Basic Question About IP Links

Answered Question
Jun 28th, 2007

Hi All,

Can anybody explain to me why you would configure a L2 switchport as a L3 interface and put an IP address on it?

I have these at my new company, mainly on links between 3750G switches and 6500 switches.

Cheers,

Dan

I have this problem too.
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Correct Answer by Amit Singh about 9 years 5 months ago

Dan,

There is a difference the way both the connection works. Although they will have the same purpose but there is the difference the way the switch ports will work.

In case of a SVI you will have eventually a Layer-2 link between the switches. This will run your normal STP and other control traffic between the switches.This will extend your STP domain from a switch to the other switch.The ports will go thorugh the normal STP states and in case of a link flap or link going dowm/coming back the recovery time will be a little high.

In case of a routed port between the switches,you will have a layer 3 link between the switches and will work as a normal router port. There will no STP running on the ports and the STP domain will not be extended beyond the downstream switches.Applring layer3 features like ACL's,PBR's will be a ltille easy in this case.

The Disadvantage of running routed ports is that each port will be a separate network and you will have to manage a large number of IP subnets on the network. Running a routing protocol will be a good idea in this case.

My suggestion for running a point-point link between 2 layer3 device would always be a layer-3 link. But this also depends on the kind of network topology you have.

HTH,Please rate if it does.

-amit singh

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Edison Ortiz Thu, 06/28/2007 - 06:52

Usually to avoid spanning-tree issues.

Also, if you have redundant connections, you will fully utilize both connections since one port won't be blocking while the other is forwarding, if etherchanneling is not an option.

abdel_n Thu, 06/28/2007 - 08:21

Hi,

SVI serves as a gateway for a multiple devices within the same VLAN (sub-network) connected to the switch respectively through multiple switch ports and this facilitate inter-VLAN routing.

In contrast, a routed port is not associated to any VLAN (as with SVI or access ports), it behaves like regular router interface except that it doesn?t support VLAN sub-interfaces.

So it can provide a layer3 path into the switch for a number of devices on a specific subnet (accessible from a single physical switch port).

I guess in you case those main links are used to aggregate traffic to the distribution/core switch.

I hope this will help,

Abdel

daniel.bowen Thu, 06/28/2007 - 08:32

Thanks for that.

What I dont understand is why you wouldnt just configure a VLAN interface and use that to facilitate the routing for switchports. I dont understand the need for a "physical" interface for Layer3.

Cheers,

Dan

Correct Answer
Amit Singh Thu, 06/28/2007 - 08:45

Dan,

There is a difference the way both the connection works. Although they will have the same purpose but there is the difference the way the switch ports will work.

In case of a SVI you will have eventually a Layer-2 link between the switches. This will run your normal STP and other control traffic between the switches.This will extend your STP domain from a switch to the other switch.The ports will go thorugh the normal STP states and in case of a link flap or link going dowm/coming back the recovery time will be a little high.

In case of a routed port between the switches,you will have a layer 3 link between the switches and will work as a normal router port. There will no STP running on the ports and the STP domain will not be extended beyond the downstream switches.Applring layer3 features like ACL's,PBR's will be a ltille easy in this case.

The Disadvantage of running routed ports is that each port will be a separate network and you will have to manage a large number of IP subnets on the network. Running a routing protocol will be a good idea in this case.

My suggestion for running a point-point link between 2 layer3 device would always be a layer-3 link. But this also depends on the kind of network topology you have.

HTH,Please rate if it does.

-amit singh

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