this was an interesting exercise, and it's not pretty... below is before dynamic desirable and before changing the native vlan...
Bridge ID Priority 20481 (priority 20480 sys-id-ext 1)
Address 0009.b752.d780
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/4 Desg FWD 19
now to change to dynamic desirable on both ends...
sw2950_01(config-if-range)#sw mode dyn des
sw2950_02(config-if-range)#sw mode dyn des
sw2950_02#sh int trunk
Port Mode Encapsulation Status Native vlan
Fa0/1 desirable 802.1q trunking 1
Fa0/2 desirable 802.1q trunking 1
sw2950_01#sh int trunk
Port Mode Encapsulation Status Native vlan
Fa0/1 desirable 802.1q trunking 1
Fa0/2 desirable 802.1q trunking 1
Fa0/3 desirable 802.1q trunking 1
Fa0/4 desirable 802.1q trunking 1
dtp packets are good...
1d23h: DTP-pkt:Fa0/2:Good DTP packet received: ../dyntrk/dyntrk_core.c:1451
1d23h: DTP-pkt:Fa0/2: Domain: ozlan ../dyntrk/dyntrk_core.c:1454
1d23h: DTP-pkt:Fa0/2: Status: TOS/TAS = TRUNK/DESIRABLE ../dyntrk/dyntrk_core.c
:1457
1d23h: DTP-pkt:Fa0/2: Type: TOT/TAT = 802.1Q/802.1Q ../dyntrk/dyntrk_core.c:145
9
1d23h: DTP-pkt:Fa0/2: ID: 0009B752D784 ../dyntrk/dyntrk_core.c:1462
1d23h: DTP-pkt:Fa0/1:Good DTP packet received: ../dyntrk/dyntrk_core.c:1451
1d23h: DTP-pkt:Fa0/1: Domain: ozlan ../dyntrk/dyntrk_core.c:1454
1d23h: DTP-pkt:Fa0/1: Status: TOS/TAS = TRUNK/DESIRABLE ../dyntrk/dyntrk_core.
now change the native vlan...
sw2950_02(config)#int range fa0/1 - 2
sw2950_02(config-if-range)#sw trunk native vlan 50
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg BKN*19 128.1 P2p *PVID_Inc
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/4 Desg BKN*19 128.4 P2p *PVID_Inc
dtp packets are still good
sw2950_01#
3w2d: DTP-pkt:Fa0/4: ID: 0009B73FCE82 ../dyntrk/dyntrk_core.c:1462
sw2950_01#
3w2d: DTP-pkt:Fa0/3:Good DTP packet received: ../dyntrk/dyntrk_core.c:1451
3w2d: DTP-pkt:Fa0/3: Domain: ozlan ../dyntrk/dyntrk_core.c:1454
3w2d: DTP-pkt:Fa0/3: Status: TOS/TAS = TRUNK/ON ../dyntrk/dyntrk_core.c:1457
3w2d: DTP-pkt:Fa0/3: Type: TOT/TAT = 802.1Q/802.1Q ../dyntrk/dyntrk_core.c:1459
3w2d: DTP-pkt:Fa0/3: ID: 000F8FFE0983 ../dyntrk/dyntrk_core.c:1462
here comes the nasty vlan errors...
sw2950_01#
3w2d: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthern
et0/1 (1), with sw2950_02 FastEthernet0/1 (50).
and this doesn't look too good... i couldn't ping and lost connectivity...
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg BKN*19 128.1 P2p *PVID_Inc
Fa0/2 Desg BKN*19 128.2 P2p *PVID_Inc
my conclusion is that although dtp is passing good packets, doesn't matter... the trunk is no good... then i changed the native vlan back, and...
sw2950_02#
1d23h: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/1 on VLAN0001.
Port consistency restored.
1d23h: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/1 on VLAN0050.
Port consistency restored.
1d23h: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/2 on VLAN0001.
Port consistency restored.
1d23h: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/2 on VLAN0050.
Port consistency restored.
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Altn BLK 19 128.2 P2p
so dtp still works, but the trunk is broken...
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