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Wan Failover & Load Balancing FAQs
03/26/2020 
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Description
Wan Failover & Load Balancing FAQs
Resolution
Q1: If you have 4 interfaces with each at 25% LB ratio, what happens when we lose one of the interfaces? The remaining three share the unallocated 25% in a round-robin fashion?
Ans: Yes, the load is shared based on score and credit. Distribution may be round-robin if zero score was achieved prior to losing an interface.
However, it is not exactly 25% since we allocate based on “new outbound flow” but we compute load based on “in/out SNMP bytes of interface”. When LB assigns a flow to an outbound interface, LB couldn’t predict how much in/out bytes will actually be transmitted within he flow so LB uses an adaptive algorithm that readjusts itself to achieve the configured ratios.
Also, please note that interfaces also receive new flows that are not part of LB (e.g. inbound flows, stack-generated such as VPN, management traffic). Such flows are included in “load” computation since they are part of the SNMP interface statistics gathered.
Q2: And the assumption is that when the 4th interface comes back online that it will reclaim 25% of the LB ratio as new connections are formed.
Ans: Again, the LB decision when the interface comes back is based on score and credit.
An interface newly transitioned to “Up” will have the lowest score (due to SNMP traffic stats) and so most new flows will be assigned to it to make up for the discrepancy in “Average” ratio and desired configured ratio.
When there’s a lot of new flows, this interface should pickup the slack in a timely manner.
The problem is when flows are fixed or constant, since obviously, they would already have been assigned to the old interfaces that had remained “Up” the whole time.
The newly “Up” interface would have fewer new flows, as we do not “preempt” existing flows in Ratio LB.
Q3: Is the Average Ratio calculated based on the entire duration of the system up time or is it for a specific period?
Ans: Yes, it is calculated for the entire duration since system up time.
However, the LB run-time statistics are reset on LB disable-enable so you should be able to influence how LB behaves by resetting the LB counters.
Q4: Does Traffic Flows due to PBR routes over a particular WAN Interface affect the LB Interface Selection as per the configured ratio considering that we look into the SNMP in/out information on the interface to compute the load on a particular interface to assign score
Ans: Yes, PBR route traffic as well as inbound management traffic on the interface will affect LB Interface Selection.
An interface having a lot of PBR route traffic and/or management traffic will appear busy due to the amount of in/out bytes associated with such traffic.
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