"routing" over layer 2 connection

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Jan 7th, 2003
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My company would like to experiment with a L2 wireless technology from Motorola called Canopy. Canopy creates high-speed, layer 2, point-to-point links in a metro are using antennas with line-of-sight connections to our buildings. We currently have routers, subnets and serial circuits connecting these buildings. I don't want to convert my network to one huge subnet. I would like to keep my subnets and replace the current T1 lines with DSL lines for backup. I need a routing solution that would use the layer 2 links under normal circumstances to connect subnets but could switch to using the DSL and VPN if the Canopy network went down.

I am thinking using a L3 capable switch somehow but I am not sure how.

Any suggestions?



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mark-obrien Wed, 01/08/2003 - 08:27
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You don't specify the type of physical interface that the Canopy presents to a router. However, you should be able to avoid making your network one huge subnet by assigning a network layer address to each Canopy link. This will make Canopy look the same, at layer three, as any point-to-point WAN link.

Good luck.


DIEGO ALONSO Wed, 01/08/2003 - 10:33
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Canopy devices have an ethernet interface on the LAN side but no IP address therefore I don't know how the router would know that the Canopy is down. I was thinking of having a subnet that would be used for all router ethernet interfaces connected to the Canopy network. This way, the next hop IP for the router would be on the other side of the wireless cloud. If the router can't talk to the "remote" IP then the wireless must be down and the router would switch to a higher cost route that would point to the DSL. The question here is: What event makes the router drop a route? The next hop is not physically connected to the router, its just an IP address on a layer 2, ethernet like segment. Will the router ping or ARP for that next hop IP to determine its up/down state?



mark-obrien Wed, 01/08/2003 - 10:50
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If you run a routing protocol over the Canopy connection, your router will establish a neighbor relationship with the router on the other side of the wireless cloud. If a period of time goes by (40 seconds, in the case of OSPF) during which your router doesn't receive a hello (again I'm using OSPF in this example) from the far end router, all routes learned from that router will be deleted.

I suggest that you contact the manufacturer of Canopy to see if Canopy is invisible to the routers. Specifically, will one Canopy unit pass broadcasts and multicasts over the wireless link to the other unit, and will this unit forward these packest on?


vcjones Wed, 01/08/2003 - 11:03
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The Canopy devices act as a remote bridge and are transparent to your routers. The cleanest way to deal with them is to dedicate a separate ethernet interface on each router to each Canopy device and configure a routing protocol across the Canopy link to detect outages (treating the links as an Ethernet with only the routers at each end attached and configuring an appropriate routing protocol to detect problems).

If you do not have extra Ethernet interfaces that can be dedicated, the problem gets much more difficult, as the Canopy's want to make your network one big LAN while common sense wants you to keep the WAN links separate and avoid having all your LAN broadcasts consuming WAN bandwidth. Using a VLAN capable switch at each site would be one approach, if your routers support trunking (generally requires a fast ethernet port on Cisco routers).

There are, however, many approaches. For example, you could use secondary addresses on all the routers to support a separate WAN backbone "LAN" but this does not help limit your real LAN's broadcast domain. Your posting also implies that you are not distinguishing between the remote units and the backhaul unit(s) nor do you mention if you are going through a service provider (in which case you also have to deal with how the service provider network is set up to support Canopy access).

Sounds like an interesting product. Be sure to post and let everyone know how it works out :-)

Good luck and have fun!

Vincent C Jones



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