Белая книга для Cisco Cisco 1700 2600 3600 3700 Series VPN Module
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Adaptive Load Balancing
The addition or removal of a member port from a Cisco EtherChannel interface has always led to a varied amount
of traffic loss for customers. The current generation of port ASICs uses a 3-bit Result Bundle Hash (RBH) value
from the PFC or DFC result to index into a load register. This allows a packet to be transmitted if the corresponding
bit is set.
When a new port is added or deleted, the load value is reset on all the ports. A new load is then distributed on all
the ports in the Cisco EtherChannel interface, including the new member, and reprogrammed into the port ASIC for
each port. This process causes packets to be dropped during the short outage window (approximately 200 to 300
ms), an undesirable result for higher-speed interfaces such as 10 Gigabit Ethernet connections where a large
amount of traffic may be lost during this brief outage window.
This problem has led to the development of an enhanced load-distribution mechanism. When ports are added or
removed from a Cisco EtherChannel interface, the load result does not need to be reset on existing member ports,
resulting in improved traffic recovery times.
You can implement this new algorithm either globally or on a per-port channel basis, where fixed is the current
default mode and adaptive uses the enhanced mode:
vss(config)#port-channel hash-distribution ?
adaptive selective distribution of the bndl_hash among port-channel members
fixed distribution of the bndl_hash among port-channel members
vss(config)#int port-channel 4
vss(config-if)#port-channel port hash-distribution ?
adaptive selective distribution of the bndl_hash among port-channel members
fixed fixed distribution of the bndl_hash among port-channel members
The algorithm selected with these commands is applied only at the next hash-distribution instance, which usually
occurs on a port-channel member link transition event:
vss#sh etherchannel 4 summary
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use N - not in use, no aggregation
f - failed to allocate aggregator
M - not in use, no aggregation due to minimum links not met
m - not in use, port not aggregated due to minimum links not
met
u - unsuitable for bundling
d - default port
w - waiting to be aggregated
Number of channel-groups in use: 9
Number of aggregators: 9
Group Port-channel Protocol Ports
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