Unnumbered VLAN interface routing on Layer 3 Switch

[jstevensunico]

Hi All,

I am looking for some advice or indication if this can be done or not.  A client with a critical industrial control system has 4 /30 subnets at 4 sites connected via BDSL routers to a carrier.  The BDSL routers occupy 1 IP address on the LAN side interface, with a single device holding the other.   They are upgrading all the devices at each end and require larger subnets at the sites so they can run the old and new systems in parallel.  Did I mention the control system is critical?  The carrier supplying the WAN service will not allow expansion of the 4 subnets beyond the /30.  So, we need to be able to interpose a device that allows the existing /30 subnet to exist without modification, and route new subnets at each site via these devices.

I would like to use a IE3000 layer 3 switch at each site configured with 2 VLANs.  One for the existing subnet, and one for the new subnet.  The quesiton becomes this.  If say vlanOLD interface is unnumbered, can I put a route to the other subnets via the BDSL router, or run say EIGRP on each router to advertise the routes?  This would allow the IE3000 to be interposed without any large interruption to the communcations while each site is re-addressed, and would keep the Old and New subets a little separated.

Obviously, it would be easier to just re-address the "local" device into the new subnet and put it's IP address onto the new subnet, and only use the old subnets to route between the sites.  However, given the geographic separation between the sites, and the criticalness and time sensitivity of the service, this is a skilled resource hungry task.

Thoughts, advice and suggestions greatly appreciated.

Cheers

[smitesh kharecha]

Hi John,

ip unnumbered can be only be used for Point-to-point (non-multi-access) interfaces only

Regards,

Smitesh

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[Richard Burts · Accepted solution]

Smitesh

It used to be true that ip unnumbered was supported only on serial point to point interfaces. But Cisco has extended the concept and now does support ip unnumbered on LAN and VLAN interfaces. See this link for details:

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/31sga/configuration/guide/unnumber.html#wp1073291

John

One thing that link does point out as limitations of ip unnumbered on lAN and VLAN interfaces is that dynamic routing protocols are not supported. So running EIGRP there would not work. I am not quite clear whether you could get it to work with static routes.

HTH

Rick

[Peter Paluch]

Hi Rick,

Thanks for pointing this out! I was not aware of this feature.

Best regards,

Peter

[smitesh kharecha]

Hi Rick,

Apologies, even I was not aware about the same.

Seems like it IOS which I tried to replicate is not supporting the same.

I tried it on 3750's on GNS with 12.4(15)T5

Thanks for the Info

Regards,

Smitesh

[jstevensunico]

Thanks Richard,

I will review the article and make a decision about it.  I would only be using static routes in this scenario, as it is simply using the carrier based VRF for point-to-mulit-point comms.  ie, each site's /30 subnet can see each other sites. As they are limited by the carrier to the /30 subnets, using an unnumbered interface with a couple of VLANs seemed like a good idea.  I guess one other way would be to use a router with a switch module in it, with a "WAN" ethernet interface as well.  That way I could IP the "WAN" Ethernet as the new Subnet gateway, and run the VLAN interface unnumbered with static routes to the other sites via the "old" Gateway IP address.

I will see what I can come up with to make this work.

Thanks all for suggestions and help.

Cheers