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New Member

Floating Static Routes - DG detection

Dear fiends,

I am trying to implement a contingency routing scheme using floating static routes.In this case I do not want to use hot stand by.

I could not find any text about how Cisco performs dead gateway detection.

I have a Lan (DMZ) with 2 possible routers in it. Lets say route 1 and route 2. I want to configure router 3 (internal) to send traffic trhough R1 or R2.

My first action is to configure route-to-1 with administrative distance less than route-to-2.

My doubt is: if route 1 goes dowm and route 2 starts to be used, how R3-IOS will find out that route 1 became available again?

There is some time out?

What is the method?

Regards,

Paulo Prista

1 ACCEPTED SOLUTION

Accepted Solutions
Bronze

Re: Floating Static Routes - DG detection

IOS's ability to verify route availability is fairly mundane. There are features in the works to add the ability to verify routes actively (e.g., via ping), but they're currently restricted to very specific code releases on lower-end platforms (the 1700 series).

This isn't the easiest thing in the world to explain, but as it stands, IOS only knows a route has become unavailable when the route to its next-hop is removed from the routing table. The typical case is when route A points to a next-hop that is reachable via a point-to-point serial interface such as a T1. When the T1 goes down, the serial interface changes state to "down", so the connected route for that interface (i.e., the route to route A's next-hop) is removed. IOS then knows that route A is unavailable because its next-hop is unreachable, and therefore removes route A from the routing table.

The problem comes when we start to talk about Ethernet links, which sounds like what you'll be using in your case. An Ethernet link doesn't change state to "down" unless the cable is unplugged on either the local or remote side. In your case, this is fine if router-1 is directly connected to router-3 via a cross-over cable. If router-1 goes down, the Ethernet link goes down and router-3 is therefore able to detect the failure. But usually this isn't the way things are connected -- usually there are switches in between that keep the Ethernet link "up" regardless of whether or not any given router on the link is up. So in your example, if the 3 routers are connected via one or more switches, router-3 has no way of knowing when router-1 or router-2 go down unless it is running a routing protocol with them. If router-1 goes down, the route on router-3 that goes through router-1 remains in the routing table because the (connected) route to its next-hop (router-1) still exists: the Ethernet link is still up.

So this is why floating static routes tend to work fine for serial links, but are almost always useless over Ethernet links.

5 REPLIES
New Member

Re: Floating Static Routes - DG detection

are they directly connected? If so the router will see it's connected interface go down and that route will not be vaild. When you do your static routes use the interface to tell it where to go.

Like this:

0.0.0.0 0.0.0.0 s1/0 100

If they are not connected then we'll have to come up with something else.

-Adam

New Member

Re: Floating Static Routes - DG detection

You have not really explained this too well. If you are asking how to do this on non-connected routers, what you could do is configure static routes on the 2 dmz routers and redistribute them into tour internal routing protocol with different metrics. This would allow your internal router router3, to learn the routes dynamically and if router 1 loses it's route to the next hop, it will stop advertising to router 3 and router 2 will take over.

Once the route comes back on router 1, it will take back over as it will have a more preferrable metric and this way of doing it would insure that you have a path at least through the DMZ routers.

Bronze

Re: Floating Static Routes - DG detection

R3 can only detect such a link failure by running a routing protocol with R1. When R1's link goes down, R1 withdraws the route via the routing protocol and R3 can then route via R2 with a floating static route.

New Member

Re: Floating Static Routes - DG detection

Ok. But I would like to use static routes only.

Just after this question was posted, I found out, in the book "High Availability Networking with Cisco" that if I have static route-to-1 and route-to-2 (with different administrative distance) IOS will check route availabilty in "per-minute" basis.

If this is true, once route-to-1 goes down, IOS will use route-to-2, but will try, in per minute basis, if route-to-1 came back.

I have not tried this yet, but if someone has ever tested this situation I will be glad to know.

Regards,

Paulo

Bronze

Re: Floating Static Routes - DG detection

IOS's ability to verify route availability is fairly mundane. There are features in the works to add the ability to verify routes actively (e.g., via ping), but they're currently restricted to very specific code releases on lower-end platforms (the 1700 series).

This isn't the easiest thing in the world to explain, but as it stands, IOS only knows a route has become unavailable when the route to its next-hop is removed from the routing table. The typical case is when route A points to a next-hop that is reachable via a point-to-point serial interface such as a T1. When the T1 goes down, the serial interface changes state to "down", so the connected route for that interface (i.e., the route to route A's next-hop) is removed. IOS then knows that route A is unavailable because its next-hop is unreachable, and therefore removes route A from the routing table.

The problem comes when we start to talk about Ethernet links, which sounds like what you'll be using in your case. An Ethernet link doesn't change state to "down" unless the cable is unplugged on either the local or remote side. In your case, this is fine if router-1 is directly connected to router-3 via a cross-over cable. If router-1 goes down, the Ethernet link goes down and router-3 is therefore able to detect the failure. But usually this isn't the way things are connected -- usually there are switches in between that keep the Ethernet link "up" regardless of whether or not any given router on the link is up. So in your example, if the 3 routers are connected via one or more switches, router-3 has no way of knowing when router-1 or router-2 go down unless it is running a routing protocol with them. If router-1 goes down, the route on router-3 that goes through router-1 remains in the routing table because the (connected) route to its next-hop (router-1) still exists: the Ethernet link is still up.

So this is why floating static routes tend to work fine for serial links, but are almost always useless over Ethernet links.

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