HP 600 Series, Services And Applications

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Dedicated backup circuits are easy to set up and may be used with all the
routable protocols. There is no need to configure static routes, since the
circuit will be disabled once the primary circuit (leased line) is restored. The
disadvantage is that a separate switched circuit, a router port, and an adapt-
er or DSU/CSU must be provided for each leased line. Compare this backup
solution to 1xN backup in the next section.
1xN Backup—Star Network (IP Only)
Figure 13 shows a router network that uses leased lines as the primary
circuits and dial up lines for backup purposes. (The backup circuits are
not
configured as backup members of a circuit group.) In this example, two
switched circuits at the central site are used to back up four leased-line
circuits (1 for 2 backup). R
a1
and R
a2
are attached to the central site LAN
and connected to routers at four remote sites. The backup circuits become
active only in the event of a primary link failure. Additionally, the backup
circuits are only used to connect from a remote site to the central site (no
remote-site-to-remote-site connectivity). In this example, the backup circuits
only work for IP protocol, since only IP has the capability to map a phone
number to a protocol address.
Figure 13. Leased-Line Network with Circuit-Switching Backup
b
c d
e
a2 a1
Circuit
Switching
Network
A
p
p
li
c
a
ti
o
n
N
o
te
s
/
C
a
s
e
S
tu
d
ie
s
3
Improving Network Availability
Sample Networks
3-25