Configuring BGP4
December 2005
© Foundry Networks, Inc.
21 - 23
See “How BGP4 Selects a Path for a Route” on page 21-4 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 – 6.
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. 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 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.
To change the maximum number of shared paths, enter commands such as the following:
FESX424 Router(config)# router bgp
FESX424 Router(config-bgp-router)# maximum-paths 4
FESX424 Router(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.
Customizing BGP4 Load Sharing
By default, when BGP4 load sharing is enabled, both IBGP and EBGP paths are eligible for load sharing, while
paths from different neighboring ASs are not eligible. You can change load sharing to apply only to IBGP or EBGP
paths, or to support load sharing among paths from different neighboring ASs.
To enable load sharing of IBGP paths only, enter the following command at the BGP configuration level of the CLI:
FESX424 Router(config-bgp-router)# multipath ibgp
To enable load sharing of EBGP paths only, enter the following command at the BGP configuration level of the CLI:
FESX424 Router(config-bgp-router)# multipath ebgp
Summary of Contents for FastIron Edge Switch X424
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