For 6509, when to set a link as trunk and when as port-channel with a vlan?

[gwhuang5398]

Some 6509's in our network are Layer 3 switches and some are just Layer 2 switches. All 6509's are linked by 2GB or 4GB links. Some of those links are dot1q trunks and some are port-channels assigned to a Vlan. I know the link needs to be a trunk if between Layer 2 switches or between Layer 2 and Layer 3 switches, but I have seen links between Layer 3 switches as trunk in some cases, and in other cases, as port-channel assigned with a Vlan.

What's thr rule of thumb in dealing with this question? if any.

Thanks a lot

Gary

[Bobby Thekkekandam]

Hi Gary,

Let me first start off by clarifying one major point. When talking about a layer 2 versus a layer 3 switch, the only real distinction is whether that switch has routing capability. In terms of your question, one thing to keep in mind is that a layer 3 switch can still have layer 2 switchports.

So to answer your question, a link should be a trunk if it needs to carry multiple vlans, regardless of whether the switch is a pure layer 2 switch or one that has layer 3 capabilities.

The same applies to etherchannels-- If you need to carry traffic from multiple vlans between switches, then configuring that etherchannel as a trunk is the best way.

Hope that helps. Please rate helpful posts

-Bobby

[jphilope]

To add to Bobby's comments.

An Etherchannel, or port-channel is a means of binding several Ethernet ports together to increase bandwidth. Similar in concept to Multilink in a WAN circuit.

A trunk combines multiple VLANs over an Ethernet link. That Ethernet link can be a single Ethernet port or an Etherchannel.

You assign the VLAN to the port-channel (Etherchannel), not the other way around.

If the switches only support a single VLAN, you only need to assign the port(s) between the two switches to a single VLAN. You can use an Etherchannel or a single port to perform this link.

You need to trunk to a switch when the switch supports more than one VLAN and/or a VLAN is passed to a subordinate switch (downstream switch).

Notice I never spoke of a difference between layer 2 or 3 switches. It makes no difference. The only time you need to use a layer 3 switch is when you perform IVR (inter VLAN routing). That can be back at your core switch. In reality, you only need a single layer 3 switch (if you don't do the IVR in the router) in a switched network. You must trunk all the VLANs back to that switch to perform the IVR, however.

Hope this has been helpful.

Jim

[gwhuang5398]

I have two 6509 with MSFC (running hybrid mode). The link between the two is configured as a trunk and allows, for example vlan 100, 200, 300, 400, etc.

I like to enable OSPF between the two 6509 MSFC. Can I just use any allowed vlan on that trunk, for example vlan 300, and do "int vlan 300", "ip address ...." on the MSFC to set Layer 3 interfaces, and put that vlan interface into OSPF?

It works if there's only one vlan associated to that link, but when the link is a trunk with multiple vlans, I'm not sure the right way to make it a Layer 3 link for routing purpose.

Thansk

Gary

[ekiriakos]

Hi Gary,

Yes you can do that. The OSPF adjacency will be formed over that VLAN. Remember thought thet the routing engine will advertise networks that are configured under the ospf process. You may chose to do the following. Chose a vlan which will be purely used for ospf between the two devices and keep it dedicated to that (i.e no hosts on that VLAN. Then advertise other interface netwoks under the OSPF proces but make them passive so you don't have multiple adjavencies. Assuming you do not have other routers on those VLANs which need to exchange routing info with your L-3 switches.

Here is an example:

int vlan 300

desc OSPF interface

ip add 1.1.1.1 255.255.255.252

int vlan 200

desc User vlan

ip add 2.2.2.2 255.255.255.0

etc...

router ospf 1

network 1.1.1.1 0.0.0.0 area x

network 2.2.2.2 0.0.0.0 area x

passive-interface vlan 200

or you can use the passive-interface default and make

no passive-interface vlan 300.

HTH

E.

[nhedhili]

Hi,

if we have other routers in the same Vlan 200 connected to one chassis, do they form adjacency with the two MSFC or no ?

the second question, if I connect another L2 switch with a trunc link to one chassis than I connect a router to that switch in the vlan 200, does the router

form adjacency with the two chassis or no.

Thanks for help

[ekiriakos]

Hi,

A VLAN is a broadcast domain. OSPF routers communicate with each other using multicasts, which will be sent to all devices on that particular VLAN. Therefore in answer to your question, yes all routers will form an adjacency on that VLAN regardless if the routers connect to different switches. Note that in the earlier post-example vlan 200 was made passive, in which case the l-3 devices will not run OSPF on that interface.

To learn more about ospf here is a good doct.

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml

Rgds

E.