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Configuring BGP4
Changing the Maximum Number of Paths for BGP4 Load Sharing
Load sharing enables the routing switch to balance traffic to a route across multiple equal-cost paths of the same
type (EBGP or IBGP) for the route.
To configure the routing switch to perform BGP4 load sharing:
• Enable IP load sharing if it is disabled.
• Set the maximum number of paths. The default maximum number of BGP4 load sharing paths 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 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 last comparison the routing switch performs is a comparison of the internal paths.
• When IP load sharing is disabled, the routing switch prefers the path to the router with the lower router ID.
• When IP load sharing and BGP4 load sharing are enabled, the routing switch balances the traffic across the
multiple paths instead of choosing just one path based on router ID.
See “How BGP4 Selects a Path for a Route” on page 10-3 for a description of the BGP4 algorithm.
When you enable IP load sharing, the routing switch can load balance BGP4 or OSPF routes across up to four
equal paths by default. You can change the number of IP load sharing paths to a value from 2 – 8.
How Load Sharing Works
Load sharing is performed in round-robin fashion and is based on the destination IP address only. The first time
the routing switch receives a packet destined for a specific IP address, the routing switch uses a round-robin
algorithm to select the path that was not used for the last newly learned destination IP address. Once the routing
switch associates a path with a particular destination IP address, the routing switch will always use that path as
long as the routing switch contains the destination IP address in its cache.
NOTE: The routing switch does not perform source routing. The routing switch is concerned only with the paths
to the next-hop routers, 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.
If the administrative distance of the paths is lower than the administrative distance of paths from other sources
(such as static IP routes, RIP, or OSPF), the BGP4 paths also are installed in the IP route table. The IP load
sharing feature then distributes traffic across the equal-cost paths to the destination.
If an IGP path underlying a BGP4 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 router for a particular destination, the software adds the additional
path to the BGP4 route table and the IP route table.
Changing the Maximum Number of Shared BGP4 Paths
When IP load sharing is enabled, BGP4 can balance traffic to a specific destination across up to four equal paths.
You can set the maximum number of paths to a value from 1 – 4. The default is 1.
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