The snapshot client interface configuration command specifies the
following variables:
• The length of the active period (which must match the length specified on the central router)
• The length of the quiet period
• Whether the router can dial the central router to exchange routing updates in the absence of
regular traffic
• Whether connections that are established to exchange user data can be used to exchange routing
updates
When the backbone routing protocol is not supported by snapshot routing (for example, OSPF or
EIGRP), standard routing redistribution techniques can be used to ensure that routing updates are propagated between routing protocols, as required. Care should be taken to ensure redistribution of subnets if needed and to avoid routing loops.
Figure 10-6
AppleTalk snapshot routing.
R1
Zone: WAN
R2
Cable-range: 700-700
E0
B0
E0
B0
Zone: HQ
Zone: Branch
Cable-range: 101-200
Cable-range: 201-300
10-14
Cisco CCIE Fundamentals: Network Design
Dial Backup for Leased Lines
• R1 configuration is as follows:
username R2 password SECRET
appletalk routing
isdn switch-type basic-5ess
!
interface BRI0
encapsulation ppp
appletalk cable-range 700-700 700.1
appletalk zone WAN
dialer map appletalk 700.2 name R2 speed 56 broadcast 5552222
dialer map snapshot 2 name R2 speed 56 broadcast 5552222
dialer-group 1
snapshot client 5 60 dialer
isdn spid1 5550066
ppp authentication chap
!
dialer-list 1 protocol appletalk permit
• R2 configuration is as follows:
username R1 password SECRET
appletalk routing
isdn switch-type basic-5ess
interface BRI0
encapsulation ppp
appletalk cable-range 700-700 700.2
appletalk zone WAN
dialer wait-for-carrier-time 60
dialer map appletalk 700.1 name R1 speed 56 broadcast 5550066
dialer-group 1
snapshot server 5 dialer
isdn spid1 5552222
ppp authentication chap
!
dialer-list 1 protocol appletalk permit
For a further examination of snapshot routing, see Chapter 21, “Using ISDN Effectively
in Multiprotocol Networks.”
Dial Backup for Leased Lines
Dial backup protects against wide-area network (WAN) downtime by allowing a dedicated serial
connection to be backed up by a circuit-switched connection. Dial backup can be performed in
several ways: either with floating static routes or with backup interfaces.
Dial backup challenges the designer with different traffic patterns than DDR-supported SOHO and
ROBO sites. When designing Dial backup port densities, consider how many links might fail
concurrently in a mass-failure scenario, as well as how many ports will be required on the central site in a worst-case scenario. Typical design involves selecting only dial-in or dial-out to avoid contention when both sides are trying to re-establish connectivity.
Backup Interfaces
A primary/dedicated serial line is configured to have a backup interface in the event of link failure or exceeded load thresholds. If the interface line or line protocol state goes down, the backup
interface is used to establish a connection to the remote site.
Once configured, the dial backup interface remains inactive until one of the following conditions is met:
1
Line Protocol on the primary link goes down. The backup line is then activated, re-establishing
the connection between the two sites.
Designing DDR Internetworks 10-15
Routing Strategies
2
The traffic load on the primary line exceeds a defined limit—The traffic load is monitored and a
five-minute moving average is computed. If the average exceeds the user-defined value for the
line, the backup line is activated. Depending on how the backup line is configured, some or all of the traffic flows onto it.
A Cisco IOS interface is placed into backup mode by applying the backup interface command:
• The backup interface interface configuration command specifies the interface that is to act as the backup.
• The backup load command specifies the traffic threshold at which the backup interface is to be activated and deactivated.
• The backup delay command specifies the amount of time that is to elapse before the backup interface is activated or deactivated after a transition on the primary interface.
Backup interfaces traditionally lock the backup interface into BACKUP state so it is unavailable for other use. Dialer Profiles eliminates this lock and allows the physical interface to be used for multiple purposes. Floating Static Route DDR design also eliminates this lock on the dialer interface. In
Figure 10-7, a leased line connects Router A to Router B, and BRI 0 on Router B is used as a backup line.
Figure 10-7
Example of dial backup over ISDN.
171 69 158 18
171 69 159 49
255 255 255 248
255 255 255 249
E0 Router A
Router B E0
171 69 159 25
171 69 159 26
255 255 255 249
255 255 255 249
ISDN
Using the configuration that follows, BRI 0 is activated only when serial interface 1/0 (the primary line) goes down. The backup delay command configures the backup connection to activate 30
seconds after serial interface 0 goes down and to remain activated for 60 seconds after the serial interface 1/0 comes up:
interface serial 1/0
ip address 172.20.1.4 255.255.255.0
backup interface bri 2/0
backup delay 30 60


 

 

 

 

 

 

 

 

 

 

 

 

   
 
  The remote routers are configured for snapshot routing by applying the snapshot client command to each ISDN interface...
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