Link Failure And Acquisition; Protecting Against Physical Link Failure; Protecting Against Virtual Link Failure - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - LINK LAYER CONFIGURATION GUIDE 2010-10-13 Configuration Manual

Link Failure and Acquisition

Copyright © 2010, Juniper Networks, Inc.
Configuration and status of LACP changes during link failure and acquisition.
Configuration of the endpoints of the member links is different.
Configuration is LAG to non-LAG in an RSTP network.
Link failure on the local system occurs when the active link is no longer active.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 217 . Optionally, you can force the failover automatically by
issuing "redundant-port force-failover" on page 218 .

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 215.
Chapter 6: Configuring 802.3ad Link Aggregation and Link Redundancy
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