[c-nsp] question about rspan sessions

[ovidiu neghina]

on cisco web site :
SPAN and RSPAN Session Limits

You can configure (and store in NVRAM) one local SPAN session or
multiple RSPAN sessions on a switch. The number of active sessions and
combinations are subject to these restrictions:
SPAN or RSPAN source (rx, tx, both): 1 active session limit. (SPAN and
RSPAN are mutually exclusive on a source switch).
RSPAN source sessions have one destination per session with an RSPAN
VLAN associated for that session.
Each RSPAN destination session has one or more destination interfaces
for each RSPAN VLAN that they support.
RSPAN destination sessions are limited to two, or one if a local SPAN or
a source RSPAN session is configured on the same switch.
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what does it mean a source session and  destionation session ?
they say that : "An RSPAN session is an association of source ports
across your network with an RSPAN VLAN. The destination source is the
RSPAN VLAN."
but i don't undersand what is a source session and a destination session


help please