VLAN access across a routed port

Answered Question
Apr 23rd, 2012

Hello all. The scenario is this: I have an old Nortel network with a bunch of servers attached. Connected to it is the new Cisco core, by way of a routed port. My task is to migrate servers over to the Cisco side of the network, with minimal downtime, and have full network connectivity, retain IP addresses/remain on the same subnet, and retire the Nortels. The Nortels are running VRRP, so I can fail the gateway over by becoming part of that group and later dropping the Nortels, but I can't seem to get a host on the Cisco side to participate in the original subnet. The routed port kills VLAN traffic, so I tried bridging the VLAN with the routed port, to no avail.


Any help would be appreciated!

I have this problem too.
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Correct Answer by dancicioiu about 1 year 11 months ago

Ok then.

Use another port from your Cisco core as trunk for the vlans configured on Nortel.

Connect the port between Cisco and Nortel.

Configure SVIs on the Cisco core for the vlans that should be moved from Nortel -> Cisco ( but the one that is used as routed port , if this has also servers, could be used like this ) , but in shutdown, with VRRP.

Activate each SVI at a time , and check the server behaviour.

I expect :

In both directions the traffic will go to the Cisco Core :

        - To the servers : CIsco will have the SVI configured and UP => directly connected

        - From the servers : Cisco will be the VRRP active.

Dan

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Edison Ortiz Mon, 04/23/2012 - 08:23

You need to change the routed port to a switchport.

If the link requires multiple Vlans, the port must be configured as trunk.

If the link requires just one Vlan, the port can be configured as access.

cco@monitronics.com Mon, 04/23/2012 - 08:28

I'd love to, but I was trying to get around this without making that change.  The network is really sensitive to outages, and I was trying to accomplish this migration with minimal downtime.  Is there no other way to have the same VLAN on both sides of a routed port?  It seems like I would be able to bridge the VLAN on the cisco side, with no SVI, to the routed port, and wind up on the same segment as the Nortel side. 

Ultimately I may have to make switch to trunked links, but I was hoping for some alternative.  Thanks for your reply.

Edison Ortiz Mon, 04/23/2012 - 08:35

No, you can't do as proposed.

By changing the port configuration from routed to trunk, you are 'bridging' the L2 domain between switches.

The method you want to implement is not supported.

dancicioiu Mon, 04/23/2012 - 08:39

Hi Derek, Edison,

Your Cisco L3 port and the Nortel Server SVI are in the same IP class ? Or is just an interconnect prefix to Nortel ?

If not , you can

            - use another port from your new Cisco core, as trunk

            - configure on cisco the new vlans and SVI but in shutdown mode + VRRP high priority

            - when you are prepered no shut on SVI on cisco

Dan

cco@monitronics.com Mon, 04/23/2012 - 08:55

Dan,

Yes, the routed port is in the same subnet as the SVI on the Nortel.  Furthermore, there are many different VLAN's and SVI's on the Nortel, and the routed port only has an IP from one of the subnets.

dancicioiu Mon, 04/23/2012 - 09:04

You also use this routed port in order to route toward the Nortel network ?

Dan

cco@monitronics.com Mon, 04/23/2012 - 09:07

Yes, that is correct.  There is actually another switch in-between that is configured with access ports and the same VLAN as the one I've been trying to light up on the Cisco side.  Everything on the Nortel side is default routed to the routed port on the Cisco side.

It's a somewhat baffling setup to be honest.  I've inherited it, and have been trying to figure out how to migrate everything to Cisco without downtime.  Not sure how I would move the other VLAN's though without a trunk...

Correct Answer
dancicioiu Mon, 04/23/2012 - 09:17

Ok then.

Use another port from your Cisco core as trunk for the vlans configured on Nortel.

Connect the port between Cisco and Nortel.

Configure SVIs on the Cisco core for the vlans that should be moved from Nortel -> Cisco ( but the one that is used as routed port , if this has also servers, could be used like this ) , but in shutdown, with VRRP.

Activate each SVI at a time , and check the server behaviour.

I expect :

In both directions the traffic will go to the Cisco Core :

        - To the servers : CIsco will have the SVI configured and UP => directly connected

        - From the servers : Cisco will be the VRRP active.

Dan

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Posted April 23, 2012 at 7:08 AM
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Tags: bridge, vlan, routed
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