User's Manual

536 | Secure Enterprise Mesh Dell Networking W-Series ArubaOS 6.4.x| User Guide
provisioned backup mesh cluster profiles to establish an uplink. If the configured profiles are unavailable
after searching for 5 minutes, the recovery profile is used.
l Moving to a better mesh link
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.
l Using a new mesh link if the current mesh link goes down
If an uplink goes down, the affected mesh nodes reestablish 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 101 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.
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:
l High-throughput APs add a high cost penalty for links to non-high-throughput APs.
l 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 can 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, the mesh parent's path cost, and the parent's node cost.
Mesh portals typically advertise a path-cost of zero, but high-throughput portals add an
offset penalty if they are connected to a 10/100mbps port that is too slow for the high-
throughput link capacity.
Table 101: Mesh Link Metric Computation