Brocade Communications Systems Layer 3 Routing Configuration ICX 6650 - Manual (Page 324)

306 Brocade ICX 6650 Layer 3 Routing Configuration Guide
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Optional BGP4 configuration tasks
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 Layer 3 switch performs is a comparison of the internal
paths:
• When IP load sharing is disabled, the Layer 3 switch prefers the path to the router with the
lower router ID.
• When IP load sharing and BGP4 load sharing are enabled, the Layer 3 switch balances the
traffic across the multiple paths instead of choosing just one path based on router ID.
Refer to “How BGP4 selects a path for a route” on page 283 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 through 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 through 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.