Juniper JUNOSE 11.1.X - LINK LAYER CONFIGURATION 4-7-2010, Configuration Manual

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The following sections describe link redundancy behavior when the:
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Configuration and status of LACP changes during link failure and acquisition.
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Configuration of the endpoints of the member links is different.
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Configuration is LAG to non-LAG in an RSTP network.
Link Failure and Acquisition
Link failure on the local system occurs when the active link is no longer active. Failures
can be characterized as physical link failure or virtual link failure.
Each type of link failure has different requirements for detection, failover, and link
acquisition. In all cases, you configure the link to fail over when it fails by issuing
“redundant-port” on page 221 . Optionally, you can force the failover automatically
by issuing “redundant-port force-failover” on page 221 .
Protecting Against Physical Link Failure
Physical link failures can occur when a cable is cut.
To protect against physical link failure, issue the transmitter keyword with the
redundant-port command to enable or disable the local redundant link. When the
redundant link needs to be down, the link behavior in failure detection and failover
follows a similar port redundancy scheme available with line modules such as the
GE-2 line module. Disabling the transmitter also enables the remote end of the
redundant link to be in the operational Down state, which might be a requirement
for third-party equipment when supporting redundancy over LAG.
Enabling the transmitter provides for a quick LAG failover in the event one of the
non-redundant links in the LAG fail. This is particularly true when LACP has been
enabled on the LAG, because it can take several seconds for LACP to converge on a
link. When the transmitter on the remote end is enabled on the redundant link before
it fails over, the local system considers the redundant link to be viable and enables
the transmitter if it is disabled. If the remote end is disabled, the local end must
enable the transmitter and wait for the remote end to enable.
Protecting Against Virtual Link Failure
A virtual link failure can occur when the active link is no longer used by the network
because of topology changes caused by physical failure in the network. Topology
changes can occur when, for example, a link is blocked because of network protocols
such as RSTP blocking the port leading to selection of the redundant port connected
to the receiver.
To protect against virtual link failure in conjunction with network protocols, use the
packet-sampling keyword with the redundant-port command to detect link the
viability. For example, when there is a network protocol decision that changes the
topology and blocks a link to compensate for failures in the network, the system
monitors the traffic to detect the change in network topology and fails over to the
redundant port if necessary. It also determines whether the failover is successful.
For more information, see “Member Link with Non-LAG Partner” on page 218.
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Ethernet Link Redundancy Behavior
JUNOSe 11.1.x Link Layer Configuration Guide