background image
Design Options 365
Figure 10-24
ISDN Backup
On the LA router, you put in a backup interface BRI0 command on the serial interface, and
when the serial link goes down or stops receiving keepalives from the Frame Relay switch, the
router places an ISDN call to the New York router and brings up the link.
interface Serial0/0
backup interface BRI0/0
-- When serial line fails, ISDN will activate
ip address 192.168.100.3 255.255.255.0
encapsulation frame-relay
ISDN is usually time-sensitive as far as price in most areas. Decisions need to be made to avoid
high phone bills, and backup strategies need to be developed. Do you bring up the link, which
is simple, or do you configure the dial-on-demand routing, which brings up the ISDN only
as needed?
Here are some benefits of ISDN backup:
·
Very fast circuit establishment
·
Bandwidth on demand can be utilized
·
Backup capabilities up to 128 K
Other decisions involve long-distance calls. In Figure 10-24, would it be more effective to have
LA dial NY or SF? San Francisco is much closer to Los Angeles geographically, so long-
distance charges probably are cheaper. Do you call SF and sacrifice some bandwidth by routing
from NY to LA via SF? These are the decisions a CCDP must make. Sometimes there is no
NY
BRI0/0
ISDN
BRI0/0
LA
SF
64 k
64 k
interface Serial0/0
backup interface BRI0/0
ip address 192.168.100.3
255.255.255.240
encapsulation frame-relay
Cloud
87200333.book Page 365 Wednesday, August 22, 2001 2:53 PM