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Difference between SVI & Root Ports and use there·of

Posted on 2010-09-23
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Morning Experts !

I have been doing some reasearch and have struggled to pick out a satisfying explanaton on the appropriate or best practice use of SVI vs Router ports on multilayer switches.

From what I gather there are much the same in function but have different ramifications once in use.

SVI are used on L2 designs and STP extends to downstream switches

Routed ports are easier to manage filtering and ACLs.

But my real question is what are the situations and scenarios would/should they be used in.

For example would you use one over the other depending on what device you were connection to.

like Switch<----------->Switch    &  Switch<------Router------->Switch

would it depend on the design locations (core,distribution,access)


Please provide some scenarios where you would use one or the other and the reasons

I have attached an image to give an oportunity of clarification of any forthcoming explanations.

But please don't let this limit you to telling me what you would do in just this scenario.

Many thanks
T4K

 example Scenario
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Question by:Thirst4Knowledge
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by:
rfc1180 earned 500 total points
ID: 33743243
>SVI are used on L2 designs and STP extends to downstream switches

No, an SVI represents a logical Layer 3 interface on a switch

A routed port is a physical interface (i.e GigabitEthernet0/1) the one has been converted from a switchport on a switch to a layer 3 interface called a routed interface/port, or two is a physical interface on a router. Yes, typically, managing ACLs on layer 3 interfaces does tend to become easier over lets say Vlan access lists.

>But my real question is what are the situations and scenarios would/should they be used in.

Good question, and that depends on several things; your network design and application. Typically,
typically, where do you use them (SVIs), how do you use them, and when you use them.

Where:
Typically at the distribution and sometimes at the access layer depending on the network design (Service Provider vs Enterprise).

How do you use them:

Routing Features
Security Features
Quality-of-Service (QoS) Features

When: anytime you need layer 3 configuration on a VLAN. (Typically called inter-vlan routing)

I am not going to discuss all scenarios, but one; routing.

Take your question: "would it depend on the design locations (core,distribution,access)" Yes, typically you core is used for fast routing/switching of packets an no SVIs should be used here, all connectivity should be routed interfaces. The distribution layer is where you typically use SVIs at; As you create VLANs that extend down to the access layer, the hosts need a gateway, and an SVI can provide this from a layer 3 perspective and also allowing the vlan to extend to the host to to the access layer.

more information:

http://www.cisco.com/en/US/prod/collateral/routers/ps5853/prod_white_paper0900aecd8064c9f4.html
http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/37sg/configuration/guides/l3_int.html
http://www.ciscopress.com/articles/article.asp?p=358549&seqNum=4
http://en.wikipedia.org/wiki/Switch_virtual_interface

Billy
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Author Comment

by:Thirst4Knowledge
ID: 33743317
So SVI are just logical constructs that offer layer 3 functionality on Layer 2 LAN enviroments while routed ports give you L3 functionality depending on where and how they connect ?
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by:rfc1180
rfc1180 earned 500 total points
ID: 33743384
>So SVI are just logical constructs that offer layer 3 functionality on Layer 2 LAN enviroments while routed ports give you L3 functionality depending on where and how they connect ?

that is correct
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Expert Comment

by:Don Johnston
ID: 33744430
Here's the way I explain it:

A multilayer switch is a Layer 2 switch with a router inside. But the router has no connection with the switch. There are two ways to connect the router to the switch.

1) SVI (the "VLAN" interface). This creates an interface on the router which can communicate with all devices in that vlan.
2) Routed (or layer 3 port). This extends the router so that it is physically connected to that particular interface.

When would you use one rather than the other?

If the router needs to communicate with more than one device on the same network, use an SVI.

If the router only has to communicate with one device on a network, you can use either (I prefer the routed interface).

Hope this helps.
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Author Closing Comment

by:Thirst4Knowledge
ID: 33744523
Makes sense
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Author Comment

by:Thirst4Knowledge
ID: 33744544
so if one side of the switch has device/hosts and the other side connects to a real physical router I would join the (non-switchport) interface to the router
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Expert Comment

by:Don Johnston
ID: 33745018
You could... I would.

But you could also create a VLAN, put that port in the VLAN and create an SVI for that VLAN.

-don
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Author Comment

by:Thirst4Knowledge
ID: 33745215
I think in keeping with a modular design school of thought I will go for the routed ports ;P
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