layer 3 switching question

Netwurk

I recently bought a 3550 for my lab and I have it set up for inter-VLAN routing, ether-channel, and EIGRP. Everything works, but some of the books I've been reading say that you need to set up a physical interface as a layer 3 interface to get everything to work. I haven't done this.

Still it all works, and the EIGRP topology table fully populates just like it does on the routers.

I'm wondering if setting up a physical interface as layer 3 is something that is just recommended (as opposed to being necessary).

Anyone have any thoughts?

CCIE_2011

hey you're the CCIE - you tell us

icon_smile.gif

we're just lowly CCNP candidates

eeh eeeh... Since i am a CCIE i'll give you some hint :P

Just kinding dude


Layer 3 port!! I bid you mean routed ports. Well it is recommended because, they just converge much faster then trunk ports where STP and other things are running.
Again it is all depending on the approach of design you will be using. Some ppl use routed ports between core area and each switch block, while others , Like my company, uses trunk ports in all areas instead.

Netwurk

Well let me ask this - if I set up a layer 3 (routed) port, it has to connect to another router or to another switch's routed port to have any benefit. Right?

Netstudent

right...because routing protocols aren't going create an adjacency between a L3 port and a L2 port.

Netwurk

Netstudent wrote:

right...because routing protocols aren't going create an adjacency between a L3 port and a L2 port.

It's interesting that I have no L3 ports at all and it works.

The EIGRP table shows routers that are 4 hops away, so no downside so far.

The switch has no directly connected routers, it links up to the routers from other switches through trunking. At some point, I'll have to try hooking up a router directly to an L2 port and see what happens.

dtlokee

Technically a SVI is a "routed" port in that it is layer 3. You don't need a physical layer 3 interface (that is an interface with the "no switchport" command). If you are using SVI's make sure you have a trunk port or a physical L2 interface that belongs to the VLAN that the SVI is connected to. If there are no physical interfaces in the VLAN the SVI will remain in a shutdown state, the same as if the VLAN was not created on the switch.

Netwurk

dtlokee wrote:

Technically a SVI is a "routed" port in that it is layer 3. You don't need a physical layer 3 interface (that is an interface with the "no switchport" command). If you are using SVI's make sure you have a trunk port or a physical L2 interface that belongs to the VLAN that the SVI is connected to. If there are no physical interfaces in the VLAN the SVI will remain in a shutdown state, the same as if the VLAN was not created on the switch.

Makes sense. I figured the SVI was serving as my "L3 port".

Funny that the books tell you to make a physical one. Must be one of the those "best practice" things.

dtlokee

Creating a physical L3 interface is not going to be the best wayy if the goal is to provide inter-vlan routing, unless you are connecting them to another switch. Since a physical L3 interface is not part of a VLAN it can't provide a connection the thte VLAN for inter-VLAN routing. If the switch is connected to another switch (say to an interface in VLAN 10) then the switch ca act like a router for VLAN 10. Not really a best practice to do it like that, but it would work. As CCIE_2011 said you can use them as uplinks to the distribution/core if you do not want your L2 VLANs to extend beyond the local switch. Back in the days before Rapid PVST engineers would make the connections to the distribution/core routed because the routing protocols could detect and recover from failed links faster than the typical STP delay of 30-50 seconds.