Link Failover

There are two contexts in which link failover applies:

• Physical link failover allows the CNA system to spread load from a failed link to others within the same group. The thresholds of the functional links within that group are increased to accommodate the change. Note that the failover mechanism assumes that all physical links have the same capacity.
• BGP link failover is used to shift the load of a failed BGP link onto other links in a predefined manner use the on-failure keyword (see load threshold-table) inside a load threshold table to specify the set of links that fail over to each other. When a BGP link fails, the load thresholds associated with the remaining BGP links specified in the failover set that are functional are increased with a proportional part of the threshold of the failed link. This addition occurs in all levels of a threshold table, and the thresholds can be increased up to the threshold specified in the highest level.

NOTE: Thresholds cannot be incremented beyond the value of the threshold of the highest level because this thresholds is assumed to reflect the capacity of the link.

For example, consider the load threshold table discussed earlier in this chapter, with the following failover set applied to that table where all the links are included:

on-failure all
 
 link L1
 
 link L2
 
 link L3
 
end
 

If the BGP link L1 fails, then based on this defined failover set, all of its load will be accommodated by links L2 and L3, up to its capacity (we assume that both L2 and L3 are functional). The load thresholds of links L2 and L3 would be incremented by 50% of the threshold assigned to L1. The first level of the load threshold table would then be changed to the following:

level 88
 
link L1 -
 
link L2 44
 
link L3 44
 

The thresholds of L2 and L3 have been increased by 14 Mbps, to account for 28Mpbs out of the 30Mbps originally assigned to L1. Note that L2 and L3 have not been able to accommodate the entire 30 Mbps because their thresholds cannot be increased beyond the value of the thresholds at the highest level, 44Mbps in this case. This also means that the higher levels specified in the original table (levels 104, 118 and 132) now all become levels with 88 Mbps, given that the load thresholds cannot be further increased. For this example, the load threshold table has been effectively reduced from four levels to just one.