This is a design I need a few weeks ago to help with a redundancy issue. Currently we have a client that occupies two buildings separated by about 500 hundred feet. Soon they will start construction to add a structure right in the middle , connecting the two buildings. But guess what runs right in the middle of this area? The fiber connecting the two buildings. We are thinking that the construction will most certainly cut the fiber causing an outage, whether planned or not.
We decided to have a backup wireless bridge link to help with redundancy. Ruckus’s P300 AC bridges works great, and that is what we decided to do .
Currently the link between the buildings is a Layer2 Trunk, and we are routing over Vlan 254 which traverses the trunk. OSPF is used to advertise each building’s local subnets, and redistribute the default route.
The goal is that routing/layer 2 will only come active on the wireless bridge in case of a failure in the Fiber connections. So Spanning-tree will block all vlans other than the native 200 – going through the bridge. If there is a failure, those vlans will come online over the bridge, routing will come up, and all should work great.
The switching/routing that is used is a Nexus 9500 and 3850 stack.
To accomplish the above, we enable OSPF on vlan 254, and make sure all routing is correct – including redistribution. Vlan 254 our routing vlan is allowed along with a few other vlans – At some point this will be fixed and we will only route over this link, but for now we have to stretch (I know, not the best practice). Building 1 is currently the STP root for all vlans stretching over layer 2 link. The Spanning-tree path cost is increased on links connecting to the bridge, and special commands are enabled on the wireless bridge to disable the Ruckus loop detection mechanism. This is very important because it will stop STP from flowing by default. Follow this link to help with that:
After the bridge was setup, path cost modified, and Cisco port configs set correctly – it is time to test. First we needed to make sure STP was indeed blocking the vlans that were needed. Yes! STP is blocking the redundant path.
Testing/Results
We tested fail over in two ways. 1 – just shutting down fiber links in CLI, and 2- physically unplugging the links. During fail over we saw that 2 pings were lost and then they were back up. I actually thought that OSPF drop, and then re-converge, but that did not happen. Instead, since the Hello-Dead timers were never reached, OSPF never dropped – fail over time was much better than I expected. The only way I could really tell we had failed over was a small increase in latency, and of course we were limited to around 300 Mbits.
Some notes on this – Make 100% sure that the Ruckus loop detection is disabled before even starting actual bridge configuration. Also create some kind of alerts VIA Prtg/Solarwinds/Cacti to send an alert if links go down, or their is a big increase in bandwidth on wireless bridges.
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