Backhaul Algorithm; Passive Beaconing (Anti-Stranding) - Cisco aironet 1522 Design And Deployment Manual

5.
Note

Backhaul Algorithm

A backhaul is used to create only the wireless connection between mesh access points.
The backhaul interface by default is 802.11a. You cannot change the backhaul interface to 802.11b/g.
The 24 Mb/s data rate is selected by default for AP1520s.
The backhaul algorithm has been designed to fight against stranded mesh access point conditions. This
also adds a high-level of resiliency for each mesh node.
The algorithm can be summarized as follows:
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Keeping the roles of mesh nodes distinct using the above algorithm greatly helps avoid a mesh access
point from being in an unknown state and becoming stranded in a live network.

Passive Beaconing (Anti-Stranding)

When enabled, passive beaconing allows a stranded mesh access point to broadcast its debug messages
over-the-air using a 802.11b/g radio. A neighboring mesh access point that is listening to the stranded
mesh access point and has a connection to a controller, can pass those messages to the controller over
CAPWAP. Passive beaconing prevents a mesh access point that has no wired connection from being
stranded.
Debug logs can also be sent as distress beacons on a non-backhaul radio so that a neighboring mesh
access point can be dedicated to listen for the beacons.
Cisco Aironet 1520, 1130, 1240 Series Wireless Mesh Access Points, Design and Deployment Guide, Release 6.0
OL-20213-01
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Cisco recommends a Fast Ethernet connection between the MAP and controller when changing
from a MAP to RAP. After a RAP-to-MAP conversion, the MAP's connection to the controller
is a wireless backhaul rather than a Fast Ethernet connection. It is the responsibility of the user
to ensure that the Fast Ethernet connection of the RAP being converted is disconnected before
the MAP starts up so that the MAP can join over the air.
MAP always sets the Ethernet port as primary backhaul if it is UP, otherwise it is the 802.11a radio.
(This gives the network administrator the ability to configure it as a RAP the first time and recover
it in-house). For fast convergence of the network, Cisco recommends that you not connect any
Ethernet device to the MAP for its initial joining to the mesh network.
A MAP failing to connect to a WLAN controller on an Ethernet port that is UP, sets the 802.11a
radio as the primary backhaul. Failing to find a neighbor or failing to connect to a WLAN controller
via any neighbor on 802.11a radio causes the primary backhaul to be UP on the Ethernet port again.
MAP will give preference to the parent which has the same BGN.
A MAP connected to a controller over an Ethernet port does not build a mesh topology (unlike a
RAP).
A RAP always sets the Ethernet port as the primary backhaul.
If the Ethernet port on a RAP is DOWN, or a RAP fails to connect to a controller on an Ethernet port
that is UP, the 802.11a radio is set as the primary backhaul. Failing to find a neighbor or failing to
connect to a controller via any neighbor on the 802.11a radio will make the RAP go to SCAN state
after 15 minutes and starts with the Ethernet port first.
Troubleshooting
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