Locality Bias in a Virtual Chassis

How Locality Bias Works

Locality bias conserves Virtual Chassis port bandwidth in a two-member MX Series Virtual Chassis by directing unicast transit traffic for equal-cost multipath (ECMP) groups and aggregated Ethernet bundles to egress links in the same (local) member router, provided that the local member router has an equal or larger number of available egress links than the remote member router. Because locality bias directs all of the traffic towards the local member router, Virtual Chassis ports do not use bandwidth.

However, if the number of available remote member router egress links exceeds the number of available local member router egress links, the system reduces the amount of traffic in the local member router by using a ratio that is based on the number of remote versus local links. The amount of traffic that the system does not direct toward egress links on the local member router is then split evenly across the egress links in the remote member router.

If either the local member router or remote member router do not have available egress links, then the traffic forwarding state across the Virtual Chassis ports does not change.

Locality Bias Percentages

The router uses the following algorithms to determine the percentage of traffic that is directed toward the local member router egress links, where L is the number of egress links on the local member router and R is the number of egress links on the remote member router.

• If L >= R, then Locality Bias Percentage = 100 percent and the local member router receives all egress traffic.

  For example, if the local member router and remote member router each contain one egress link, then the locality bias is 100 percent. The router directs all unicast transit traffic that is destined for an ECMP group or aggregated Ethernet bundle to the local member router.

• If L < R, then Locality Bias Percentage = 200 * (L / ( R + L ))

  For example, if the local member router (L) contains one link and the remote member router (R) contains two links, the locality bias percentage calculation is

  200 * ( 1 / ( 2 + 1)) = 66

  This means that the system directs 66 percent of the unicast transit traffic destined for an ECMP group or aggregated Ethernet bundle toward the local member router. The system splits the remaining 34 percent of the unicast transit traffic equally between the remote member router egress links. Each of the two remote egress links in the example receives 17 percent of the traffic.

  Note:

  The actual amount of traffic that the local member router receives can vary slightly from the percentages in the algorithm calculations.

• If L = 0 or R = 0, then locality bias does not change the forwarding state.

For both the L < R and L >= R algorithms, locality bias percentages are recalculated on each line card whenever one of the aggregated Ethernet child links goes up or down, or whenever a link is added to or removed from an ECMP bundle.