Cisco Cisco Packet Data Gateway (PDG)
System Settings
▀ Configuring ASR 5000 Link Aggregation
▄ ASR 5000 System Administration Guide, StarOS Release 16
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Figure 9.
LAG with L2 Redundancy, Two Ethernet Switches
An LACP implementation with L2 redundancy cannot pass traffic even though standby ports have link up. For example,
with two QGLCs connected to two different Ethernet switches configured as a top-bottom pair and all ports in the same
LAG, failure of ports would not trigger a LAG switch until the active port number ratio flipped (more ports down than
up).
with two QGLCs connected to two different Ethernet switches configured as a top-bottom pair and all ports in the same
LAG, failure of ports would not trigger a LAG switch until the active port number ratio flipped (more ports down than
up).
To handle this case without requiring standby ports to pass LACP packets, separate instances of LACP are started on
redundant cards. The LAG manager monitors the two LACP instances and port link states to decide whether to auto-
switch (including automatic L2 port switching).
redundant cards. The LAG manager monitors the two LACP instances and port link states to decide whether to auto-
switch (including automatic L2 port switching).
Port States for Auto-Switch
Ports are classified in one of four states to determine whether to start auto-switching. See the table below.
For counters, State(x) represents the number of ports on a card in that state.
Table 5. Auto-Switch Port States
State
Counter
Description
Link
L(x)
Physical link up
Standby
S(x)
Link up but in standby mode
Waiting
W(x)
Waiting for Link Aggregation Control Protocol negotiation
Aggregated
A(x)
Aggregation formed
Hold Time
Once the LAG manager switches to another LACP instance, it does not consider another change for a short period to let
link and LACP negotiation settle down. This “hold time” is configurable.
link and LACP negotiation settle down. This “hold time” is configurable.
The LAG manager also enters/extends the hold period when an administrator manually switches ports to trigger a card
switch.
switch.