Configuring Snapshot Routing

Configuring Snapshot Routing
This chapter describes how to configure snapshot routing. It includes the following main sections:
•
Snapshot Routing Overview
•
How to Configure Snapshot Routing
•
Monitoring and Maintaining DDR Connections and Snapshot Routing
•
Configuration Examples for Snapshot Routing
To identify the hardware platform or software image information associated with a feature, use the
Feature Navigator on Cisco.com to search for information about the feature or refer to the software
release notes for a specific release. For more information, see the “Identifying Supported Platforms”
section in the “Using Cisco IOS Software” chapter.
For a complete description of the snapshot routing commands mentioned in this chapter, refer to the
Cisco IOS Dial Technologies Command Reference. To locate documentation of other commands that
appear in this chapter, use the command reference master index or search online.
Snapshot Routing Overview
Snapshot routing enables a single router interface to call other routers during periods when the line
protocol for the interface is up (these are called “active periods”). The router dials in to all configured
locations during such active periods to get routes from all the remote locations.
The router can be configured to exchange routing updates each time the line protocol goes from “down”
to “up” or from “dialer spoofing” to “fully up.” The router can also be configured to dial the server router
in the absence of regular traffic if the active period time expires.
Snapshot routing is useful in two command situations:
•
Configuring static routes for dial-on-demand routing (DDR) interfaces
•
Reducing the overhead of periodic updates sent by routing protocols to remote branch offices over
a dedicated serial line
When configuring snapshot routing, you choose one router on the interface to be the client router and
one or more other routers to be server routers. The client router determines the frequency at which
routing information is exchanged between routers.
Routing information is exchanged during an active period. During the active period, a client router dials
all the remote server routers for which it has a snapshot dialer map defined in order to get routes from
all the remote locations. The server router provides information about routes to each client router that
calls.
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How to Configure Snapshot Routing
At the end of the active period, the router takes a snapshot of the entries in the routing table. These entries
remain frozen during a quiet period. At the end of the quiet period, another active period starts during
which routing information is again exchanged; see Figure 59.
Active and Quiet Periods in Snapshot Routing
Active
period
Active
period
Quiet period
S3105
Figure 59
Time (minutes)
When the router makes the transition from the quiet period to the active period, the line might not be
available for a variety of reasons. For example, the line might be down or busy, or the permanent virtual
circuit (PVC) might be down. If this happens, the router has to wait through another entire quiet period
before it can update its routing table entries. This wait might be a problem if the quiet period is very
long—for example, 12 hours. To avoid the need to wait through the quiet period, you can configure a
retry period. If the line is not available when the quiet period ends, the router waits for the amount of
time specified by the retry period and then makes the transition to an active period. See to Figure 60.
Active
period
Retry Period in Snapshot Routing
Retry
period
Active
period
Quiet period
Active
period
S3106
Figure 60
Time (minutes)
The retry period is also useful in a dialup environment in which there are more remote sites than router
interface lines that dial in to a PRI and want routing information from that interface. For example, a PRI
has 23 DS0s available, but you might have 46 remote sites. In this situation, you would have more dialer
map commands than available lines. The router will try the dialer map commands in order and will use
the retry time for the lines that it cannot immediately access.
The following routed protocols support snapshot routing. Note that these are all distance-vector
protocols.
•
AppleTalk—Routing Table Maintenance Protocol (RTMP)
•
Banyan VINES—Routing Table Protocol (RTP)
•
IP—Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP)
•
Internet Protocol Exchange (IPX)—RIP, Service Advertisement Protocol (SAP)
How to Configure Snapshot Routing
To configure snapshot routing, perform the tasks in the following sections:
•
Configuring the Client Router (Required)
•
Configuring the Server Router (Required)
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You can also monitor and maintain interfaces configured for snapshot routing. For tips on maintaining
your network with snapshot routing, see the section “Monitoring and Maintaining DDR Connections and
Snapshot Routing” later in this chapter.
For an example of configuring snapshot routing, see the section “Configuration Examples for Snapshot
Routing” at the end of this chapter.
Configuring the Client Router
To configure snapshot routing on the client router that is connected to a dedicated serial line, use the
following commands beginning in global configuration mode:
Command
Purpose
Step 1
Router(config)# interface serial number
Specifies a serial interface.
Step 2
Router(config-if)# snapshot client
active-time quiet-time
[suppress-statechange-updates] [dialer]
Configures the client router.
To configure snapshot routing on the client router that is connected to an interface configured for DDR,
use the following commands beginning in global configuration mode:
Command
Purpose
Step 1
Router(config)# interface serial number
Specifies a serial interface.
Step 2
Router(config-if)# dialer rotary-group number
Configures a dialer rotary group.
Step 3
Router(config-if)# interface dialer number
Specifies a dialer interface.
Step 4
Router(config-if)# snapshot client
active-time quiet-time
[suppress-statechange-updates] [dialer]
Configures the client router.
Step 5
Router(config-if)# dialer map snapshot
sequence-number dial-string
Defines a dialer map.
Repeat these steps for each map you want to define. Maps must be provided for all the remote server
routers that this client router is to call during each active period.
Because ISDN BRI and PRI automatically have rotary groups, you need not define a rotary group when
configuring snapshot routing.
To configure snapshot routing on the client router over an interface configured for BRI or PRI, use the
following commands beginning in global configuration mode:
Command
Purpose
Step 1
Router(config)# interface bri number
Specifies a BRI interface.
Step 2
Router(config-if)# snapshot client
active-time quiet-time
[suppress-statechange-updates] [dialer]
Configures the client router.
Step 3
Router(config-if)# dialer map snapshot
sequence-number dial-string
Defines a dialer map.
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Monitoring and Maintaining DDR Connections and Snapshot Routing
Configuring the Server Router
To configure snapshot routing on the server router that is connected to a dedicated serial line, use the
following commands beginning in global configuration mode:
Command
Purpose
Step 1
Router(config)# interface serial number
Specifies a serial interface.
Step 2
Router(config-if)# snapshot server
active-time [dialer]
Configures the server router.
To configure snapshot routing on the associated server router that is connected to an interface configured
for DDR, use the following commands beginning in global configuration mode:
Command
Purpose
Step 1
Router(config)# interface serial number
Specifies a serial interface.
Step 2
Router(config-if)# interface dialer number
Specifies a dialer interface.
Step 3
Router(config-if)# snapshot server
active-time [dialer]
Configures the server router.
The active period for the client router and its associated server routers should be the same.
Monitoring and Maintaining DDR Connections and Snapshot
Routing
To monitor DDR connections and snapshot routing, use any of the following commands in privileged
EXEC mode:
Command
Purpose
Router# show dialer [interface type number]
Displays general diagnostics about the DDR interface.
Router# show interfaces bri 0
Displays information about the ISDN interface.
Router# clear snapshot quiet-time interface
Terminates the snapshot routing quiet period on the client
router within 2 minutes.
Router# show snapshot [type number]
Displays information about snapshot routing parameters.
Router# clear dialer
Clears the values of the general diagnostic statistics.
Configuration Examples for Snapshot Routing
The following example configures snapshot routing on an interface configured for DDR on the client
router. In this configuration, a single client router can call multiple server routers. The client router dials
to all different locations during each active period to get routes from all those remote locations.
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The absence of the suppress-statechange-updates keyword means that routing updates will be
exchanged each time the line protocol goes from “down” to “up” or from “dialer spoofing” to “fully up.”
The dialer keyword on the snapshot client command allows the client router to dial the server router in
the absence of regular traffic if the active period time expires.
interface serial 0
dialer rotary-group 3
!
interface dialer 3
dialer in-band
snapshot client 5 360 dialer
dialer map snapshot 2 4155556734
dialer map snapshot 3 7075558990
The following example configures the server router:
interface serial 2
snapshot server 5 dialer
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