Distribution to Core connection via Layer 3 question

Unanswered Question
Aug 27th, 2008

I want to connect the distribution switch to core switch via Layer 3. The distribution switch has 6-7 VLANS and I don't want to bring the VLAN to the core switch. How I can configure both distribution and core switch for layer 3 connection, should I configure IP address on physical interface or on VLAN1? If anybody can help with the example.

Thanks

I have this problem too.
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sachinraja Wed, 08/27/2008 - 23:00

Hello Rahman

you need to configure the port directly as a Layer 3 port !! You dont need to configure VLANs on these ports... you can use a "no switchport" command and disable layer 2 access on that port..

sample configuration :

interface gig5/1

description **** Connectivity to Core *****

no switchport

ip address x.x.x.x 255.255.255.252

no cdp enable

no shutdown

You can configure static/dynamic routing over this interface to enable routing on this interface.

Hope this helps.. rate replies if found useful..

Raj

AJAZ NAWAZ Thu, 08/28/2008 - 03:39

Personally I would recommend using L3 SVI. Changing a switchport to L3 is restricting. The SVI method gives best of both worlds. Sometimes later perhaps the business might have a requirement for L2 span - this is where the SVI and trunk will place you in pole position to make the change.

2cents

Ajaz

mrahman0302 Thu, 08/28/2008 - 09:51

Hi,

Thanks for your reply. So, do I have to create L3 SVI interface on distribution switch and core switch for each vlan, like for vlan 100

on distribution switch:

10.1.1.2 255.255.255.252

On Core Switch :

10.1.1.1 255.255.255.252

For vlan 200

on distribution switch:

10.1.2.2 255.255.255.252

On Core Switch :

10.1.2.1 255.255.255.252

Thanks

AJAZ NAWAZ Thu, 08/28/2008 - 10:07

Yes. The point I was making is to avoid using the 'no switchport' command as it is restrictive. Just make sure the L2 is not extended unintentionally. You can do this by keeping each IP subnet (/30), in it's own vlan. Further, prune your vlans on any trunks (if there are any), this is spanning-tree best practice anyway.

With an SVI, you have the freedom to add further L2 or L3 networks on that interface. It's advantageous in my experience to have this flexibility and applies for any L3 switch-to-switch connection.

If you need more clarification - just don't hesitate to get back to us.

Ajaz

mrahman0302 Wed, 09/24/2008 - 00:02

Hi Ajaz,

Thanks for your expert opinion. From Distribution to Core there will be 10G fibre uplink connection. Also I will use LMS and I want to push the same config on the remaining 14 distribution switch. So I need to make unique configuration for all distribution switches. So If I create SVI on distribution switch and core switch what will be the default gateway on the distribution switch? Also I have 8 server switch stack which is connected to Core via 10G fibre uplink. The connection between the servers and the server switch is via layer2 and server switch to core will be layer3. I will create the layer 2 and layer 3 interface on Server switch and configure the default gateway pointing to SVI interface address of Core switch. Please give your expert opinion on this.

Thanks

cowetacoit Wed, 09/24/2008 - 04:31

If you decide to go with the SVI on the distribution for a routed vlan the link between the core and dist will still be in layer 2, and you will not be able to take full advantage of dynamic routing protocols. Also you will have to rely on spanning-tree for redundancy. Do you have dual fiber links connecting the core and dist? If so i would NOT go with layer 2. Protocols such as EIGRP can give you equal cost load balancing and redundancy. anawaz has a good suggestions, its really what kind of physical links you have in my opinion.

mrahman0302 Wed, 09/24/2008 - 07:54

Hi Michael,

I do have dual fiber link in between distribution and core. I will use OSPF for routing.

Thanks

cowetacoit Wed, 09/24/2008 - 08:07

I don't have a lot of experience with OSPF. Eigrp will load balance across those links though. If you have l2 links then you are stuck with Spanning-tree. With p2p routed links there is no wait time for convergence and I'm pretty sure OSPF will load balance on your dual uplinks between dist and core.

glen.grant Wed, 09/24/2008 - 08:03

If you decide to use SVI's as opposed to a routed port , yes that is what you would do .

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