Solved: layer 3 interface

suthomas1 · ‎03-05-2009

what is the difference between a proper L3 interface on an L3 switch & a switched virtual Lan interface on L3?

which situations would justify either of them.

Thanks!

Joseph W. Doherty · ‎03-05-2009

Yes, having the two links terminate on different modules would avoid failure of one module dropping both links.

If you go with SVIs (assuming you have spare ports), you would have two SVIs, each defined with two ports, one SVI port (of each SVI) defined on each module. Each external link to it's own SVI but initially not on the same module.

i.e.

SVI A - ports mod1/1 mod2/1

SVI B - ports mod1/2 mod2/2

Link X connects to mod1/1

Link Y connects to mod2/2

Routing shouldn't be impacted.

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Joseph W. Doherty · ‎03-05-2009

A "proper" L3 interface would only have one port on the L3 subnet. A SVI could have many ports on the L3 subnet. When supporting multiple hosts on a subnet, you would use a SVI. If the port connects to another router, you might just use "proper" dedicated L3 ports.

A SVI can also only have one defined port, then it generally behaves much like your "proper" port. This being so, why defined a port to only support one L3 subnet? Precludes the mistake of defining multiple ports to a subnet. The big advantage of a single port subnet, when the port is down, the device considers the subnet unreachable.

suthomas1 · ‎03-05-2009

ok..lets say if i have 2 different PE-CE link terminating on a L3 switch. Would it be advisable to terminate it on L3 as 2 different routed ports or create SVI's for both?

Joseph W. Doherty · ‎03-05-2009

Since CE-PE link is likely router-to-router and p-2-p, most would prefer defining the ports as routed.

However, if your L3 switch has multiple blades or multiple stack members, if you define two ports to the SVI, each on a different component, it provides the option to quickly repatch if the primary component fails without needing to touch the config.

suthomas1 · ‎03-05-2009

Oh ok..thats a great piece of info..yes the layer 3 switch has 2 modules, so u mean both links should be terminated on each of the seperate modules to have sort of resilience in case one of them goes down?

And then it has to be on 2 different SVI's on the 2 modules correct?would this affect the routing that would be run on the switch?