Edge Switch design/configuration issue

Unanswered Question
Nov 19th, 2007

Hello All,

What is the recommended way of configuring the switch in the following scenario ?

Internal N/w--- > 3560/3750 switch --> ISP router

Options

1.configure a vlan SVI with WAN ip address provided by ISP

2.Configure a routed port and assign the IP address.

What parameters are considered to use one of the these options?

Thanks

Padmanabhan

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andrew.butterworth Mon, 11/19/2007 - 15:40

Personally I would use a routed port every time. This removes STP from the equation so the port will not have to wait for any STP timers. If you have any VLAN trunks you won't have to prune off the VLAN and you can't accidentally put an access port in the same VLAN (since it isn't a VLAN).

It's your choice at the end of the day and you should configure what you are happiest with.

Andy

padramas Tue, 11/20/2007 - 09:12

Hello Andy ,

Thanks for your response.

Can you explain more about how STP is going to affect the SVI ( which is a layer 3 interface) ?

From my understanding do you mean for SVI to be active, any associated ports of that vlan needs to be physically active which is influenced by STP timers?

andrew.butterworth Tue, 11/20/2007 - 09:46

If you create a VLAN then there is by default an STP instance created (or if you are using MST then it will be in a region). If any layer-2 ports (access or trunks) that are configured for this VLAN go up or down then there will be an STP event, potentially if the layer-2 topology isn't stable then you will have network instability.

The delays are associated with the STP states - listening, learning, forwarding & blocking. You can of course minimise these delays with the use of port-fast.

If you use a routed interface these issues are removed.

HTH

Andy

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