Connecting L3 switch to a Router

Answered Question
Dec 17th, 2009

I have a question. I have a 4500 Series Switch which i am trying to connect it to a Router. An expert told me

that when i have to connect a L3 switch to a Router i should connect them through a link network subnet,

meaning configure on the Switch port (no switchport) and put an ip address. Also the same network on

the Fast Ehternet port of the Router.

Since he is an expert i have no doupts that he is right. But what i am going to gain from that? What actually is

the reason ? Why is better using this than just a normal connection with the Router? Do i need to do that?

Thanks a lot

Moses

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Correct Answer by Jon Marshall about 6 years 11 months ago

moses12315 wrote:

I have a question. I have a 4500 Series Switch which i am trying to connect it to a Router. An expert told me

that when i have to connect a L3 switch to a Router i should connect them through a link network subnet,

meaning configure on the Switch port (no switchport) and put an ip address. Also the same network on

the Fast Ehternet port of the Router.

Since he is an expert i have no doupts that he is right. But what i am going to gain from that? What actually is

the reason ? Why is better using this than just a normal connection with the Router? Do i need to do that?

Thanks a lot

Moses

Just to add to Reza's comments.

Given the choice i would agree with Reza that it is more common and personally i think better to use a routed link. However this is not a hard and fast rule. Lets say for example you have a L3 switch connecting to 2 routers and you wanted them all to be in the same subnet. With that requirement you could not use routed ports on the L3 switch, you would have to use a vlan with a L3 vlan interface and the switchports connecting to the 2 routers would be in that vlan.

So it's a common thing but it's not always applicable.

Jon

Correct Answer by Reza Sharifi about 6 years 11 months ago

Hello Moses,

Connecting the 2 L-3 devices as a routed port is common.  This would be a point-to-point routed link and you normally use a /30 subnet to connect the 2 devices together.  Since this is a routed link, you do not need to configure switch port and since there is no VLAN involved you eliminate STP, BPDU, etc.

HTH

Reza

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Correct Answer
Reza Sharifi Thu, 12/17/2009 - 06:27

Hello Moses,

Connecting the 2 L-3 devices as a routed port is common.  This would be a point-to-point routed link and you normally use a /30 subnet to connect the 2 devices together.  Since this is a routed link, you do not need to configure switch port and since there is no VLAN involved you eliminate STP, BPDU, etc.

HTH

Reza

Correct Answer
Jon Marshall Thu, 12/17/2009 - 07:12

moses12315 wrote:

I have a question. I have a 4500 Series Switch which i am trying to connect it to a Router. An expert told me

that when i have to connect a L3 switch to a Router i should connect them through a link network subnet,

meaning configure on the Switch port (no switchport) and put an ip address. Also the same network on

the Fast Ehternet port of the Router.

Since he is an expert i have no doupts that he is right. But what i am going to gain from that? What actually is

the reason ? Why is better using this than just a normal connection with the Router? Do i need to do that?

Thanks a lot

Moses

Just to add to Reza's comments.

Given the choice i would agree with Reza that it is more common and personally i think better to use a routed link. However this is not a hard and fast rule. Lets say for example you have a L3 switch connecting to 2 routers and you wanted them all to be in the same subnet. With that requirement you could not use routed ports on the L3 switch, you would have to use a vlan with a L3 vlan interface and the switchports connecting to the 2 routers would be in that vlan.

So it's a common thing but it's not always applicable.

Jon

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