12-27-2022 07:48 AM
Hi,
I am referring to following image from Cisco Oficial Cert Guide:
What makes SW3 to change G1/0/2 from blocking to forwarding state? SW3 still receives the BPDUs from SW2, right? So based on what it changes its port state?
12-27-2022 08:01 AM - edited 12-27-2022 08:09 AM
SW2-SW3
SW2 send BPDU declare that it root SW for STP domain.
SW3 receive the BPDU but it already have superior BDPU (toward SW1 root) so the BLK -> FWD and SW3 start send BPDU to SW2.
I will try run lab and show you debug message.
12-27-2022 08:45 AM
What made previously (before link failure) sent BPDUs from SW2 to SW3 superior?
12-27-2022 09:11 AM
SW1 is root
SW2 receive the BPDU from SW1
SW3 receive the BPDU from SW1
SW2 and SW3 forward the BPDU to each other after modify BPDU.
NOW
SW2 have BPDU from SW1 (superior) and from SW3 (inferior)
SW3 have BPDU from SW1 (superior) and from SW2 (inferior)
but what make BPDU superior or inferior ?? the SW2/SW3 when modify the BPDU add cost to SW1, and as role root-port is the port with lowest cost-path.
cost from SW2/SW3 to SW1 << cost from SW2/SW3 to SW2/SW3 to SW1.
but how BLK is select ?
since SW2 and SW3 exchange BPDU to each other
the SW2(inferior) compare to SW3(inferior) and the port receive best inferior will be BLK and port receive worse will be Designated FWD.
now
what happened in your case
the SW2 dont receive any BPDU anymore from SW1 so it assume that it now root of STP so it start send BPDU (with SW2 as roor bridge ID).
the SW2 send BPDU to SW3 BUT SW3 already receive superior BPDU from SW1 root port. hence the SW3 refuse this new BPDU.
here the SW3 start send BPDU (superior BPDU receive from SW1) to SW2, and the port now change it status from BLK to FWD because ""port receive best inferior will be BLK and port receive worse will be Designated FWD. ""
12-27-2022 08:57 AM
Rather we invent the wheel again, let's take a look at good video from Keith he explained in more detail with the example below :
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