background image
586 Chapter 8: WAN Protocols and Design
DDR Step 4: Determining When the Connection Is Terminated
The dialed link acts just like a leased line while it is up. If a particular Layer 3 protocol is
enabled on the link, it can be routed across the link. Transparent (encapsulated) bridging can be
used just like any other point-to-point link. Routing updates, IPX SAPs, AppleTalk ZIP, and
other broadcasts are sent across the link if the broadcast keyword is coded. Most importantly,
any access list used to define which packets are interesting does not filter the traffic on the
interface. If packet filters are desired, an access list must be enabled on the interface.
Additional parallel links can be dialed if more capacity is desired. To do so, dialer profiles must
be configured. Additionally, the dialer load-threshold load command is used to define the link
utilization that must be exceeded for another link to be dialed.
The decision to take the link down is the most intriguing part about what happens while the link
is up. Although any type of packets can be routed across the link, only interesting packets are
considered worthy of keeping the link up. An idle timer counts the time since the last interesting
packet went across the link. If that time expires, the link is brought down.
Two idle timers can be set. With the dialer idle-timeout seconds command, the idle time as
previously described is set. If interesting traffic that needs to flow to another dial site is
occurring, however, another shorter idle timer can be used. The dialer fast-idle seconds
command enables you to configure a typically lower number than the idle timer so that when
other sites need to be dialed, the link that is currently up but can be brought down earlier in the
process of idling out as per the dialer idle-timeout command.
ISDN Configuration
Example 8-30 and Example 8-31 show the DDR configuration for the network in Figure 8-31.
ISDN configuration details have been added; the text following these two examples describes
the ISDN commands shown.
Example 8-30
SanFrancisco Configuration--Complete
ip route 172.16.3.0 255.255.255.0 172.16.2.1
ip route 172.16.4.0 255.255.255.0 172.16.2.3
! Added usernames for CHAP support!
username LosAngeles password Clark
username GothamCity password Bruce
!
access-list 101 permit tcp any host 172.16.3.1 eq 80
access-list 101 permit tcp any host 172.16.4.1 eq 21
!
dialer-list 2 protocol ip list 101
!
interface bri 0
encapsulation ppp
ppp authentication chap
isdn spid1 555555111101
isdn spid2 555555222202
dialer idle-timeout 300
ch08.fm Page 586 Monday, March 20, 2000 5:17 PM