Table 5, How ip load sharing works – Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) User Manual
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Configuring IP parameters
4
How IP load sharing works
On the Virtual ADX, if there is more than one path to a given destination, a hash is calculated based
on the source MAC address, destination MAC address, source IP address, destination IP address,
TCP/UDP source port and destination port, if the packet is also a TCP and UDP packet. This hash is
used to select one of the paths.
Path redundancy
If a path to a given destination becomes unavailable, the Virtual ADX provides redundancy by using
another available equal-cost path to the destination, as described in the following sections.
Response to path state changes
If one of the load-balanced paths to a cached destination becomes unavailable, or the IP route
table receives a new equal-cost path to a cached destination, the software removes the
unavailable path from the IP route table. Then the software selects a new path:
•
For host-based IP load sharing, the next load-balancing cache entry uses the first path to the
destination. The first path is the path that entered the IP route table first.
on page 79 describes the host-based load-sharing mechanism.
•
For network-based IP load sharing, the next load-balancing cache entry uses the next available
path is then calculated based on the current number of paths and the maximum number of
paths allowed.
Host-based IP load sharing
The host-based load sharing method uses a simple round-robin mechanism to select an equal-cost
path for traffic to a destination host. When the Virtual ADX receives traffic for a destination host
and the IP route table has multiple equal-cost paths to the host, the Virtual ADX checks the IP
forwarding cache for a forwarding entry to the destination.
•
If the IP forwarding cache contains a forwarding entry for the destination, the device uses the
entry to forward the traffic.
•
If the IP forwarding cache does not contain a forwarding entry for the destination, the software
selects the next path in the rotation (the path after the one the software used for the previous
load sharing selection). The software then creates an IP forwarding cache entry that associates
the destination host IP address with the selected path (next-hop IP address).
A cache entry for host-based IP load sharing has an age time of ten minutes. If a cache entry is not
used before the age time expires, the device deletes the cache entry. The age time for IP load
sharing cache entries is not configurable.
shows an example of host-based IP load sharing. In this example, the Virtual ADX has two
equal-cost paths to hosts H1 – H9. For simplicity, this example assumes that the Virtual ADX does
not have any entries in its IP forwarding cache to begin with, and receives traffic for the destination
hosts (H1 – H9) in ascending numerical order, beginning with H1 and ending with H9.
TABLE 5
Default load sharing parameters for route sources
Route source
Default number of paths
Default maximum
number of paths
Maximum number of
paths
See page...
Static IP route
4
4
8
1
1.
This value depends on the value for IP load sharing, and is not separately configurable.