Foundry Networks Switch and Router Installation And Configuration Manual page 711

See "How BGP4 Selects a Path for a Route" on page 19-3 for a description of the BGP4 algorithm.
When you enable IP load sharing, the Layer 3 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 router receives a packet destined for a specific IP address, the router uses a round-robin algorithm to select
the path that was not used for the last newly learned destination IP address. Once the router associates a path
with a particular destination IP address, the router will always use that path as long as the router contains the
destination IP address in its cache.
NOTE: The Layer 3 Switch does not perform source routing. The router 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.
NOTE: The maximum number of BGP4 load sharing paths cannot be greater than the maximum number of IP
load sharing paths. To increase the maximum number of IP load sharing paths, use the ip load sharing <num>
command at the global CONFIG level of the CLI or use the # of Paths field next to Load Sharing on the IP
configuration panel of the Web management interface.
USING THE CLI
To change the maximum number of shared paths, enter commands such as the following:
BigIron(config)# router bgp
BigIron(config-bgp-router)# maximum-paths 4
BigIron(config-bgp-router)# write memory
Syntax: [no] maximum-paths <num>
The <num> parameter specifies the maximum number of paths across which the Layer 3 Switch can balance
traffic to a given BGP4 destination. You can change the maximum number of paths to a value from 2 – 4. The
default is 1.
USING THE WEB MANAGEMENT INTERFACE
1. Log on to the device using a valid user name and password for read-write access. The System configuration
panel is displayed.
2. Click on the plus sign next to Configure in the tree view to expand the list of configuration options.
3. Click on the plus sign next to BGP in the tree view to expand the list of BGP option links.
4. Click on the General link to display the BGP configuration panel, shown in Figure 19.2 on page 19-8.
December 2000
Configuring BGP4
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