7705 SAR Interfaces
88
Interface Configuration Guide
3HE 11011 AAAC TQZZA Edition: 01
3.2.9.1 Line and Internal Ethernet Loopbacks
A line loopback loops frames received on the corresponding port back towards the
transmit direction. Line loopbacks are supported on ports configured for access or
network mode.
Similarly, a line loopback with MAC addressing loops frames received on the
corresponding port back towards the transmit direction, and swaps the source and
destination MAC addresses before transmission. See MAC Swapping for more
information.
An internal loopback loops frames from the local router back to the framer. This is
usually referred to as an equipment loopback. The transmit signal is looped back and
received by the interface. Internal loopbacks are supported on ports configured in
access mode.
If a loopback is enabled on a port, the port mode cannot be changed until the
loopback has been disabled.
A port can support only one loopback at a time. If a loopback exists on a port, it must
be disabled or the timer must expire before another loopback can be configured on
the same port. EFM-OAM cannot be enabled on a port that has an Ethernet loopback
enabled on it. Similarly, an Ethernet loopback cannot be enabled on a port that has
EFM-OAM enabled on it.
When an internal loopback is enabled on a port, autonegotiation is turned off silently.
This is to allow an internal loopback when the operational status of a port is down.
Any user modification to autonegotiation on a port configured with an internal
Ethernet loopback will not take effect until the loopback is disabled.
The loopback timer can be configured from 30 s to 86400 s. All non-zero timed
loopbacks are turned off automatically under the following conditions: an adapter
card reset, DSL module reset, GPON module reset, an activity switch, or timer
expiry. Line or internal loopback timers can also be configured as a latched loopback
by setting the timer to 0 s, or as a persistent loopback with the persistent keyword.
Latched and persistent loopbacks are enabled indefinitely until turned off by the user.
Latched loopbacks survive adapter card resets and activity switches, but are lost if
there is a system restart. Persistent loopbacks survive adapter card resets and
activity switches and can survive a system restart if the admin-save or
admin-save-detail command was executed prior to the restart. Latched loopbacks
(untimed) and persistent loopbacks can be enabled only on Ethernet access ports.
Persistent loopbacks are the only Ethernet loopbacks saved to the database by the
admin-save and admin-save-detail commands.
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