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Cytoplast2281
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Port with disabled RSTP

Mon Dec 05, 2022 5:09 pm

Hi guys,

I'm now using a CRS326-24G and a CRS312-4C+8XG with both running SwOS 2.13. Currently I'm connecting one Ethernet port on CRS326-24G to our ISP. Due to the restriction from ISP, no STP traffic should be sent to ISP via that port. So I disabled RSTP for that port. No VLAN tag to ISP.
However, I found that CRS312-4C+8XG cannot learn the MAC of ISP gateway via RSTP. The two switches are connected with VLAN tag 100. It's important for me, because my virtual router is attached to CRS312-4C+8XG. Here are questions:
  • Is it possible for SwOS to make the MAC address attached to an edge port propagated to other switches via RSTP? Dynamical or statical approaches are both welcome.
  • If not possible, can I assign that ISP Ethernet port as the root of RSTP? In this way, any STP traffic won't be sent through that port.
This is the topology of my homelab. Your input is always appreciated.
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mkx
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Re: Port with disabled RSTP

Mon Dec 05, 2022 5:47 pm

STP (and RSTP in particular) has nothing to do with MAC learning. It's about loop detection ...
Each switch performs own MAC learning. And virtual router has to learn ISP gateway's MAC address using some of standard ways (e.g. by using "ARP who has" handshake or by PPPoE PADO reply or ...).
 
Cytoplast2281
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Re: Port with disabled RSTP

Tue Dec 06, 2022 11:37 am

STP (and RSTP in particular) has nothing to do with MAC learning. It's about loop detection ...
Each switch performs own MAC learning. And virtual router has to learn ISP gateway's MAC address using some of standard ways (e.g. by using "ARP who has" handshake or by PPPoE PADO reply or ...).
Thank you very much for your explanation. I did make a mistake about RSTP, and it's indeed just for loop detection / prevention. My bad :)
My ISP actually uses static IP address to access without any authentication, so I guess only ARP is involved there. At least MAC address learning is still very important in my scenario.
Actually I did notice that RSTP might have something to do with MAC address learning. When I disable RSTP for that ISP port on CRS326-24G , the ISP network works without problem ( I tested with a router directly connected to CRS326-24G). However, the MAC address of ISP port cannot be learned by CRS312-4C+8XG. And when I enable RSTP on that ISP port, the ISP gateway MAC address is not even learnt by CRS326-24G. That's the reason why I thought there might be an issue with RSTP for that.
Maybe it's better for me to convert it to another question: what's the best practice to configure a switch port as an edge port (access-only port) on SwOS? Simply disabling RSTP?
 
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mkx
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Re: Port with disabled RSTP

Tue Dec 06, 2022 7:35 pm

Yes, quite a few ISPs use CPEs which get very upset upon receiving LLDP frames and quickly disable that port.

I've no idea about how to configure a port as edge port in SwOS. Disabling xSTP on that port sounds quite right. If you don't find a better way and you're confident that there won't be a loop involving that CRS326, then you can entirely disable (all dialects of) STP on the switch.

Regarding MAC learning: if switches don't learn MAC address even though nothing is filtered, then they'll flood all ports with traffic destined to "not learned" MAC address. Not ideal, but at least traffic should not be blocked. There's something wrong if switches don't learn MACs though and you really should investigate why. And most probably the fault lies in the switch which doesn't learn .... sometimes a cold boot helps.

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