Information Technology Reference
In-Depth Information
Resetting the routing table and CEF tables might cause brief packet drops.
NOTE
Load Sharing
CEF has two forms of load sharing:
•
Per-session load sharing
—This is commonly, although incorrectly, called per-
destination load sharing. This form of load sharing is the default behavior and does
not require special configuration. A session is a traffic flow that has the same source
and destination IP addresses.
•
Per-packet load sharing
—Load is shared on a per-packet basis. To enable it, enter
ip cef load-sharing per-packet
in interface configuration mode. For per-packet load
sharing to work properly, all outgoing interfaces must have the command configured.
Per-Session Load Sharing
Per-session load sharing allows the router to use multiple paths to distribute the traffic.
Packets for a given source-destination host pair are guaranteed to take the same path, even
if multiple paths are available. Traffic destined for different pairs tends to take different
paths. Per-session load sharing is enabled by default when you enable CEF; it's the method
of choice in most situations. Because per-session load sharing depends on the statistical
distribution of traffic, load sharing becomes more effective as the number of source-
destination pairs increases. Per-session load balancing can be used to ensure that packets
for a given host pair arrive in order, because all packets for the same host pair are routed
over the same link (or links).
For each session of the source and destination addresses, an active path is assigned. Each
path carries an equally loaded number of sessions. A hash function using the source and
destination addresses, number of active paths, and router ID is run to assign sessions to
paths. Sixteen hash buckets are numbered 0 through 15. Evenly filled buckets are assigned
to paths depending on the number of paths and each path's weight.
In Example 2-7, 16 buckets from 0 through 15 are evenly filled with three equal paths
(0 through 2). (Bucket 15 is not used in this example.)
Load Sharing for Three Paths
Example 2-7
Bucket number 0 1 2 3 4 5 6 7 8 9 10 11 12
13 14 15
Path number 0 1 2 0 1 2 0 1 2 0 1 2 0
1 2
