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dnordenberg
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RSTP question about backup port?

Fri Jun 11, 2021 5:38 pm

Hi!
I have three switches, first one is root bridge. To that one I have the two other switches connected via ethernet and a DSL line. Then I have a 4G modem connected to each MT device which each runs a EoIP tunnel in parallel back to root bridge switch. I set higher port path cost on each EoIP interface connected to the bridge in each MT device and setup works fine, EoIP over 4G is used when DSL fails but how can I control which which end get its ports forwarding and not forwarding? Right now the edge switches puts their EoIP interface in alternative role (not forwarding). I would rather make the root bridge switch put its EoIP ports in backup role (not forwarding) so broadcasts from root bridge switch does not need to be sent over 4G when path is not active. But I find no way how to do that, I cant get a port into backup role no matter what :( It is always the far side ports that get into their non forwarding role.
Best would be if both sides did not forward over EoIP/4G when not needed but RSTP might not be able to do that? Is it always just one side that is not forwarding?
Another thing about this behavior is that I can not easily tell from root bridge switch side which path is the active one :(

Kind regards
David
Last edited by dnordenberg on Fri Jun 11, 2021 8:10 pm, edited 1 time in total.
 
tdw
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Re: RTSP question about backup port?  [SOLVED]

Fri Jun 11, 2021 6:27 pm

Presumably you meant RSTP (Rapid Spanning Tree Protocol) and not RTSP (Real-Time Steaming Protocol) in the topic title.

Port designation is per the protocol design:
root-port - port that is facing towards the root bridge and will be used to forward traffic from/to the root bridge.
alternate-port - port that is facing towards root bridge, but is not going to forward traffic (a backup for root port).
backup-port - port that is facing away from the root bridge, but is not going to forward traffic (a backup for non-root port).
designated-port - port that is facing away from the root bridge and is going to forward traffic.
disabled-port - disabled or inactive port.

From your description working as intended, non-forwarding ports are always furthest away from the root.
 
dnordenberg
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Re: RSTP question about backup port?

Fri Jun 11, 2021 8:20 pm

Ah sorry fixed that :)

Ah, the last statement... I never understood that, great info :) But when is "backup port" used then? The ports facing the root bridge ports can never be the blocking ones then according to that so the role backup port is never ever used??
 
tdw
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Re: RSTP question about backup port?

Fri Jun 11, 2021 8:58 pm

I don't immediately have access to a complex enough setup to check, but I expect backup would apply to blocked links between leaf nodes.
 
dnordenberg
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Re: RSTP question about backup port?

Sun Jun 13, 2021 11:54 pm

Its ok, just curious :)
So from the root bridge device, there is absolutly no way to tell which way is active? From there I would like to have a script that notifies me when the 4G backup path goes active but I guess that can not be done easily then?
 
tdw
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Re: RSTP question about backup port?

Mon Jun 14, 2021 2:17 pm

AFAIK no, devices with ports facing towards root bridge elect which of those is passing traffic and which is blocking.
 
dnordenberg
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Re: RSTP question about backup port?

Mon Jun 14, 2021 5:49 pm

I ended up with a a ping monitoring script on the root bridge device which pings remote mt devices plus a ICMP L2 fw blocking rule for the EoIP interface so if ping is timed out then the DSL must be down and links are running on 4G backup. I already had a similar script which monitored the 4G/EoIP instead to make sure the backup path is online even when not used...

Thank you for your tips tdw :)

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