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serial interfaces


I would just like a clarification on an issue while doing some configurations. i want to configure the serial interfaces on three different routers, basically so as to be able to configure static routes and dynamic routes. R1 has two serial interfaces, R2 has one serial interface and R3 has one serial interface. R1 is the hub that spokes R2 and R3 connect to. the ip subnet is I am able to configure the serial interfaces on R2 with ip and R3 with i can configure s0 on R1 with the address but whenever i try to configure s1 on R1 with i get the error message  * overlaps with Serial0/2/1*

kindly explain what the issue can be? does this mean that  the differnet serial interfaces on a rtr have to be in different subnets, are they to be treated as two different entiities even if they are on the same rtr?

also when i give s1 the address (being a different subnet) i can not ping this interface even from R1 that it is directly configured. kindly provide an explanation to the issue.




It would be great if you post R1 config. I'm assuming you're using Frame Relay. Most of times the router won't be able to ping itself unless you manually add a "frame-relay map" config.

About interfaces, every interface has to be in different networks (no matter if they're serial, ethernet or subinterfaces).


Yes routed interfaces on a given router cannot have addresses that overlap.

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Assuming you are not doing frame relay,

What you really have is 2 separate networks, each with 2 layer 3 interfaces.

R1 s0 to R2 is one network.

R1 s1 to R3 is one network,

Then R1 does it's job by routing between the 2 networks.

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To put it another way:

The addresses you are trying to assign to different router interfaces are in the same network/subnet.

even if you were just working with one router it wouldnt let you give one interface an address of then another interface

If you think about it this makes sense as the whole point of routers is to route between different networks. If this isnt what you want then you need a switch.

you need to make sure the interfaces are in different networks so &

or & for example

It sounds like you need to be more confortable with subnetting. but you can use
to help


In hub and spoke topology, hub router has one physical interface with one or more subinterface and this topology is used when layer2 is frame relay. in frame relay you can use one subnet for all routers but you should defined neighbors and set encapsulation to frame relay. you are using 2 physical interface on central router, so this show that you are not using frame relay. in this situation you cannot use one subnet and each interface should be in different subnet because router does not let you to set ip and also routing protocols cannot work in this case. you need one subent for R2 and the associated interface on R1 and second subnet for R3 and associated subnet on R1(this subnet can be /24 or /30)

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