Root Bridge has Backup port.

[Aravind002]

Hi Everyone,

Cisco switch (Catalyst 9000 series switch running is IOS v17.x.x) was making one of the port to be the backup port even though it was a root bridge on specific VLAN (that back port was running in full duplex, it is the fiber port connected using GBIC transceiver) and sometimes, it is making all the ports as Designated port while it was a root bridge which it is the usual STP behavior but two fiber ports in the switch was in blocking state! why these are happening?

This Cisco switch was connected with third-party switches, I know the Cisco switch will generate standard BPDUs which are untagged and will be received on native-vlan, this will be used by third-party switches because third party switches will not understand the BPDUs generated from Cisco for specific VLANs and those BPDUs will only be understood by other Cisco switches. please correct If I'm wrong.

Thank You!

[Aravind002]

Yes, I guessed the same, it leads to the third-party switch (these third-party switches are interconnected in Ring)

[MHM Cisco World]

Show spanning tree g1/1/4 detail

Share this let see if port send recieve bpdu or not

[Aravind002]

I do checked using debug command, this switch is not receiving any BPDUs on VLAN 1 but receiving BPDU on VLAN 128 and 129!

[MHM Cisco World]

SW#debug spanning-tree bpdu receive

can you share this please ?
also please share the 
show vlan 
show interface trunk 

[Aravind002]

Unfortunately, I will not be near this switch rest of the day. So, please find the O/P of debug spanning-tree bpdu receive command below:

Spanning Tree BPDU Received debugging is on

TXO-AJAXCNTLRM-9200-S01#STP: pak->vlan_id: 3

STP: pak->vlan_id: 666

STP: pak->vlan_id: 2177

STP: pak->vlan_id: 130

STP: pak->vlan_id: 132

STP: pak->vlan_id: 134

STP: pak->vlan_id: 128

*Apr 19 17:55:06.319: STP: VLAN0128 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/1/4  , linktype SSTP , enctype 3, encsize 22

*Apr 19 17:55:06.319: STP: enc  1  0  C CC CC CD 3C 26 E4 DD A4 B3  0 32 AA AA  3  0  0  C  1  B

*Apr 19 17:55:06.320: STP: Data      0 0 2 23C 0803C26E4DDA480 0 0 0 0 0803C26E4DDA4808033 0 014 0 2 0 F 0

*Apr 19 17:55:06.321: STP: VLAN0128 Gi1/1/4: 0 0  2  2 3C  0803C26E4DDA480  0 0 0 0  0803C26E4DDA480 8033  0 0 14 0  2 0  F 0

*Apr 19 17:55:06.321: RSTP(128): Gi1/1/4 repeated msg

*Apr 19 17:55:06.321: RSTP(128): Gi1/1/4 rcvd info remaining 6STP: pak->vlan_id: 129

*Apr 19 17:55:06.322: STP: VLAN0129 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/1/4  , linktype SSTP , enctype 3, encsize 22

*Apr 19 17:55:06.322: STP: enc  1  0  C CC CC CD 3C 26 E4 DD A4 B3  0 32 AA AA  3  0  0  C  1  B

*Apr 19 17:55:06.323: STP: Data      0 0 2 23C 0813C26E4DDA480 0 0 0 0 0813C26E4DDA4808033 0 014 0 2 0 F 0

*Apr 19 17:55:06.323: STP: VLAN0129 Gi1/1/4: 0 0  2  2 3C  0813C26E4DDA480  0 0 0 0  0813C26E4DDA480 8033  0 0 14 0  2 0  F 0

*Apr 19 17:55:06.324: RSTP(129): Gi1/1/4 repeated msg

TXO-AJAXCNTLRM-9200-S01#

*Apr 19 17:55:06.324: RSTP(129): Gi1/1/4 rcvd info remaining 6STP: pak->vlan_id: 30

*Apr 19 17:55:06.537: STP: VLAN0030 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/0/47  , linktype IEEE_SPANNING , enctype 2, encsize 17

*Apr 19 17:55:06.537: STP: enc  1 80 C2  0  0  0 BC DB  9 90 C1 22  0 27 42 42  3

*Apr 19 17:55:06.537: STP: Data      0 0 2 23C80 0BCDB 990C0F8 0 0 0 080 0BCDB 990C0F8802A 0 014 0 2 0 F 0

*Apr 19 17:55:06.538: STP: VLAN0030 Gi1/0/47: 0 0  2  2 3C 80 0BCDB 990C0F8  0 0 0 0 80 0BCDB 990C0F8 802A  0 0 14 0  2 0  F 0

*Apr 19 17:55:06.538: RSTP(30): Gi1/0/47 repeated msg

*Apr 19 17:55:06.539: RSTP(30): Gi1/0/47 rcvd info remaining 6STP: pak->vlan_id: 1

STP: pak->vlan_id: 1

STP: pak->vlan_id: 3

STP: pak->vlan_id: 666

STP: pak->vlan_id: 2177

STP: pak->vlan_id: 130

STP: pak->vlan_id: 132

STP: pak->vlan_id: 134

STP: pak->vlan_id: 128

*Apr 19 17:55:08.320: STP: VLAN0128 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/1/4  , linktype SSTP , enctype 3, encsize 22

*Apr 19 17:55:08.320: STP: enc  1  0  C CC CC CD 3C 26 E4 DD A4 B3  0 32 AA AA  3  0  0  C  1  B

*Apr 19 17:55:08.321: STP: Data      0 0 2 23C 0803C26E4DDA480 0 0 0 0 0803C26E4DDA4808033 0 014 0 2 0 F 0

*Apr 19 17:55:08.321: STP: VLAN0128 Gi1/1/4: 0 0  2  2 3C  0803C26E4DDA480  0 0 0 0  0803C26E4DDA480 8033  0 0 14 0  2 0  F 0

*Apr 19 17:55:08.322: RSTP(128): Gi1/1/4 repeated msg

*Apr 19 17:55:08.322: RSTP(128): Gi1/1/4 rcvd info remaining 6STP: pak->vlan_id: 129

*Apr 19 17:55:08.323: STP: VLAN0129 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/1/4  , linktype SSTP , enctype 3, encsize 22

*Apr 19 17:55:08.324: STP: enc  1  0  C CC CC CD 3C 26 E4 DD A4 B3  0 32 AA AA  3  0  0  C  1  B

*Apr 19 17:55:08.324: STP: Data      0 0 2 23C 0813C26E4DDA480 0 0 0 0 0813C26E4DDA4808033 0 014 0 2 0 F 0

*Apr 19 17:55:08.325: STP: VLAN0129 Gi1/1/4: 0 0  2  2 3C  0813C26E4DDA480  0 0 0 0  0813C26E4DDA480 8033  0 0 14 0  2 0  F 0

*Apr 19 17:55:08.325: RSTP(129): Gi1/1/4 repeated msg

*Apr 19 17:55:08.325: RSTP(129): Gi1/1/4 rcvd info remaining 6STP: pak->vlan_id: 30

*Apr 19 17:55:08.564: STP: VLAN0030 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/0/47  , linktype IEEE_SPANNING , enctype 2, encsize 17

*Apr 19 17:55:08.565: STP: enc  1 80 C2  0  0  0 BC DB  9 90 C1 22  0 27 42 42  3

*Apr 19 17:55:08.565: STP: Data      0 0 2 23C80 0BCDB 990C0F8 0 0 0 080 0BCDB 990C0F8802A 0 014 0 2 0 F 0

*Apr 19 17:55:08.565: STP: VLAN0030 Gi1/0/47: 0 0  2  2 3C 80 0BCDB 990C0F8  0 0 0 0 80 0BCDB 990C0F8 802A  0 0 14 0  2 0  F 0

*Apr 19 17:55:08.566: RSTP(30): Gi1/0/47 repeated msg

*Apr 19 17:55:08.566: RSTP(30): Gi1/0/47 rcvd info remaining 6STP: pak->vlan_id: 1

STP: pak->vlan_id: 1

STP: pak->vlan_id: 3

STP: pak->vlan_id: 666

STP: pak->vlan_id: 2177

STP: pak->vlan_id: 130

STP: pak->vlan_id: 132

STP: pak->vlan_id: 134

STP: pak->vlan_id: 128

*Apr 19 17:55:10.319: STP: VLAN0128 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/1/4  , linktype SSTP , enctype 3, encsize 22

*Apr 19 17:55:10.320: STP: enc  1  0  C CC CC CD 3C 26 E4 DD A4 B3  0 32 AA AA  3  0  0  C  1  B

*Apr 19 17:55:10.320: STP: Data      0 0 2 23C 0803C26E4DDA480 0 0 0 0 0803C26E4DDA4808033 0 014 0 2 0 F 0

*Apr 19 17:55:10.321: STP: VLAN0128 Gi1/1/4: 0 0  2  2 3C  0803C26E4DDA480  0 0 0 0  0803C26E4DDA480 8033  0 0 14 0  2 0  F 0

*Apr 19 17:55:10.321: RSTP(128): Gi1/1/4 repeated msg

*Apr 19 17:55:10.321: RSTP(128): Gi1/1/4 rcvd info remaining 6STP: pak->vlan_id: 129

*Apr 19 17:55:10.322: STP: VLAN0129 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/1/4  , linktype SSTP , enctype 3, encsize 22

*Apr 19 17:55:10.322: STP: enc  1  0  C CC CC CD 3C 26 E4 DD A4 B3  0 32 AA AA  3  0  0  C  1  B

*Apr 19 17:55:10.322: STP: Data      0 0 2 23C 0813C26E4DDA480 0 0 0 0 0813C26E4DDA4808033 0 014 0 2 0 F 0

*Apr 19 17:55:10.323: STP: VLAN0129 Gi1/1/4: 0 0  2  2 3C  0813C26E4DDA480  0 0 0 0  0813C26E4DDA480 8033  0 0 14 0  2 0  F 0

*Apr 19 17:55:10.323: RSTP(129): Gi1/1/4 repeated msg

*Apr 19 17:55:10.323: RSTP(129): Gi1/1/4 rcvd info remaining 6STP: pak->vlan_id: 1

STP: pak->vlan_id: 30

*Apr 19 17:55:10.586: STP: VLAN0030 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/0/47  , linktype IEEE_SPANNING , enctype 2, encsize 17

*Apr 19 17:55:10.587: STP: enc  1 80 C2  0  0  0 BC DB  9 90 C1 22  0 27 42 42  3

*Apr 19 17:55:10.587: STP: Data      0 0 2 23C80 0BCDB 990C0F8 0 0 0 080 0BCDB 990C0F8802A 0 014 0 2 0 F 0

*Apr 19 17:55:10.587: STP: VLAN0030 Gi1/0/47: 0 0  2  2 3C 80 0BCDB 990C0F8  0 0 0 0 80 0BCDB 990C0F8 802A  0 0 14 0  2 0  F 0

*Apr 19 17:55:10.588: RSTP(30): Gi1/0/47 repeated msg

*Apr 19 17:55:10.588: RSTP(30): Gi1/0/47 rcvd info remaining 6STP: pak->vlan_id: 1

STP: pak->vlan_id: 3

STP: pak->vlan_id: 666

STP: pak->vlan_id: 2177

STP: pak->vlan_id: 130

STP: pak->vlan_id: 132

STP: pak->vlan_id: 134

STP: pak->vlan_id: 128

O/P of show vlan brief:

128  SCADA                            active    Gi1/0/1, Gi1/0/2, Gi1/0/3, Gi1/0/4, Gi1/0/5, Gi1/0/6, Gi1/0/7, Gi1/0/8, Gi1/0/9, Gi1/0/10, Gi1/0/11, Gi1/0/12, Gi1/0/13, Gi1/0/14, Gi1/0/15, Gi1/0/16

                                                Gi1/0/17, Gi1/0/18, Gi1/0/19, Gi1/0/20, Gi1/0/21, Gi1/0/22, Gi1/0/23, Gi1/0/24, Gi1/0/25, Gi1/0/26, Gi1/0/27, Gi1/0/28, Gi1/0/29, Gi1/0/30, Gi1/0/31

                                                Gi1/0/32, Gi1/0/33, Gi1/0/34, Gi1/0/35, Gi1/0/36, Gi1/0/37, Gi1/0/38, Gi1/0/39, Gi1/0/40, Gi1/0/41, Gi1/0/42, Gi1/0/43, Gi1/0/44, Gi1/0/45, Gi1/0/46, Gi1/1/2

129  VLAN0129                         active   

253  VLAN0253                         active   

254  VLAN0254                         active   

1002 fddi-default                     act/unsup

1003 token-ring-default               act/unsup

1004 fddinet-default                  act/unsup

1005 trnet-default                    act/unsup

2178 VLAN2178                         active   

2179 VLAN2179                         active   

O/P of sh interfaces trunk:

Port           Mode             Encapsulation  Status        Native vlan 

Gi1/0/48       on               802.1q         trunking      1

Gi1/1/1        on               802.1q         trunking      1

Gi1/1/3        on               802.1q         trunking      1

Gi1/1/4        on               802.1q         trunking      1

Port           Vlans allowed on trunk

Gi1/0/48       1-4094

Gi1/1/1        1-4094

Gi1/1/3        128-129

Gi1/1/4        128-129

Port           Vlans allowed and active in management domain

Gi1/0/48       1,30,128-129,253-254,2178-2179

Gi1/1/1        1,30,128-129,253-254,2178-2179

Gi1/1/3        128-129

Gi1/1/4        128-129

Port           Vlans in spanning tree forwarding state and not pruned

Gi1/0/48       1,30,128-129,253-254,2178-2179

Gi1/1/1        1,30,128-129,253-254,2178-2179

Gi1/1/3        128-129

Gi1/1/4        none

Note: I removed some information from these outputs above due to security reasons.

[MHM Cisco World]

*Apr 21 16:35:05.510: STP: VLAN0001 rx BPDU: config protocol = rstp, packet from Ethernet0/0 , linktype IEEE_SPANNING , enctype 2, encsize 17

*Apr 21 16:35:06.356: STP: VLAN0010 rx BPDU: config protocol = rstp, packet from Ethernet0/0 , linktype SSTP , enctype 3, encsize 22

*Apr 21 16:35:06.356: STP: VLAN0020 rx BPDU: config protocol = rstp, packet from Eth

I run lab 
there is four PVST bpdu receive, three for each VLAN (1,10,20) and other send without any tag (native VLAN) 
now we can not see 128 in debug I think this happened because the SW is root and it send not receive the bpdu 

so check the bpdu in other SW.

[Aravind002]

Thanks for running this test in your lab but I already noted all this behavior and I do checked the BPDUs in other Cisco switches which working fine as I expected. I don't know why these Root switch is acting weird, especially these two fiber ports in the switch. I cannot able to share more information on this due to security reasons, thanks for all your help.

[MHM Cisco World]

can I know what is brand of 3rd SW?

[MHM Cisco World]

PVST+ side (Cisco switch) sends untagged IEEE STP BPDUs corresponding to VLAN 1 (CST) STP to IEEE MAC address across the link. This is done for the purpose of joining the local VLAN 1 instance and the IEEE instance into CST. At the same time, VLAN 1 BPDUs are replicated to SSTP multicast address, tagged with VLAN 1 number (to inform other Cisco switches that VLAN 1 is non-native on our switch). Finally, BPDUs of the native VLAN instance (VLAN 100 in our case) are sent untagged using SSTP encapsulation and destination address. Of course, native VLAN100 BPDUs, (even though they are untagged) carry VLAN number inside a special TLV SSTP header.

*Apr 19 17:55:06.324: RSTP(129): Gi1/1/4 rcvd info remaining 6STP: pak->vlan_id: 30

*Apr 19 17:55:06.537: STP: VLAN0030 rx BPDU: config protocol = rstp, packet from GigabitEthernet1/0/47  , linktype IEEE_SPANNING , enctype 2, encsize 17