Users Guide

220 | Secure Enterprise Mesh Dell PowerConnect W-Series ArubaOS 6.1 | User Guide
If the existing uplink quality degrades below the configured threshold, and a lower cost or more preferable
uplink is available on the same channel and cluster, the mesh point reselects that link without re-scanning. In
some cases, this invalidates all of the entries that have this mesh point as a next hop to the destination and
triggers new learning of the bridge tables.
 Using a new mesh link if the current mesh link goes down
If an uplink goes down, the affected mesh nodes re-establish a connection with the mesh portal by re-scanning
to choose a new path to the mesh portal. If a mesh portal goes down, and a redundant mesh portal is available,
the affected mesh nodes update their forwarding tables to reflect the path to the new mesh portal.
Link Metrics
Mesh points use the configured algorithm to compute a metric value, or “path cost,” for each potential uplink
and select the one with the lowest value as the optimal path to the mesh portal.Table 41 describes the
components that make up the metric value: node cost, hop count, link cost and 802.11 capacity.
The link metrics indicate the relative cost of a path to the mesh portal. The best path (lowest metric value) is
used to create the uplink.
Optimizing Links
You can configure and optimize operation of the link metric algorithm via the mesh radio profile. These
configurable mesh link trigger thresholds can determine when the uplink or mesh path is dropped and another is
chosen, provide enhanced network reliability, and contain flapping links. Although you can modify the behavior
of the link metric algorithm, Dell recommends the default values for most deployments. For information, see
“metric algorithm” on page229.
Table 41 Mesh Link Metric Computation
Component Description
Node cost Indicates the amount of traffic expected to traverse the mesh node. The more traffic, the higher the node cost.
When establishing a mesh link, nodes with less traffic take precedence. The node cost is dependent on the
number of children a mesh node supports. It can change as the mesh network topology changes, for example
if new children are added to the network or old children disconnect from the network.
Hop count Indicates the number of hops it takes the mesh node to get to the mesh portal. The mesh portal advertises a
hop count of 0, while all other mesh nodes advertise a cumulative count based on the parent mesh node.
Link cost Represents the quality of the link to an active neighbor. The higher the Received Signal Strength Indication
(RSSI), the better the path to the neighbor and the mesh portal. If the RSSI value is below the configured
threshold, the link cost is penalized to filter marginal links. A less direct, higher quality link may be preferred
over the marginal link.
The following factors also affect mesh link metrics
 High-throughput APs add a high cost penalty for links to non-high-throughput APs.
 Multi-stream high-through APs add proportional cost penalties for links to high-throughput APs that
support fewer streams.
802.11 capacity High-throughput APs can send 802.11 information elements (IEs) in their management frames, allowing high-
throughput mesh nodes to identify other mesh nodes with a high-throughput capacity. High-throughput mesh
points prefer to select other 802.11-capable mesh points in their path to the mesh portal, but will use a legacy
path if no high-throughput path is available.
Path Cost Path cost is calculated by analyzing the other components in this table, and adding the link cost plus mesh
parent's path cost plus the parent's node cost.
Mesh portals typically advertise a path-cost of zero, but high-throughput portals will add an offset penalty if
they are connected to a 10/100mbps port that is too slow to for the high-throughput link capacity.