It is currently Fri Aug 17, 2018 5:24 pm


All times are UTC




Post new topic Reply to topic  [ 6 posts ] 
Author Message
 Post subject: STP root port election error?
PostPosted: Mon Jun 10, 2013 8:11 pm 
Offline

Joined: Sat Nov 10, 2012 8:51 pm
Posts: 6
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 3786 times ]
Top
 Profile  
 
 Post subject: Re: STP root port election error?
PostPosted: Mon Jun 10, 2013 8:35 pm 
Offline
Site Admin

Joined: Sat Oct 11, 2008 1:41 pm
Posts: 2668
Location: Canada
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,

_________________
Jeremy, GNS3 Programmer & Benevolent Dictator for Life.


Top
 Profile  
 
 Post subject: Re: STP root port election error?
PostPosted: Mon Jun 10, 2013 9:19 pm 
Offline

Joined: Sat Nov 10, 2012 8:51 pm
Posts: 6
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?


Top
 Profile  
 
 Post subject: Re: STP root port election error?
PostPosted: Mon Jun 10, 2013 10:18 pm 
Offline
Site Admin

Joined: Sat Oct 11, 2008 1:41 pm
Posts: 2668
Location: Canada
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/

_________________
Jeremy, GNS3 Programmer & Benevolent Dictator for Life.


Top
 Profile  
 
 Post subject: Re: STP root port election error?
PostPosted: Tue Jun 11, 2013 7:40 am 
Offline

Joined: Sat Nov 10, 2012 8:51 pm
Posts: 6
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


Top
 Profile  
 
 Post subject: Re: STP root port election error?
PostPosted: Tue Jun 11, 2013 5:48 pm 
Offline

Joined: Tue Jul 17, 2012 1:34 pm
Posts: 214
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



_________________
J.Neumann, GNS3 Contributor & Forum Moderator (Minion ID: sin.zx81.1981ad)
Author: Cisco Routers for Small Business (Apress)


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 6 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 3 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group

phpBB SEO