The
lowest-cost path from a router to a destination network is determined by the
router's redirectors. Redirectors exist for each of the different network
protocols—IP, Novell IPX, AppleTalk, DECnet, XNS, and the learning bridge.
Redirectors react to network changes by sending, receiving, and processing
routing updates. They calculate a forwarding table from the available routing
information. The forwarding table indicates the cost to the destination network
and the interface (circuit group) on which data bound for the destination
network will be transmitted by the forwarder (the process within each
redirector that routes data packets).
During normal router operation, the forwarding table indicates only the best
(lowest- cost) path to a destination network. When link 1 fails, router A and
router B will exchange routing information with router C. Router A will
learn about its new path to the site B LAN (through router C), and router B
will learn about its new path to the site A LAN (again through router C).
Note that it takes a short period of time for the routers to detect the link
failure and discover new routes to all available networks. This time is
referred to as convergence time. Small networks will generally converge
within one minute.
The permanent alternate path added in figure 1 has several advantages:
Immediate availability. It can be used (to route data from site A to
site B) as soon as the routers determine that it is the best path to a
network that is unreachable due to another link failure. There are no
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SITE B
b
Link 2
Link 1
a
SITE A SITE C
c