Articles & Tutorials

Setting Up Dial Backup Connections
By John Trepp

In this day and age, making sure data reaches its intended destination is of utmost importance regardless of company and network size. In a large company, one would expect to find enormous and complex networks where the backup circuit is a high speed redundant link, The high cost of a dedicated WAN (Wide Area Network) circuit may be high but the cost of a broken circuit far exceeds this reoccurring monthly expense. This same scenario may not economically feasible for many small to midsize companies, so a modem backup solution may be the desirable route to take.

Minimizing solution and implementation cost while still providing adequate bandwidth is something that cannot be determined here, because we recognize everyone has unique needs. We will however show you how to configure two different types of DBU, that provide varying degrees of bandwidth. As you will see in the examples, this is an easily adaptable solution, which can be modified to suit your requirements.

ISDN solution: Low cost, and suitable for backing up 56 – 128k leased lines

To accomplish this, all that’s needed is two ISDN lines, and two DBU capable DSU’s (data service unit), something like an Adtran DSU ESP IV. Outfitted with a DBU card, the modem (if you want to call it that) terminates the leased line as well as the ISDN, and provides automatic switchover. Because of the quick call establishment for ISDN, backup can be realized in 5-10 seconds.

Now a not-so-obvious variation of the above is probably worth mentioning here.
An async cable can be connected to the serial port of the router, and then connected to an analog modem in place of the digital modems indicated above. The router can then be configured to dial the modem in the event of an outage. One router forces a dial out, the other enables it’s modem to receive a call - which provides security (if both routers are able to sense loss of the leased line). This works well and has been tested with Cisco 1601R routers. If the solution were deployed as shown above (two routers and two analog modems) the line costs would be very inexpensive so the recurring costs would be low. By utilizing a networking equipment reseller, the one time equipment costs can also be reduced. If the link that’s being backed up is one that provides Internet connectivity, then a CHAT Script can be configured on the router that will automatically provide the login credentials necessary to dial into an ISP.

In the following example, we will use the Cisco 1601R. This router has a serial port, 10bT Ethernet, and a slot for a WAN interface (WIC) card. There is quite an assortment of WIC cards to choose from, supporting just about any link you choose, from ISDN to T1, making this platform quite flexible.

For LAN to LAN DBU:

The operation of this solution is completely controllable through the configuration on the routers, enabling you to have the link establish on your terms. The flexibility of the router having an expansion slot allows for tailoring the backup link to your needs.

Most of this document has been dealing with dial backup, but another somewhat related application for this setup is to have bandwidth-on-demand. It’s almost like having the best of both worlds – dial backup when needed and added bandwidth when needed. A side benefit of using it in this manner is that the “dial backup” would be tested on a periodic basis. If it didn’t work, more than likely it would be in a non-critical situation where a less bandwidth could be tolerated in lieu of no connectivity at all. I have seen situations where a DBU solution was installed and tested, only to have it fail when needed because something locked up due to inactivity. It is important to test any DBU solution on a regular basis.

Getting started configuration
Mentioned above was a DBU solution that dialed an ISP for Internet connectivity. In the configuration shown below, this router will dial out when it sees “interesting traffic”. In this case, any traffic that the router cannot resolve, it will dial out and attempt to route. This is for a 1601R router, which has an async cable connected between the serial port and an analog modem. The chat script enters the login information, and the router dials out and starts a PPP session with the ISP. This is a handy configuration to have on a router connected top a PC. Instead of having to manually dial, the router will do it all for you as soon as you try to access the Internet.

! version 12.0 service timestamps debug datetime msec localtime service timestamps log uptime service password-encryption ! hostname MY-Router ! logging buffered 100000 debugging enable secret whateveryouwant ! ip subnet-zero no ip domain-lookup ! chat-script mychat ABORT ERROR ABORT BUSY ABORT NO "" \pATZE0 OK \d[modem init string here] OK \dATX0 OK \dATDT\T TIMEOUT 60 CONNECT \d [login credentials here] ! process-max-time 200 ! interface Ethernet0 ip address xxx.xxx.xxx.xxx 255.255.xxx.xxx [put your IP adressing here] no ip directed-broadcast no cdp enable ! interface Serial0 physical-layer async ip unnumbered Ethernet0 no ip directed-broadcast encapsulation ppp ip tcp header-compression dialer in-band dialer idle-timeout 600 [set your timeout in seconds] dialer string [first ISP number to dial] dialer-group 1 async default routing no peer neighbor-route no peer default ip address ! ip classless ip route 0.0.0.0 0.0.0.0 Serial0 permanent ip http server ! no cdp enable ! line con 0 password reseller login transport input none line 1 password reseller autoselect arap script dialer mychat login modem InOut transport input telnet stopbits 1 speed 115200 flowcontrol hardware line vty 0 4 password reseller login ! end

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