DSL and cable are both becoming popular options for backup links since they aren’t very expensive, and are routinely much higher speed than ISDN dial-up lines.
Both cable and DSL connect to some sort of modem usually, and then are plugged into an Ethernet card. Certain routers can interface with DSL lines directly (a WIC-1ADSL for your Cisco Router for example). Once the connection is complete, you typically have a single IP address available to you. (This can be negotiated differently with your ISP, but we’ll work with the basics here.)
Be aware that cable and DSL are public Internet connections and inherently don’t offer you any protections like a pseudo-private frame-relay connection does. Be very aware of the security on your Cisco Routers at this point! You should always be well protected, but just in case it has slipped your mind, pay extra attention to security here!
So assuming your cable/DSL lines are set up and ready to go, let’s look at the Cisco Routers perspective for things.
You’ll need to configure NAT or better yet an IPSec tunnel for your internal traffic since it’ll be traversing the Internet over cable or DSL. I’ll let you research those functions separately so this column doesn’t turn into a novel!
After those things are complete, there are a few different ways to do things, all of them the same technologies as we use for Dial on Demand Routing (DDR) applications! The usefulness of each depends on how you have configured the rest of your Cisco Router though.
The first is floating static routes. Look at your routing table; perhaps you have static routes already configured. If the Cisco Router you’re configuring is a spoke or branch office, then most likely you have a 0/0 route (0.0.0.0 network and 0.0.0.0 mask). In which case, you can enter:
Ip route 0.0.0.0 0.0.0.0 (dsl/cable connection interface) 200
The “200” on the end signifies the administrative distance of the route. If your existing line goes down, the route would disappear and then this alternate would take effect. Bear in mind though, that for the static route to disappear, the physical line needs to go down. If your PVC goes down (and you are NOT using sub-interfaces), then it will have no effect.
The next option you can use on your Cisco Router is the backup-interface command. This means when the interface goes down, then the backup interface the router should use next will be listed. You can apply backup-interface to a physical interface (again, PVCs going down would have no effect) or to the sub-interface itself in order to monitor PVCs.
(config-if)#backup interface (dsl/cable connection interface)
You may also want to specify a delay factor to prevent flapping of interfaces (or to cope with the flapping of interfaces). Typically I recommend that you activate a backup right away, but deactivate it after a minute or so of the primary being back up. This will help reduce problems within your network.
(config-if)#backup delay 1 60
Those are two methods that will most definitely work with your cable or xDSL interfaces. The next method I’m going to lay out may or may not work. Some of the documents I’ve seen in the past say that they rely upon other Dial on Demand functions and operating code, which would only be for a dial-up environment (ISDN or Analog). But, just in case code changes or in case you want to use this method for your dial-up lines, here we go!
The newer, sexy way to do backup lines is with dialer-watch! This will watch your routing table for specific routes that you list. If those routes disappear, then the backup interface will be initiated. This method works great with dynamic routing protocols. (Note: If you’re using dynamic routing protocols and an IPSec tunnel for your backup interface, you need to research GRE within IPSec. IPSec tunnels don’t inherently support routing protocols.)
The first step is to decide what routes you’re watching:
(config)#dialer watch-list 1 ip 172.16.1.0 255.255.255.0 (config)#dialer watch-list 1 ip 172.16.2.0 255.255.255.0 etc.
Depending on the number of interfaces you’re supporting, you can have more than one dialer-watch list. The lists are then assigned to a particular interface that will handle the backup, in this case the cable/dsl interface:
(config-if)#dialer watch-group 1
Like the backup interface commands, I recommend specifying a delay value here as well to prevent flapping:
(config-if)#dialer watch-disable 60
Somewhere between your implementation of one of these backup interface command sets, you should find it relatively simple to use cable or DSL to back up your primary connections. A few things I would keep in mind though: