3-5 Routing Over Dialup Networks | Cisco Field Manual[c] Router Configuration

Snapshot routing reduces the time a DDR connection is kept up by only periodically exchanging routing updates.
Snapshot routing is based on the concept of active periods, when routing information can be exchanged, and quiet periods, when a snapshot of the routing information is kept frozen.
During a quiet period, the routing entries are frozen and cannot be changed. They also are treated as static routes and are not aged until the next active period.
A retry period can be defined to keep the client router from waiting another complete quiet period if a DDR interface is not available. In this case, the client router can retry the active period call after the retry period has elapsed.
A snapshot client router can dial remote server routers only during an active period to collect routing information.
Only distance vector routing protocols are supported: AppleTalk RTMP, Banyan Vines RTP, IP RIP and IGRP, and IPX RIP and SAP.
On-Demand Routing (ODR) allows a centralized "hub" router to collect route advertisements from "spoke" routers on stub networks. The stub routers use only static or default routes and do not run any dynamic routing protocols.
ODR uses the Cisco Discovery Protocol (CDP) to dynamically learn about the stub routers and their directly connected networks. Therefore, no routing information is exchanged by routing protocols.

Snapshot Routing Configuration

Define snapshot routing on the client router.
1. Select an interface to reach a server router:
```
 (global)  interface   type slot/port  
```
  The interface is usually a logical interface dialer or a physical DDR interface.
2. Enable client mode snapshot routing:
```
 (interface)  snapshot client   active-time quiet-time  [  suppress-statechange-updates  ] [  dialer  ] 
```
  On the client router, the active-time (5 to 100 minutes; no default; typically 5 minutes) and quiet-time (8 to 100000 minutes) intervals are defined. The suppress-statechange-updates keyword can be used to disable routing exchanges each time the DDR interface state changes from "down" to "up" or from "dialer spoofing" to "fully up." In that case, routing information is exchanged only when the active period begins, regardless of whether the DDR interface just came up. The dialer keyword is used to cause the client router to dial the server routers even if there is no active traffic to trigger a call. Otherwise, the client router waits for the call to be made from normal DDR activity.
3. Define one or more server routers to contact:
```
 (interface)  dialer map snapshot   sequence-number dial-string  
```
  A server router is listed with a unique sequence-number (1 to 254), specifying the order in which the routers are to be called. The dial-string (string of digits) required to reach the server router must also be given.
4. Make sure DDR is configured on the dialer interfaces. You don't have to configure the triggers for interesting traffic, because the snapshot routing triggers DDR and provides the dial string. Refer to Section 3-3 for more configuration details if needed.
Define snapshot routing on the server router.
1. Select an interface for incoming calls from the client router:
```
 (global)  interface   type slot/port  
```
  The interface is usually a physical DDR interface that can accept incoming calls.
2. Enable server mode snapshot routing:
```
 (interface)  snapshot   active-time  [  dialer  ] 
```
  The server router must have the same active-time (5 to 100 minutes) configured as that of the client router. The dialer keyword can be used to allow the server router to accept calls from the client router, even when regular DDR traffic is not present.

Snapshot Routing Example

Snapshot routing is configured on a client router at a central site. The snapshot client maintains periodic routing updates with a list of three server routers at branch sites. Snapshot routing is configured with an active period of 5 minutes and a quiet period of 480 minutes, or 8 hours. The server routers are dialed during the active period, even if a DDR call is not already in progress.

The following example shows the configuration of the client router:

  interface dialer 1   ip address 192.168.1.1 255.255.255.0   encapsulation ppp   dialer pool 5   snapshot client 5 480 dialer   interface bri 0   encapsulation ppp   dialer pool-member 5   dialer map snapshot 1 8598851234   dialer map snapshot 2 8598855678   dialer map snapshot 3 8598859999   router rip   network 192.168.1.0   network 192.168.100.0

The following example shows the configuration of one of the server routers:

  interface bri 0   encapsulation ppp   ip address 192.168.1.2 255.255.255.0   snapshot server 5 dialer   router rip   network 192.168.1.0   network 192.168.200.0

ODR Configuration

Enable ODR on the hub router:
```
 (global)  router odr   process-id  
```
ODR is started and is assigned a unique process number.

NOTE

Routes can be filtered as they are collected by ODR. Refer to Section 8-4 for more information. Routes can also be redistributed into or out of ODR by using the IP route redistribution commands. Refer to Section 8-3 for more information.
(Optional) Tune the ODR timers.
1. Adjust the CDP timer on the stub routers:
```
 (global)  cdp timer   seconds  
```
  ODR depends on CDP advertisements to collect IP route prefixes from stub routers. By default, CDP advertisements occur every 30 seconds. Set the CDP interval to seconds for more frequent advertisements and faster route convergence.
2. Adjust the ODR timers:
```
 (router)  timers basic   update invalid holddown flush  [  sleeptime  ] 
```
  The frequency that routing updates are expected from stub routers can be adjusted to provide faster convergence. The update parameter (default 90 seconds) sets the rate in seconds at which updates are expected. This is the fundamental timing parameter of the ODR protocol. It should be set to match the CDP advertisement interval. The invalid parameter (default 270 seconds) sets the interval of time in seconds after which a route is declared invalid. It should be at least three times the value of update. The holddown parameter (default 280 seconds) sets the interval in seconds during which routing information regarding better paths is suppressed. It should be at least three times the value of update. The flush parameter (default 630 seconds) is the amount of time in seconds that must pass before the route is removed from the routing table. The interval specified must be at least the sum of invalid and holddown. If it is less than this sum, the proper holddown interval cannot elapse. This results in a new route's being accepted before the holddown interval expires . The sleeptime parameter (default 0 milliseconds) sets the interval in milliseconds for postponing routing updates in the event of a Flash update. The sleeptime value should be less than the update time. If the sleeptime is greater than the update time, routing tables become unsynchronized.
3. Define a default route on the stub routers:
```
 (global)  ip route 0.0.0.0 0.0.0.0   interface-name  
```
  A stub router needs only a statically defined default route pointing to an interface that connects to the hub router.