Configuring Layer 3 link between multilayer switch

Answered Question
May 29th, 2008

hello

I am planning to connected catalyst 6500 switch with MSFC card through Layer 3 link. Can I use the vlan created in switch to make Switched VLAN interface (SVI) or I have to create the vlan in routing module (MSFC) of the switch..

Please help ! ! !

subharose

I have this problem too.
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Correct Answer by Edison Ortiz about 8 years 6 months ago

Let's start by saying you won't have L3 links in a Hybrid configuration. Your inter-switch connection will be Layer2 due to the limitation in the CatOS portion of the config.

You can have a /30 subnet between the switches while still carrying L2 Vlans for other uses.

If you want to limit the traffic just for that /30 subnet, then use an access-vlan method.

HTH,

__

Edison.

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Edison Ortiz Thu, 05/29/2008 - 06:34

It sounds like you are running in hybrid mode. The Vlans configured in the Layer2 portion (CatOS) will work in conjunction with the Layer3 portion (MSFC IOS), thus you can create SVIs.

HTH,

__

Edison.

subharojdahal Thu, 05/29/2008 - 06:48

Thanks Edision

Let me extend my question little further. switches are connected like A <-->B <--->C <---> D. Each of having MSFC card in SUP ( Hybrid mode).

When I issue show cdp neighbor in routing module of swiches B, C, D, E, that shows local interface on neighbor device is in vlan 3. But when I issue the same command in router A it doesn't show anythings ( considering cdp enabled.)

I didn't understand the local interface on specific vlan ?? what does that mean ? I check the interface of MSFC--> 15/1 is not in any vlan ( i.e I didn't see any native vlan assignment for 15/1)

Please help

Edison Ortiz Thu, 05/29/2008 - 06:58

In Hybrid mode, CDP Neighbor is only displayed in the CatOS portion not in the IOS portion.

In Hybrid mode, a port is created between the MSFC and the Supervisor for reachability. That port is 15/1 and you can also see 16/1 if running dual Supervisors.

In the CDP output, the local interface represents the egress interface the CDP packets are originating the packets from.

In the same output, the last column represents the interface from the remote device.

HTH,

__

Edison.

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subharojdahal Thu, 05/29/2008 - 08:19

Edison

Thanks a lot.

If I want to add one port lets say 3/12 in the SVI then do I need to make it trunk or access.

Lets say I have created a vlan 2 and create SVI for VLAN 2 to connected A <--> B by Layer 3 trunk. And I want to use port 3/12 for SVI, then what should I do for port 3/12.

i) access port for vlan 3

or

ii) make it trunk and assign native vlan as 3

or

iii) make it trunk and allow only vlan 3.

Please help

subharose

Edison Ortiz Thu, 05/29/2008 - 08:42

That's a design decision.

Just understand the differences; if you are planning to carry more than one Vlan over this link, make it a trunk port. If you are planning to carry just one Vlan, make it an access port.

Personally, I prefer option (ii) as it gives you the ability to forward other Vlans via this link while keeping Vlan 3 traffic untagged (similar to an access port behavior).

HTH,

__

Edison.

subharojdahal Thu, 05/29/2008 - 10:04

Edision

I understand what you saying.

But in my case, I want the layer 3 link between two CatOS + MSFC running switch in /30 subnet that terminates all the L2 frames and forward traffic on the basis of EIGRP routing protocols.

So, you still recommend me to make trunk and assign native vlan to same vlan having /30 block of IP or You could see other possibilities ! !

subharose

Correct Answer
Edison Ortiz Thu, 05/29/2008 - 10:11

Let's start by saying you won't have L3 links in a Hybrid configuration. Your inter-switch connection will be Layer2 due to the limitation in the CatOS portion of the config.

You can have a /30 subnet between the switches while still carrying L2 Vlans for other uses.

If you want to limit the traffic just for that /30 subnet, then use an access-vlan method.

HTH,

__

Edison.

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