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venkatesh Gotur
Beginner

Migreting L2 to L3 with OSPF in LAN

Hi,

We have an existing network with a core 6500 as a VSS connecting 4 buildings with 4500 chassis under which number of L2 switches are connected. Currunlty we are using RSTP in ring for redundancy but we want to use OSPF in LAN for faster conversion.

All the VLAN's are created on 6500.

Attached is the Network Diagram.

How do we achive this.

Any suggestions will be a great help.

Thanks.

4 REPLIES 4
NES IT
Beginner

OSPF is a routing protocol, RSTP is a layer 2 redundancy protocol.

What exactly are you trying to do?

Dan

dominic.caron
Contributor

If I understand correctly, you want to change your topologie from L2 to L3. In the routed access model, a network is only present on one switch. If your network span multiple acces switch, you need to correct that.

If this part of the network is ok, you can start defining new vlan and build your ospf adjacency. (you also need to fine tune ospf)

Once done, you define a L3 interface in access vlan and transfer the actice HSRP to the access switch. Then close the access vlan interfaces on the 6500. You then remove the access vlan from the trunk between the 6500 and the 4500 leaving only the vlan you use with ospf.

In the end, you also need to convert your ospf vlans interface to L3 interface with the no switchport command.

It's a lot of work to get from 1 second to 200ms convergence time and you will have less flexibility in your network. I did it in one of my building and it did not scale well. There is always one application that is L2 only like SCADA (building control) and it's a pain to support in L3 access. (I did a rollback)

Hi Dominic,

Greetings....

Thanks for the responce.

Yes, network is present on only 6500 switch, 6500 is the VTP server from where VLAN's gets distributed.

**Once done, you define a L3 interface in access vlan and transfer the  actice HSRP to the access switch. Then close the access vlan interfaces  on the 6500. You then remove the access vlan from the trunk between the  6500 and the 4500 leaving only the vlan you use with ospf.**

Can you please elaborate.

Thanks once again.

.

Vivek Ganapathi
Enthusiast

Hi Venkatesh,

Below are the things you may need to do

1) Move all your Layer 3 VLANs to 4500's

2) You would need to build a Multichassis Etherchannel (MEC) between your VSS switches & the downstream 4500's.

3) Create a Layer 3 port-channel & run OSPF.

4) Enable Non-Stop Forwarding (NSF) within OSPF to statefully sync the routing table to the backup sup module.

4) You would require to tweak OSPF configurations to allow you to achieve sub-second convergence. You may require to tune the LSA Arrival, Pacing timers & run incremental SPF.

Hope this helps.

Regards

Vivek