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STP root port election error?
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Author:  AldoBr [ Mon Jun 10, 2013 8:11 pm ]
Post subject:  STP root port election error?

Hi all,

I would like to ask you if I'm correctly understanding root port election in STP, and if GNS3 is doing it wrong.
The network I'm using is shown in attached Topology-STP.png

I am using 2691 routers with NM-16ESW cards and using them just as switches.
Really, the configuration of all of them is just a basic one where I just configured the trunks, the speed of links, hostnames, and STP priorities. Nothing more.

So let's focus on Switch3.

Switch3 is directly connected to Switch1 via f1/13, and this should have a cost of 100.
f1/14 instead sees an accumulated path cost of 19+19=38
Thus my opinion is that for Switch3 the root port is f1/14, while f1/13 should be blocking.

Well, according to GNS3 I'm wrong as I see this:

-------------------------------------------------------------------------------------------------------------------------
Switch3#show spanning-tree vlan 1 brief

VLAN1
Spanning tree enabled protocol ieee
Root ID Priority 8192
Address c036.3cd4.0000
Cost 19
Port 54 (FastEthernet1/13)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 65535
Address c038.3cd4.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/13 128.54 128 19 FWD 0 8192 c036.3cd4.0000 128.54
FastEthernet1/14 128.55 128 19 BLK 19 16384 c037.3cd4.0000 128.55
FastEthernet1/15 128.56 128 19 BLK 19 32768 c039.3cd4.0000 128.56

-------------------------------------------------------------------------------------------------------------------------

So, is GNS3 completely wrong when calculates port costs, or should I study this stuff deeper (better)?....

Attachments:
Topology-STP.png
Topology-STP.png [ 12.19 KiB | Viewed 4435 times ]

Author:  grossmj [ Mon Jun 10, 2013 8:35 pm ]
Post subject:  Re: STP root port election error?

Hi,

Nothing wrong in your logic, just make sure f1/13 on Switch3 is configured as a 10MB link:

Code:
Switch3(config)#int f1/13
Switch3(config-if)#bandwidth 10000

Switch3#show int f1/13
FastEthernet1/13 is up, line protocol is up
  Hardware is Fast Ethernet, address is c202.1be1.f10d (bia c202.1be1.f10d)
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 100 usec,

Switch3#show spanning-tree vlan 1 brief

VLAN1
  Spanning tree enabled protocol ieee
  Root ID    Priority    8192
             Address     c200.1be1.0000
             Cost        38
             Port        55 (FastEthernet1/14)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    65535
             Address     c202.1be1.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface                                   Designated
Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet1/13     128.54   128   100 BLK     0  8192 c200.1be1.0000 128.54
FastEthernet1/14     128.55   128    19 FWD    19 16384 c201.1be1.0000 128.55
FastEthernet1/15     128.56   128    19 BLK    19 32768 c203.1be1.0000 128.56


Cheers,

Author:  AldoBr [ Mon Jun 10, 2013 9:19 pm ]
Post subject:  Re: STP root port election error?

Ok my friend, that's the point.

But I thought that manually changing the speed to 10Mb should have automatically adjusted bandwidth.

So if I impose 10Mb speed, exported bandwidth is still 100Mb.
Is this true also for real switches?

Author:  grossmj [ Mon Jun 10, 2013 10:18 pm ]
Post subject:  Re: STP root port election error?

The bandwidth setting is used by protocols. Speed is a hardware setting (same thing with clock rate for serial interfaces).

Changing the speed on an interface does not change the configured bandwidth in GNS3.

http://www.firstdigest.com/2010/08/cisc ... e-command/

Author:  AldoBr [ Tue Jun 11, 2013 7:40 am ]
Post subject:  Re: STP root port election error?

mmmm.... no this time I have to disagree.
At least for a real Catalyst 4500 on which I just run a test.
Speed command automatically changed bandwidth.
After all I think it is logical that you cannot have a BW value higher than configured speed:



rtcc014#sho
rtcc014#show inte
rtcc014#show interfaces gi
rtcc014#show interfaces gigabitEthernet 1/30
GigabitEthernet1/30 is down, line protocol is down (notconnect)
Hardware is Gigabit Ethernet Port, address is d0d0.fd03.905d (bia d0d0.fd03.905d)
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto-speed, link type is auto, media type is 10/100/1000-TX
input flow-control is off, output flow-control is off
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out
rtcc014#
rtcc014#
rtcc014#
rtcc014#conf t
Enter configuration commands, one per line. End with CNTL/Z.
rtcc014(config)#
rtcc014(config)#
rtcc014(config)#
rtcc014(config)#inte
rtcc014(config)#interface gig
rtcc014(config)#interface gigabitEthernet 1/30
rtcc014(config-if)#spe
rtcc014(config-if)#speed 10
rtcc014(config-if)#end
rtcc014#
rtcc014#
rtcc014#
rtcc014#show interfaces gigabitEthernet 1/30
GigabitEthernet1/30 is down, line protocol is down (notconnect)
Hardware is Gigabit Ethernet Port, address is d0d0.fd03.905d (bia d0d0.fd03.905d)
MTU 1500 bytes, BW 10000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Half-duplex, 10Mb/s, link type is auto, media type is 10/100/1000-TX
input flow-control is off, output flow-control is off
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out
rtcc014#
rtcc014#




Anyway no big problem, since now on I will always manually set BW accordingly when using GNS3

Author:  Jason [ Tue Jun 11, 2013 5:48 pm ]
Post subject:  Re: STP root port election error?

Here's another good article on the topic if anyone's interested: Clarifying The Cisco IOS Bandwidth Command

http://www.techrepublic.com/article/clarifying-the-cisco-ios-bandwidth-command/6093972

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