Solved: Question About L2 Trunks and Port Channels

mrashby · ‎07-23-2008

All,

In my studying I have been instructed that in order to pass Vlan traffic between 2 devices you need a L2 trunk and if you want to pass L3 data you need a routed link. I have also been instructed that L3 data doesn't pass over L2 trunks and vice versa. I was recently working with another engineer and he proposed that I can pass L3 and L2 data over the same link using a L2 port channel. Is this true? I am asking because it goes against what I have been taught. Can someone shed some light on this for me. Thanks.

Jon Marshall · ‎07-23-2008

Mario

I think it is the terminology that is a little confusing here.

If you connect your 2 switches with a L2 trunk then

1) You can pass vlan traffic across the trunk link. So vlan 10 on sw1 is the same as vlan 10 on sw2.

2) You can pass vlan traffic from one vlan to another ie.

vlan 10 on sw1

vlan 20 on sw2

As long as both vlans are allowed on the L2 trunk packets arriving on sw1 from vlan 10 can be routed on sw1 onto vlan 20 and then switched across the L2 trunk link to the destination in vlan 20.

Again this assumes that vlans 10 & 20 are the same on both switches.

3) Routing protocol information can pass along the L2 link ie vlan 10 on sw1 can form an EIGRP neighborship with vlan 10 on sw2.

All of the above is possible with a L2 link.

If you connect the 2 switches up with a L3 connection only then vlan 10 on sw1 is not the same vlan as vlan 10 on sw2 and you cannot use the same IP subnet for vlan 10 on both switches if you wanted the vlans to communicate.

You can run multiple links as you say but a single link cannot be both L2 and L3.

Hope this makes sense.

Jon

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lee.reade · ‎07-23-2008

Hi,

You are correct in saying that in order to pass multiple vlan traffic between 2 switches you require a trunk port, the trunk port operates a L2.

However all you would need to do is create a switch virtual interface (SVI) on each switch for a certain vlan, in order to pass L3 information across the L2 trunk.

This trunk passes data for a number of vlans, which you can specify which ones.

A L3 routed port is a direct connection between two switches, that is instead of being a L2 trunk connection, it is configured as a L3 routed port, via the "no switchport" and "ip address x.x.x.x" command.

HTH

LR

mrashby · ‎07-23-2008

Lee,

You have cleared up my basic quesiton but what I am getting at is I have 4 6509's 2 core switches and 2 distribution switches. I have to put a VLAN on my distribution switches. Now in order to pass VLAN traffic between those 2 distribution switches I have to have a L2 trunk between them. I also need a L3 connection between them to route. So what the other engineer was saying was instead of using 4 ports between the two switches-- 1L3 port and 1L2 port on both swtiches. That we can pass L2 and L3 traffic over a L2 port channel, thus utilizing only 2 ports between the two distribution switches, 1 port on Dist 1 and 1 port on Dist2. Is this true?

lee.reade · ‎07-23-2008

Hi,

If you are wanting to trunking user access vlans between the switches, then you want to have a L2 trunk between each switch, then on each switch you will create the SVI for a specific vlan and perform the L3 operations on that, (ip address/hsrp/dhcp/routing).

If you are just wanting a connection between two switches, ie you are not looking at having the same vlan on both switches, then you can just create a routed port, point to point link. This is normally done in the campus & data centre cores.

You can obvisouly channel two links together and then run either L2 or L3 across them.

HTH

LR

mrashby · ‎07-23-2008

Okay your last statement is what I am interested in. So I can "either" run L2 or L3 across the channel but not "both" L2 and L3 at the same time, correct?

Jon Marshall · ‎07-23-2008