Configuring Weighted Static Route Load Balancing - Fortinet FortiGate Series Administration Manual

ECMP route failover and load balancing
Determining of a interface has exceeded its Spillover Threshold

Configuring weighted static route load balancing

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The FortiGate unit selects an ECMP route for a new session by finding the first route in the
routing table that sends the session out a FortiGate interface that is not processing more
traffic that its configured route spill-over limit.
Note: A new session to a destination IP address that already has an entry in the routing
cache is routed using the route already added to the cache for that destination address.
See "ECMP routing of simultaneous sessions to the same destination IP address" on
page 345.
For example, consider a FortiGate unit with interfaces port3 and port4 both connected to
the Internet through different ISPs. ECMP routing is set to usage-based and route
spillover for to 100 KBps for port3 and 200 KBps for port4. Two ECMP default routes are
added, one for port3 and one for port4.
If the route to port3 is higher in the routing table than the route to port4, the FortiGate unit
sends all default route sessions out port3 until port3 is processing 10Mbps of data. When
port3 reaches its configured bandwidth limit, the FortiGate unit sends all default route
sessions out port4. When the bandwidth usage of port3 falls below 10Mbps, the FortiGate
again sends all default route sessions out port3.
New sessions to designating IP addresses that are already in the routing cache; however,
use the cached routes. This means that even of port3 is exceeding its bandwidth limit, new
sessions can continue to be sent out port3 if their destination addresses are already in the
routing cache. As a result, new sessions are sent out port4 only if port3 exceeds its
bandwidth limit and if the routing cache does not contain a route for the destination IP
address of the new session.
Also, the switchover to port4 does not occur as soon as port3 exceeds its bandwidth limit.
Bandwidth usage has to exceed the limit for a period of time before the switchover takes
place. If port3 bandwidth usage drops below the bandwidth limit during this time period,
sessions are not switched over to port4. This delay reduces route flapping.
FortiGate usage-based ECMP routing is not actually load balancing, since routes are not
distributed evenly among FortiGate interfaces. Depending on traffic volumes, most traffic
would usually be processed by the first interface with only spillover traffic being processed
by other interfaces.
If you are configuring usage-based ECMP in most cases you should add spillover
thresholds to all of the interfaces with ECMP routes. The default spillover threshold is 0
which means no bandwidth limiting. If any interface has a spillover threshold of 0, no
sessions will be routed to interfaces lower in the list unless the interface goes down or is
disconnected. An interface can go down if Detect interface status for Gateway Load
Balancing does not receive a response from the configured server.
You can use the diagnose netlink dstmac list CLI command to determine if an
interface is exceeding its Spillover Threshold. If the command displays over_bps=1 the
interface is exceeding its threshold. If over_bps=0 the interface has not exceeded its
threshold.
Configure weighted load balancing to control how the FortiGate unit distributes sessions
among ECMP routes by adding weights for each route. Add higher weights to routes that
you want to load balance more sessions to.
FortiGate Version 4.0 MR1 Administration Guide
01-410-89802-20090903
http://docs.fortinet.com/
Router Static
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