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Updated:January 7, 2014
This deployment guide provides detailed design and implementation information for deployment of Dial Backup with the Cisco
® Virtual Office. Please refer to the Cisco Virtual Office overview (
http://www.cisco.com/go/cvo) for further information about the solution, its architecture, and all of its components.
Dial Backup provides backup connectivity using dial network-to-corporate network connections if the Internet service provider (ISP) connection from spoke to hub fails. In the Cisco Virtual Office solution, which encompasses Dynamic Multipoint VPN (DMVPN) architecture for data gateway infrastructure, the Dial Backup feature provides connectivity to the data gateways using the dial network if the ISP connection fails. The bandwidth and speed provided by dial networks is low and should be used mainly to provide secondary connectivity. Whenever connection to the ISP restores, the tunnel to the data gateway through the dial network is torn down and the tunnel through the ISP is restored.
In DMVPN deployment, Dial Backup is incorporated on spokes. Figure 1 shows the connectivity between spoke and data gateways in the current Cisco Virtual Office deployment.
The dial server is introduced and a new hub is created to handle all DMVPN tunnels originating from the dial network.
Figure 1. Deploying Dial Backup in Cisco Virtual Office Environment
Initially connectivity to the internal network is through the data gateways VPN1 and VPN2 . These gateways provide redundant connectivity to the internal network. Although the primary path to reach the internal network is provided by one VPN gateway (say VPN1), VPN2 gateway takes over if VPN1 gateway is unusable because of system abnormality or router reload. By providing a dial backup, the redundancy is taken to the next level, where connectivity to the internal network is transparently provided if ISP connectivity fails.
When the spoke recognizes that connectivity to VPN1 and VPN2 gateways is unavailable, the spoke triggers the dial process and attempts to reach the dial server. The spoke has two paths to reach the VPN gateways -- the path through the ISP and the path through the dial network. On the spoke, the path through the ISP is given higher priority than the path through the dial network. When the spoke does not find the path to the gateway through the primary path (because of a missing entry in the routing table or Internet Control Message Protocol [ICMP] ping failure), it triggers the dial process. The spoke gets a dynamic address from the dial server and reaches the VPN gateway through the dial server.
The spoke then initiates Next Hop Resolution Protocol (NHRP) registration and subsequently brings up a DMVPN tunnel with VPN3 gateway and thereby provides connectivity to the internal network. When the ISP connection is up again, the DMVPN tunnel to VPN1 and VPN2 gateways is brought up and the DMVPN tunnel to VPN3 is torn down.
In testing Dial Backup in Cisco Virtual Office setup, only analog modems were used. In the spoke router, you can use platforms such as modular integrated services routers, which have internal modems; for platforms such as the Cisco 881 Integrated Services Routers, which do not have and internal modem, you must use an external modem. If you use an external modem, be sure to adjust the speed and other parameters on the spoke router to match the speed that the modem is configured to work with.
The dial server is supported on many platforms. For testing we limited the dial server and spoke platforms and images as follows:
• Dial server platform: Cisco 3845 Integrated Services Router; image: Cisco IOS® Software Release 12.4(15)T5 or later
• Spoke platform: Cisco 1841 Integrated Services Router; image: Cisco IOS Software Release 12.4(15)T5 or later
For the Cisco 880 Series Integrated Services Routers, you must use Cisco IOS Software Release 12.4(20)T or later.
• When a DMVPN tunnel is established using Dial Backup, only the hub-to-spoke topology is supported.
• The testing was performed with an in-house dial server; the setup has not been tried with dialup service provided by ISPs.
• While configuring the Cisco 800 Series for Dial Backup, do not use the console. Always perform Telnet to the router and then configure the modem commands and other commands for the Dial Backup feature. Because the Cisco 800 Series routers do not have an internal modem, connect the console port on the Cisco 800 with an external analog modem.
Because the Cisco 1841 uses a modem WAN interface card (WIC), connect the phone cable to the ports on the WIC.
Figure 2 gives the sample topology with IP address corresponding to the node, to map with the configuration that follows.
Figure 2. Sample Topology
Configuration on the Dial Server
Note: In the configuration that follows, a dedicated dial server was used. In general you can use any public ISP dial server.
! Specifies one or more AAA authentication methods for use on serial interfaces running Point-
! to-Point Protocol (PPP)
aaa authentication ppp default group dial-server
aaa session-id common
no ip domain lookup
ip domain name cisco.com
ip audit po max-events 100
no ftp-server write-enable
! The following example specifies ISDN PRI on T1 slot 1, port 0, and configures voice and data
! bearer capability on time slots 1 through 24:
isdn switch-type primary-5ess
controller T1 1/0
pri-group timeslots 1-24
ip address 10.32.200.1 255.255.255.128
ip ospf network point-to-point
ip address 10.34.200.85 255.255.255.240
ip access-group fw_acl out
! In a dedicated configuration, we assume the 24th timeslot will be used by ISDN.
! Serial interface 0:23 is created for configuring ISDN parameters.
no ip address
dialer rotary-group 1
isdn switch-type primary-5ess
isdn incoming-voice modem
no isdn outgoing ie redirecting-number
no isdn incoming alerting add-PI
no cdp enable
! Create an asynchronous group interface
ip unnumbered Loopback0
async mode interactive
peer default ip address pool dialin_pool
ppp authentication chap callin
group-range 97 114
router ospf 5
area 24 nssa
network 10.32.200.0 0.0.0.127 area 24
network 10.34.200.80 0.0.0.7 area 24
! configure a local pool of IP addresses to be used when a remote peer connects to a
! point-to-point interface.
ip local pool dialin_pool 10.32.200.2 10.32.200.126
! Define a DDR dialer list to control dialing by protocol or by a combination of a protocol and
! a previously defined access list
dialer-list 1 protocol ip permit
no cdp log mismatch duplex
! Configure line interface for the AUX port. enable incoming and outgoing calls.
line con 0
exec-timeout 0 0
line 97 114
exec-timeout 0 0
modem autoconfigure discovery
line aux 0
ntp clock-period 17180022
ntp server 192.168.203.5
Configuration on the VPN3 Data Gateway
Note: The following configuration is mainly for the Dial Backup feature. For a complete DMVPN gateway configuration, please refer to the Cisco Virtual Office overview available at http://www.cisco.com/go/cvo.
service timestamps debug uptime
service timestamps log uptime
ip domain name dmvpn.com
crypto pki trustpoint certificate-server
enrollment url http://192.168.203.12:80
! The certificates are generated automatically after the registration with the CA
crypto pki certificate chain TEST-CA
certificate ca <removed>
!--- Certificate is abbreviated for easier viewing
crypto ipsec transform-set t3 esp-3des esp-sha-hmac
mode transport require
! Create an IPSec profile to be applied dynamically to the GRE over
! IPSec tunnels.
crypto ipsec profile dialbackup
set transform-set t3 t1 t2
ip address 10.3.0.67 255.255.255.255
! This is the mGRE interface for dynamic GRE tunnels via the dial
ip address 10.7.9.4 255.255.254.0
no ip redirects
ip mtu 1400
ip pim nbma-mode
ip pim sparse-dense-mode
ip multicast rate-limit out 768
ip nhrp map multicast dynamic
ip nhrp network-id 1234
ip nhrp holdtime 600
ip nhrp server-only
ip tcp adjust-mss 1360
no ip split-horizon eigrp 7
ip nhrp redirect
no ip mroute-cache
tunnel source Loopback1
tunnel mode gre multipoint
tunnel key 100
tunnel protection ipsec profile dialbackup
ip address 172.17.0.1 255.255.255.252
ip address 192.168.0.1 255.255.255.0
! Enable a routing protocol to send/receive dynamic updates about the private networks over the
router eigrp 7
network 10.7.8.0 0.0.1.255
distribute-list split_out in Tunnel300
ntp server 192.168.203.5
Configuration on the Spoke Router
Note: For this configuration the Cisco 1841 is used as a spoke. The following configuration is mainly for the Dial Backup feature. For a complete spoke configuration, please refer to the Cisco Virtual Office overview at http://www.cisco.com/go/cvo.