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This may be a dumb question, but I have a triangle connection using point to point 128k serial lines. What’s a triangle connection, you ask? Let’s say you have a connection from NY to DC and from DC to Atlanta, and finally from Atlanta to NY. Each are 128k point to point connections. My headquarters is in NY.

My question: When a connection from point A to point B goes down, how does the router pass traffic to the other path? I've tried to make it work, but right now all it does is load balancing, which does make my connections faster, but provides no fault tolerance.

Thanks.

-- Manuel Couverthie
Puerto Rico

Answer:

Manuel,

Well, I’m assuming that since you’re experiencing load balancing at the moment, you have solved the problem using static routes.

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With static routes, there will be two “equal cost” paths and your router will load balance between them. You can engineer your “preferred” choices by setting a different administrative distance on separate routes. For example:

Ip route 10.1.0.0 255.255.0.0 10.0.1.1
Ip route 10.1.0.0 255.255.0.0 10.0.1.2

This will cause you to have two directions to go in order to reach the 10.1.0.0 network and will load balance between them.

Ip route 10.1.0.0 255.255.0.0 10.0.1.1
Ip route 10.1.0.0 255.255.0.0 10.0.1.2 10

This, on the other hand will assign an administrative distance of 10 to the second route. Static routes by default have an AD of 1, and the lower the AD, the more preferred the route is. So in this method, your router will route all 10.1.0.0 traffic to the first router. This is called a floating static route.

So what happens if the first router goes away? Nothing -- because you manually told the router where to send things.

Let me clarify things a little further before people jump up and down about this one! Notice that I put the destinations on the same subnet! If you have two separate layer 3 links (separate sub-interfaces if you are running Frame Relay PVCs), then you could, indeed, have some failover that way.

Being on two separate interfaces, if a physically connected interface goes down, the router would know that it couldn’t route out that direction; therefore, the second route would enter the routing table.

So if you had:

Ip route 10.1.0.0 255.255.0.0 serial 0/0.1
Ip route 10.1.0.0 255.255.0.0 serial 0/0.2 10

then you would find some failover there if the PVC representing Serial 0/0.1 went away.

You can also use a dynamic routing protocol to learn about networks automatically. You would then always have the “best path” in your routing table no matter what the status of your physical links. All routing protocols have some sort of shortest-path or lowest-cost or best-choice algorithm to help you weigh all the options available.

Note that with either of these options (floating static or dynamic routing protocol), you will have only one route to that network represented in your routing table at any one point in time. It would represent the best path or best choice to that destination.

In the grand scheme of things, it’s better to run a routing protocol and let your router handle the decisions like it’s supposed to! Especially as your company/network may grow, you would find that administering a series of static routes would become quite cumbersome and often outdated. And you generally don’t realize this until something breaks!

Hope that helps.

-- Scott

Send your toughest CCIE-level technical questions to editor@tcpmag.com.

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Scott Morris, quadruple CCIE, JNCIE and all-around uber-geek, can often be seen traveling around the world consulting and delivering CCIE training. He recently accepted a new Senior CCIE Instructor position with Internetwork Expert! For more information on him check out http://www.uber-geek.net or for CCIE training check out http://www.internetworkexpert.com. You can contact Scott via editor@tcpmag.com. You can contact Scott about "Floating Static vs. Dynamic Routing Protocol " at editor@tcpmag.com.