i'm in the middle of a spanning tree lab and because we are using plain old spanning tree with defaults on everything, my output is not exactly like that of the lab... the mac addresses of my switches are simply different...
we know that in spanning tree the switch with the lowest mac address will be root if the bridge priorities are the same (or in this case the default)... remember the priority is the first indicator, and in case of a tie, the mac address will determine root (the lowest wins)
big friggin deal, not much easier than that...
but now the question is, determine which switch will win the election without using spanning tree command output, or telnet between the switches to collect and compare mac's...
the sh ver command will give us the base mac of the switch we are on as below...
Base ethernet MAC Address: 00:0B:5F:C9:00:00... ok
cdp neighbor detail is no help... and mac-address table dynamic is confusing for our purpose here...
dls2#sh mac-add dynamic
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0009.b73f.ce80 DYNAMIC Fa0/9
1 0009.b73f.ce89 DYNAMIC Fa0/9
1 0009.b752.d780 DYNAMIC Fa0/9
1 000f.8ffe.098b DYNAMIC Fa0/11
1 000f.8ffe.098c DYNAMIC Fa0/12
Total Mac Addresses for this criterion: 5
dls2#
got it yet?
dls2#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.104 24 0009.b752.d780 ARPA Vlan1
Internet 10.1.1.102 - 000b.5fc9.0000 ARPA Vlan1
Internet 10.1.1.103 25 0009.b73f.ce80 ARPA Vlan1
Internet 10.1.1.101 25 000f.8ffe.0980 ARPA Vlan1
dls2#sh spann root
Root Hello Max Fwd
Vlan Root ID Cost Time Age Dly Root Port
---------------- -------------------- --------- ----- --- --- ------------
VLAN0001 32769 0009.b73f.ce80 19 2 20 15 Fa0/9
dls2#
sh spann root from dls2 tells us that it's NOT the root (good command by the way)
but looking back at the arp output and since we can add and subtract, we know 10.1.1.103 is the root...
dls2#sh spann
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0009.b73f.ce80
Cost 19
Port 9 (FastEthernet0/9)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000b.5fc9.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/7 Altn BLK 19 128.7 P2p
Fa0/8 Altn BLK 19 128.8 P2p
Fa0/9 Root FWD 19 128.9 P2p
Fa0/10 Altn BLK 19 128.10 P2p
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
sh spann from dls2 doesn't tell us it is root... the root id, and THIS bridge id are not the same... all ports are not designated and not forwarding....
looking at the output of spann root on als1 we see...
als1#sh spann root
Root Hello Max Fwd
Vlan Root ID Cost Time Age Dly Root Port
---------------- -------------------- ------ ----- --- --- ----------------
VLAN0001 32769 0009.b73f.ce80 0 2 20 15
cost 0 means this switch must be root and...
als1#sh spann
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0009.b73f.ce80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0009.b73f.ce80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/7 Desg FWD 19 128.7 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/9 Desg FWD 19 128.9 P2p
Fa0/10 Desg FWD 19 128.10 P2p
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
this bridge is the root, the mac's are the same and all ports are designated and forwarding...
spanning tree very pretty...
network cisco ccna gns3 certification arteq
a network runs through it
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