L2 vs L3 links between Distribution switch and Core switch.

Answered Question
Apr 18th, 2010

Hi all,

        I have the setup which consists of two core and 4 distribution switches, and 4 distribution switches is connected with both core switches in redundant links.

First of all let me tell u what ever concepts i have, if wrong please correct me!

I have configured management vlan1 interface with 10.1.1.1/24 in core1 and 10.1.1.2/24 in core2 and my hsrp IP for vlan 1 is 10.1.1.100/24, which I have used as a gateway for DIS 1,2,3,4.

management IP for Dis1 ( 10.1.1.3) - Dis (10.1.1.4) - Hsrp IP for these distribution switch is 10.1.1.5 which i have used as a gateway in 1st two access switches.

I have configured vlan 20,30,40,50 in core 1(vtp server) and gave the ip 20.1.1.1/24(vlan 20), 30.1.1.1/24(vlan 30), 40.1.1.1/24(vlan 40), 50.1.1.1/24(vlan 50) in CORE1

&

ip 20.1.1.2/24(vlan 20), 30.1.1.2/24(vlan 30), 40.1.1.2/24(vlan 40), 50.1.1.2/24(vlan 50) in CORE2

&

HSRP IP for vlan 20 ( 20.1.1.3), vlan 30 ( 30.1.1.3), vlan 40 ( 40.1.1.3), vlan 50 ( 50.1.1.3). I have used these ip's as gateway for my pc in respective vlan's.

My concern is, If i run L3 links between distribution switch and core, what all changes I have to make?

If possible please help me with learning the diff between L2 and L3 links between L2 and L3 links between switches!!

Please advice - waiting for your valuable answers - need help!!

Thanks

I have this problem too.
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Correct Answer by Jon Marshall about 6 years 7 months ago

maddy_kavitha wrote:


This is a lot of work.

If you have L3 between the core and the distro switches then all the vlan info and L3 inter-vlan routing + HSRP needs to be migrated off the core to the distro switches.

However it's not that straightforward. At the moment because the links are L2 you can have the same vlan across all four distro switches. If you migrate to  L3 then you need to have 2 pairs of distro switches. Each pair of distro switches would be interconnected via either a L2 trunk or a L3 link. Either way you will not be able to have the same vlan on both pairs of switches because there is now no L2 adjacency between all 4 distro switches  unless you carefully plan your access-layer switch uplinks and use these as transit links but that would get very messy.

Basically you need to understand exactly how you would allocate vlans to distro switches before even starting to plan a migration otherwise you could do some serious damage.

Jon

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Correct Answer
Jon Marshall Sun, 04/18/2010 - 13:02

maddy_kavitha wrote:


This is a lot of work.

If you have L3 between the core and the distro switches then all the vlan info and L3 inter-vlan routing + HSRP needs to be migrated off the core to the distro switches.

However it's not that straightforward. At the moment because the links are L2 you can have the same vlan across all four distro switches. If you migrate to  L3 then you need to have 2 pairs of distro switches. Each pair of distro switches would be interconnected via either a L2 trunk or a L3 link. Either way you will not be able to have the same vlan on both pairs of switches because there is now no L2 adjacency between all 4 distro switches  unless you carefully plan your access-layer switch uplinks and use these as transit links but that would get very messy.

Basically you need to understand exactly how you would allocate vlans to distro switches before even starting to plan a migration otherwise you could do some serious damage.

Jon

maddy_kavitha Sun, 04/18/2010 - 13:09

Thank you so much bro!!

If i use L3 then what would be my gateway in dis and access switches?

Jon Marshall Sun, 04/18/2010 - 13:12

maddy_kavitha wrote:

Thank you so much bro!!

If i use L3 then what would be my gateway in dis and access switches?

Your gateways would be the HSRP addresses on your distro switches.

Jon

Reza Sharifi Sun, 04/18/2010 - 13:12

You would need to put your wokstations gateways on the distribution switches instead of the core

Reza Sharifi Sun, 04/18/2010 - 13:07

Hi Madhan,

Are your distribution layer switch capable of running layer-3?  If yes do you have the IOS with layer-3 capability/license installed?

If this is production do not use vlan 1 for managment use for example 10.

The benefit of running layer-3 between your distribution devices is that you do not have to worry about running spanning tree protocol since there is no loop in you network vs in a layer-2 design you would need to run STP. If you go with layer-3 then you need to deploy /30 between your distribution devices.

Overall, layer-3 is the prefered solution.

Also,in this case  HSRP would be deployed on distro switches.

HTH

Reza

maddy_kavitha Sun, 04/18/2010 - 13:16

hi reza,

         thanks for the info!! Yes DIS switches are capable of running L3. I want to make the routing for vlans to vlan's to happen in DIS and internet traffic only must cross CORE. how can i accomplish this pls ?

Reza Sharifi Sun, 04/18/2010 - 13:41

HI Madhan

The same way I explained in previous posts.  Right now, the default gateway for your workstations/printers, etc..is the core routers.  You would need to move it to the DIS and then run a layer-3 link between your DIS and between your DIS and the core devices using /30 subnets.  You also have to more the HSRP functionality to your DIS.  In this scenario all the inter vlan routing in done by your DIS.   If the traffic's destination is to Internet than that goes through your core.

HTH

Reza

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