Dell 6.2 Link Metrics, Optimizing Links

442| Secure Ente rprise Mesh DellPowerConnect W- Series ArubaOS 6.2 | User Guide

If the existing uplinkquality degrades below the configured threshold,and a lower cost or more preferableuplink

is available on the same channeland cluster, the mesh point reselects that linkwi thout re-scanning.In some

cases, this invalidates all of theentries t hat havethis mesh point as a next hop to the destination and triggers

new learningof the bridge tables.

lUsing a new mesh link if the currentmesh link goes down

If an uplinkgoes down, the affected mesh nodes re-establisha co nnection with the mesh portal by re-scanningt o

choose a new path to the mesh portal. If a mesh portal goes down, and a redundant mesh portal is available, the

affectedmesh nodes update their forwarding tables to reflect the path to the new mesh portal.

Link Metrics

Mesh points use the configuredalgorithm 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.Table127 describes the components

that makeup the metric value: node cost, hopco unt,link cost and 802.11 capacity.

The linkmetrics indicate the relative cost of a path t o the mesh portal. The best path (lowest metric value) is used

to create the uplink.

Nodecost Indicatesthe amount of traffic expectedto traverse the mesh node. The more traffic, the higher the node

cost.When establishing a m eshl ink, nodes with less traffictake precedence. The node cost is dependent

onthe number of chil dren a mesh node supports.It can change as the mesh network topology changes,

forexample i fnew childrenare added to the network or old children di sconnectfrom the network.

Hopcount Indicatesthe number of hops it takes the mesh node toget to the mesh portal. The mesh portal advertises

ahop count of 0, whil e all other mesh nodes advertisea cumul ativecount based on the parent mesh node.

Linkcost Representsthe quality of the link to an active neighbor. The higher the Received Signal S trengthIndication

(RSSI),the better the path to the neighbor and the mesh portal. If theR SSI value is below the configured

threshold,the l ink cost is penalized to filter margi nal links. A less direct, higher qualitylink may be

preferredover the marginal l ink.

Thefoll owing factors also affectm esh link metrics

lHigh-throughputAPs add a high cost penalty for links to non-high-throughput APs.

lMulti-streamhi gh-throughAPs add proportional cost penalties for links to high-throughput APs that

supportfewer streams.

802.11capacity High-throughputAPs can send 802.11 information elements (IEs)in their managem entframes, al lowing

high-throughputmesh nodes to i dentifyother mesh nodes w ith a high-throughput capacity. High-

throughputmesh points prefer to select other 802.11-capablemesh points in their path to the mesh portal,

butcan use a legacy path if no high-throughput path is available.

PathCost P ath costi s calculated by analyzing the other components in this table, andadding thelink cost plus mesh

parent'spath cost plus the parent's node cost.

Meshportals typically advertise a path-cost of zero, buthigh-throughput portals add an offset penalty if

theyare connected to a 10/100mbpsport that is too slow to for the high-throughput link capacity.

Optimizing Links

You can configureand optimize operatio no ft helink metric algorithm via the mesh radio profile. These configurable

mesh linktrigger thresholds can determine when the uplink or mesh path is dropped and anotheri s chosen, provide

enhancednetwork reliability, and contain flapping links.A lthoughyou can modify the behavior of the link metric

algorithm,Dell recommends the default values for most deployments. For information, see "Metric algorithm" on

page 452.