ISDN Overview 397
A common use of ISDN PRIs is connecting multiple remote sites using ISDN BRI ports to a
central site with a PRI port. At the central site, common ISDN equipment would be a Cisco
4000, 7000, or maybe an AS5300, as shown in Figure 11-6.
Figure 11-6 shows three remote sites connecting to the central site. All the remotes use two
64 K BRIs.
Figure 11-6
Remote Sites
NOTE
New VIP cards are available that have eight RJ-45 ports. These ports can be used as either ISDN
PRI ports or serial T1 ports.
IP Unnumbered
The Cisco feature IP unnumbered is mentioned here because it goes hand-in-hand with ISDN.
It seems this feature is often misunderstood on many networks, so that is why it will be
discussed here in detail. It is not a standard, so you could not have a Cisco router talk to another
vendor router using unnumbered interfaces. Many companies use IP unnumbered to conserve
address space, but there are other benefits as well.
Central site
AS 5300
PRI
23 - 64 Kbps channels
Remote site
128 Kbps
Cloud
128 Kbps
128 Kbps
Remote site
Remote site
87200333.book Page 397 Wednesday, August 22, 2001 2:53 PM