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STP, bridge root, and 2948 issue?

ptlinvaptlinva Member Posts: 125
in CCNA & CCENT
Today's lesson is playing around with STP.

I've setup three switches with connecting ethernet cables and I'm having problems figuring out why one of the switches was elected the 'root'.
I'm using a 2948 and (2) 2950's.

Each of the 2950's becomes the root for VLANs that are only on those switches. However, the 2948 becomes the 'root' for VLAN1.

It seems that the 2950's will use the default (32768 PLUS the vlan no#) as its priority (BID?). However, it seems that the 2948 doesn't follow these rules of adding in the VLAN number.

S1 2950 MAC: 0009.7c77.7100
S2 2950 MAC: 000a.f4a9.b5c0
S3 2948 MAC: 0050.14dc.c400

It seems to me that the S1 2950 should have won.

ps. The 2948 is running a different OS (8.4(10)GLX) and I was able to download the latest one on Cisco's site and update it.

Am I missing something here? Thanks for taking the time to look!
-Paul

Here's the output...

S1 2950 Output
=======================
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address 0050.14dc.c400
Cost 19
Port 16 (FastEthernet0/16)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0009.7c77.7100
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
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
Fa0/13 Desg FWD 19 128.13 P2p
Fa0/16 Root FWD 19 128.16 P2p
Fa0/24 Desg FWD 19 128.24 P2p
=======================

S2 2950 Output
=======================
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address 0050.14dc.c400
Cost 19
Port 16 (FastEthernet0/16)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.f4a9.b5c0
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 100 128.1 Shr
Fa0/5 Desg FWD 100 128.5 Shr
Fa0/6 Desg FWD 100 128.6 Shr
Fa0/11 Altn BLK 19 128.11 P2p
Fa0/12 Altn BLK 19 128.12 P2p
Fa0/13 Altn BLK 19 128.13 P2p
Fa0/16 Root FWD 19 128.16 P2p
Interface Role Sts Cost Prio.Nbr Type
---- ---
Fa0/24 Desg FWD 100 128.24 Shr
=======================

2948 Output
=======================
Console> (enable) show spantree
VLAN 1
Spanning tree mode RAPID-PVST+
Spanning tree type ieee
Spanning tree enabled
Designated Root 00-50-14-dc-c4-00
Designated Root Priority 32768
Designated Root Cost 0
Designated Root Port 1/0
Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Bridge ID MAC ADDR 00-50-14-dc-c4-00
Bridge ID Priority 32768
Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Port State Role Cost Prio Type
----
----
2/1 not-connected - 100 32
2/2 not-connected - 100 32
2/3 not-connected - 100 32
2/4 not-connected - 100 32
2/5 not-connected - 100 32
2/6 not-connected - 100 32
2/7 not-connected - 100 32
2/8 not-connected - 100 32
2/9 forwarding DESG 19 32 P2P, PEER(STP)
2/10 not-connected - 100 32
2/11 forwarding DESG 19 32 P2P, PEER(STP)

Thanks again!
-Paul
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Comments

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    ptlinvaptlinva Member Posts: 125
    After playing around with it a bit more, I'm convinced that it's a 2948 OS issue in the way it calculated the BID priority. That's what I get for a non-IOS router.

    I issued the command
    set spantree priority 35000 1
    and the S1 2950 was promoted to root.

    It seems to me that setting the 'root' manually would be the smart thing to do when configuring a growing network. Do you agree?

    I've got (2) more switches on the way, so I'll be glad when I the 2948 goes to the bottom of the available switches.

    BTW, if this was a CISCO switch acting this way, what does this say for NON-CISCO switches and conforming to STP guidelines/IEEE standards? I thought dot1q/stp was a "standard".

    Can I expect to see more of this as I move up in the Cisco world?

    Your friend in Virginia,
    Paul Laudenslager
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  • Options
    ptlinvaptlinva Member Posts: 125
    Some routers (software earlier than Release 12.1(9)EA1) do NOT support the extended system ID.

    Software later than Release 12.1(9)EA1 does support the extended system ID.

    Cisco quote...
    ===============
    If your network consists of switches that both do and do not support the extended system ID, it is unlikely that the switch with the extended system ID support will become the root switch. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches running older software.
    ===============

    Conclusion: From my lab experience, you should ALWAYS set the "root" manually.

    BTW, I'm sorry to tie up the list with my troubleshooting. Thanks for bearing with me.

    Your friend in Virginia,
    Paul Laudenslager
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