IBM AIX Continuous Availability Features - IBM Redbooks

2.4.4 EtherChannel
network traffic to an alternate backup route, if available. DGD is generally most useful for
hosts that use static rather than dynamic routing.
AIX releases prior to AIX V5.1 do not permit you to configure multiple routes to the same
destination. If a route's first-hop gateway failed to provide the required routing function, AIX
continued to try to use the broken route and address the dysfunctional gateway. Even if there
was another path to the destination which would have offered an alternative route, AIX did not
have any means to identify and switch to the alternate route unless a change to the kernel
routing table was explicitly initiated, either manually or by running a routing protocol program,
such as gated or routed. Gateways on a network run routing protocols and communicate with
one another. If one gateway goes down, the other gateways will detect it, and adjust their
routing tables to use alternate routes (only the hosts continue to try to use the dead gateway).
The DGD feature in AIX V5.1 enables host systems to sense and isolate a dysfunctional
gateway and adjust the routing table to make use of an alternate gateway without the aid of a
running routing protocol program.
There are two modes for dead gateway detection:
► Passive dead gateway detection
► Active dead gateway detection
Passive dead gateway detection
Passive dead gateway detection will work without actively pinging the gateways known to a
given system. Passive DGD will take action to use a backup route if a dysfunctional gateway
has been detected.
The passive DGD mechanism depends on the protocols Transmission Control Protocol
(TCP) and Address Resolution Protocol (ARP), which provide information about the state of
the relevant gateways. If the protocols in use are unable to give feedback about the state of a
gateway, a host will never know that a gateway is down and no action will be taken.
Passive dead gateway detection has low overhead and is recommended for use on any
network that has redundant gateways. However, passive DGD is done on a best-effort basis
only.
Active dead gateway detection
When no TCP traffic is being sent through a gateway, passive DGD will not sense a
dysfunctional state of the particular gateway. The host has no mechanism to detect such a
situation until TCP traffic is sent or the gateway's ARP entry times out, which may take up to
20 minutes. But this situation does not modify route costs. In other words, a gateway not
forwarding packets is not considered dead. In such cases, active DGD becomes valuable to
use.
Active DGD will ping gateways periodically, and if a gateway is found to be down, the routing
table is changed to use alternate routes to bypass the dysfunctional gateway. Active dead
gateway detection will be off by default and it is recommended to be used only on machines
that provide critical services and have high availability requirements. Because active DGD
imposes some extra network traffic, network sizing and performance issues have to receive
careful consideration. This applies especially to environments with a large number of
machines connected to a single network.
EtherChannel is a network interface aggregation technology that allows you to produce a
single large pipe by combining the bandwidth of multiple Ethernet adapters. In AIX V5.1, the
38 IBM AIX Continuous Availability Features