On 2/26/14 6:25 AM, Paul McCall wrote:
Scott,
Thanks for those suggestions. I understand and agree with what you are saying.
Eventually, we would move each one to its own BH PTP for all the reasons you mention.
Each one of the "fed" towers in our network has another solid path in the
network, so losing an AP feeding it would not really bring down the sub-towers if you
will. BW has not been a bottleneck yet. Frequency planning to do all the PTPs off the
one tower (one tower feeds 5 others) has proved to be challenging with GPS sync. That's
been a factor in wanting to tackle that before we are ready to deal with all that.
I will look at all these options and report later.
Thanks!
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Scott Reed
Sent: Wednesday, February 26, 2014 6:41 AM
To: Mikrotik discussions
Subject: Re: [Mikrotik] OSPF "problem" situation #1
From my experience and knowledge of OSPF (I am not an expert, but have read a
lot and have over 700 devices on the network running OSPF) this is to be
expected. There is no OSPF cost difference in the route to the AP and to the
other tower. From Tower A the cost to the AP is the same as the cost to the
other tower.
You may be able to "fix" it by disabling inter-client communication on the AP
wireless interface.
You may be able to "fix" it by creating a virtual interface on the AP wireless
interface and using a different subnet on that virtual interface. Then the cost from
remote tower to AP is less than the cost from remote tower to remote tower.
You may also "fix" it by setting the router IDs so that the AP gets elected as
the Designated Router. I don't recall if MT uses the highest or lowest ID, but one of
them has preference as DR.
Honestly, my opinion is the best fix is to use two PTP links to get to the 2
towers and put them on separate subnets. Since all the traffic for both towers
goes in and out one interface, there is a bandwidth limitation for each tower.
Troubleshooting gets easier with separation. A single radio failure only takes
down one tower instead of
2 towers. I realize there are lots of valid reasons to share the AP, but one
of my network design criteria is to not share backhaul links between towers.
On 2/26/2014 6:21 AM, Paul McCall wrote:
This network is routed. We have this scenario a few places in our network but
this is the only section not working.
A Single AP (single interface) connects to SMs at multiple towers to be their
primary (lowest cost) path to the internet. All on the same subnet. We have
as many as 5 towers fed like this from a single AP/single subnet/single OSPF
network.
This particular AP feeds 2 towers. One of the "fed" towers works as expected, always going to the
"AP" tower. The other "fed" tower jumps to its "brother" instead of the AP tower.
In this situation, do you have client isolation enabled on the AP?
Scott Reed suggested that but I'm not sure if you said one way or another.
I believe that the best way for this to work, with multiple links off
one radio, all in the same subnet, is to enable client isolation on the
AP, then use PTMP for the network type. PTMP is designed for networks
where everyone needs to talk to one designated router, but cannot or
should not speak directly to the other devices on the same media. It
doesn't save you any resources to have Tower B send packets directly to
Tower C. The traffic still crosses the AP twice.
NBMA is more for where everyone is supposed to talk to more than one
other designated router. You could probably make NBMA work, by limiting
the NBMA neighbors you specify on the client towers.
In a PTMP OSPF network, Tower A would build an adjacency with Tower B
and Tower C. Tower B and Tower C would never speak directly to one
another. With client isolation they should never know the other existed.
Going forward, you may want to switch to using a /30 per tower link.
You wouldn't need any virtual APs or anything. MikroTik will happily
speak OSPF PTP on multiple networks on one interface. That is something
I've taken advantage of when changing routers at key locations. It
would not work with a Cisco in the mix.
Then you can run OSPF PTP and when you switch to a dedicated link to one
tower or another, you the association should just switch to the new
interface when you move the IP address for that /30 to the new
interface, assuming the OSPF interface is already created. But that is
not necessary. You can deal with dedicated links when they happen.
I've not run PTMP on my wireless links, so there may be some bugs of
which I am unaware. If it give you fits, I would switch to PTP and see
if that stabilizes things. With just two links on the AP you don't
really save IP space having a /29 vs two /30s. If you have 3 links, you
can conserve some IP space.
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