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Home > Archive > CCNP > March 2001 > what is "floating"
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what is "floating"
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| is it equal ip route 0.0.0.0 0.0.0.0 x.x.x.x? | |
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| I saw the posting which you discussed, but still not so clear,
I know it can be a backup route
I know it can be overriden by dynamic route
and another is confusing
dmaftei£¬could you help me | |
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| quote:
Originally posted by ct
I saw the posting which you discussed, but still not so clear,
I know it can be a backup route
I know it can be overriden by dynamic route
and another is confusing
dmaftei£¬could you help me
I'll try this one and give dmaftei a break.
Simple example: Let's say RIP is your routing protocol. Router1 has a serial interface 0 with a point-to-point circuit on it. When that circuit is working, all routes to the rest of the network go out S0. The dynamic routes have a cost of 120, because 120 is the default administrative distance for RIP.
Router1 also has an ISDN circuit. You want the ISDN to dial out if (and only if) the primary circuit goes down. So you configure a "floating static" route like this:
ip route 0.0.0.0 0.0.0.0 <next hop via BRI0> 150
You could also use an IP route to your 10.0.0.0 or whatever network address is out in the rest of the WAN. In a more complex scenario, you may need to be much more specific about your static routing.
When the primary circuit is up, all traffic will route via s0, because that route has a distance of 120. But if s0 goes down, those routes drop out of the routing table. Then and only then does the router decide to use the static route with the administrative cost of 150. It then routes the traffic over the BRI interface. Because you have the dialer configured properly, the line dials out, connects, and the traffic starts routing over the BRI interface.
When s0 comes back up, RIP will update and put routes back in the routing table. The router will prefer these routes, because the administrative distance of 120 is better than your floating static route, which is 150. So traffic stops going over the ISDN link, and goes over s0. Then Router1 will see no traffic moving across the ISDN, wait for its configured timeout, and end the call.
That's why it's called "floating." The route is configured, but you won't see it in the routing tables as long as RIP routes are working. It "floats" out there, so to speak, and never "lands" in the routing table unless the RIP routes are all gone. Then, when the RIP routes are back, it just floats away again. :-)
HTH,
doctorcisco | |
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| I have seen the term 'floating address; used when two routers are running HSRP - Hot Standby Routing Protocol. Essentially, each router has an address on an ethernet interface with a priority number assigned, one router has a higher priority number than its neighbour. Additionally, each router ethernet interface shares one 'floating address' and this address is 'tagged' (in software) to the two other addresses. Incoming traffic to the interfaces is directed at the floating address, and the router interface with the higher priority takes the traffic. If that interface or router fails, then the neighbour takes over. Since the failed router has a higher priority, when it comes back on line, it will take over again.
Howzat?
Hippo. | |
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| Dr Cisco gives a great explanation!
Typically "floating" is used when referring to a backup static route (most often with ISDN) as the good Dr describes.
For HSRP I have most often heard the term "virtual interface" used for the one it creates.
Yankee | |
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| because 150>120, so 150 not work if 120 working, when 120 down, 150 work to take over the 120.
thank you so much, now I am clear. |
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