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New Member

3750 stack master failure

We have a stack of a 3750E with a 3750X, when we turn off the master, a disconnection occurs for around one minute while only one request timeout is seen when the slave is turned off. Is this normal?

Details: in other words, when the slave switch is elected as the new master all the port-channels and some physical ports become in 'down' state before reobtaining the up state again, this results in a one minute outage. The question is: is it normal to have a layer2 control traffic interruption when the master fails and re-election occurs and if yes, what is the normal downtime in seconds. the layer2 control information such as port-channels LACP protocol are not synchronised to all slave switches by the master?

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7 REPLIES
VIP Super Bronze

3750 stack master failure

One minute between failure is too long.  Here are some numbers from Cisco's documentation:

On stack master failure, another switch in the stack takes over

1:N master redundancy

Reconvergence times tested under heavy load:

Layer 1 failure is detected in several microseconds

Layer 2 failure ~ mseconds

Layer 3 link failure—sub 200 mseconds

Layer 3 member failure—sub 300 mseconds

Layer 3 master failure—up to eight seconds

HTH

New Member

3750 stack master failure

Hi Reza,

Layer 2 failure ~ mseconds is when the master switch fails?

BR

VIP Super Bronze

3750 stack master failure

Correct. This is when the master fails.

New Member

3750 stack master failure

ok, do you think that 'stack-mac persistent timer 0' command which is still not implemented will help in decreasing the downtime from 1 minute to mseconds or it will just decrease the time needed to reach the management IP again?

VIP Super Bronze

3750 stack master failure

I know enabling it defiantly helps when the devices are running layer-3 routing protocols like OSPF

I know your devices are layer-2, but I would try anyway

please keep us posted on the results.

HTH

New Member

3750 stack master failure

i think it should help also for layer2, this is what i found in my research :

With LACP, the system-id uses the stack MAC address from the stack master, and if the stack master changes, the LACP system-id can change. If the LACP system-id changes, the entire EtherChannel will flap, and there will be an STP reconvergence. Taken from the following reference:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750x_3560x/software/release/15.0_1_se/configuration/guide/3750X_scg.pdf

I will update u after doing the test

many thanks

New Member

3750 stack master failure

test done and what was mentioned in the previous post resolved the problem

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