I'm sorry for being basic in writing this question. I think the details have been covered though.

What's happening here is, two separate networks are being brought into an isolated area over a microwave radio connection. I may need to move the Layer 3 for these isolated devices to be on their side of the microwave connection (the far side).

How will I have a single Layer 3 switch on the far side of the microwave connection, be able to participate in OSPF areas for both segregated networks, and not break anything. The networks are merged at the Layer 2 switches that connect to the microwave at both ends. I use trunks and VLANs to re-separate the traffic here, but am lost about what to do with Layer 3/OSPF.

NET A  (10.142.x.x)                                                    NET A               
OSPF AREA 7                                                            OSPF AREA 7
                       L2 SWITCH (to) MICROWAVE RADIO (to) L2 SWITCH 
NET B  (10.143.x.x)                                                    NET B
OSPF AREA 107                                                          OSPF AREA 107

1 Answer 1


Just remember that OSPF areas are assigned by interface, and OSPF areas, like 7 and 107 that you show, cannot directly exchange routes. The separate VLANs will use separate SVI interfaces, and each interface can be in a different OSPF area. You could have the layer-3 switch be in both areas without any problem.

You can also have separate OSPF processes if you want, but that seems unnecessary since you have different area numbers. Separate OSPF processes will allow you to have the same area number without mixing the two separate areas.

  • from what you say it sounds pretty clear, but I wonder about how the OSPF area will know which interface/Vlan/trunk to take to the rest of the network.
    – skrap3e
    Feb 11, 2016 at 4:17
  • OSPF areas are assigned by interface. An OSPF area will use the networks for the interfaces which are assigned to that area, or you can explicitly redistribute (e.g. connected networks) into OSPF. Alternatively, you could have a separate, third OSPF area for this site and create a virtual link back to Area 0. It all depends on your overall network design and requirements.
    – Ron Maupin
    Feb 11, 2016 at 4:23
  • what about the layer 2 switch, will that be an issue with extending the OSPF area to a new layer 3 gateway (basically a new network part of the same area). Will the LSA have an issue processing through a layer two switch?
    – skrap3e
    Feb 11, 2016 at 5:25
  • LSAs are in IP packets, a layer-3 construct, which are carried as the payload of layer-2 frames. Layer-2 switches don't care about the payload of layer-2 frames, they only switch the frames based on the destination MAC address in the frame header, and VLANs are further separated by the 802.1Q tag in the frame header.
    – Ron Maupin
    Feb 11, 2016 at 5:41

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