Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 413)

• Enable IP load sharing if it is disabled.
• Set the maximum number of BGP4 load sharing paths. The default maximum number is 1, which
means no BGP4 load sharing takes place by default.
NOTE
The maximum number of BGP4 load sharing paths cannot be greater than the maximum number of IP
load sharing paths.
How Multipath load sharing affects route selection
During evaluation of multiple paths to select the best path to a given destination (for installment in the IP
route table), the device performs a final comparison of the internal paths. The following events occur
when load sharing is enabled or disabled:
• When load sharing is disabled, the device prefers the path with the lower device ID if the compare-
routerid command is enabled.
• When load sharing and BGP4 Multipath load sharing are enabled, the device balances the traffic
across multiple paths instead of choosing just one path based on device ID.
Refer to How BGP4 selects a path for a route (BGP best path selection algorithm) on page 386 for a
description of the BGP4 algorithm.
When you enable IP load sharing, the device can load-balance BGP4 or OSPF routes across up to four
equal paths by default. You can change the number load sharing paths to a value from 2 through 8.
How Multipath load sharing works
Multipath load sharing is performed in round-robin fashion and is based on the destination IP address
only. The first time the device receives a packet destined for a specific IP address, the device uses a
round-robin algorithm to select the path that was not used for the last newly learned destination IP
address. Once the device associates a path with a particular destination IP address, the device will
always use that path as long as the device contains the destination IP address in its cache.
NOTE
The device does not perform source routing. The device is concerned only with the paths to the next-
hop devices, not the entire paths to the destination hosts.
A BGP4 destination can be learned from multiple BGP4 neighbors, leading to multiple BGP4 paths to
reach the same destination. Each of the paths may be reachable through multiple IGP paths (multiple
OSPF or RIP paths). In this case, the software installs all the multiple equal-cost paths in the BGP4
route table, up to the maximum number of BGP4 equal-cost paths allowed. The IP load sharing feature
then distributes traffic across the equal-cost paths to the destination.
If an IGP path used by a BGP4 next-hop route path installed in the IP route table changes, then the
BGP4 paths and IP paths are adjusted accordingly. For example, if one of the OSPF paths to reach the
BGP4 next hop goes down, the software removes this path from the BGP4 route table and the IP route
table. Similarly, if an additional OSPF path becomes available to reach the BGP4 next-hop device for a
particular destination, the software adds the additional path to the BGP4 route table and the IP route
table.
How Multipath load sharing affects route selection
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