Peter, if allow me to say so, your job is really nice. I understand that and it is the reason I said I have a strange behavior. SW2 should be blocking on interface fast0/24 for all Vlans.
After the erase configuration, the reset and the configuration again, it result in the expected behavior. Before erase it, I tried to just reset, but it did not help. I did not configure bridge priorities per Vlan, to have redundancy, as I would do to have different states per Vlan. Peter van Oene em 14/11/2002 11:55:52 Para: "[EMAIL PROTECTED]" cc: [EMAIL PROTECTED] Assunto: Re: Strange Spanning-Tree Behavior [7:57449] Sure.. To really get into this, you need to dig into graph theory and the use of various algorithms to calculate minimal spanning trees (or shortest spanning trees etc). Doing this will help you understand much more than STP as used in 802.1D, but also most routing protocols as well. Essentially, a given STP domain (set of bridging devices sharing a broadcast domain) will elect a single root bridge. From there, it is the responsibility of every other bridge in the network to calculate their shortest cost path to that root. To do so, each bridge examines all its ports and considers the BPDU messages received on them to figure out which of its N ports is the closest to the root. This port will always be set in a forwarding state. However, additionally, per LAN segment, a bridge will be chosen as a designated bridge and will be responsible for forwarding traffic toward the root for that particular segment. Which bridge is chosen depends upon which of the candidate bridges happens to be closest to the root. The elected designated bridge will set the port connected to the segment to a forwarding state. Hence, a given non root bridge may have a number of ports available, 1 of which is root, some of which might be designated, and the rest neither. At this point, all ports on the root bridge are in a forwarding state, all ports on segments for which a bridge was elected designated will be set to forwarding, and each device will set its closest port to the root to forwarding. It is now that the pruning begins. All ports that have not been set to forwarding, that is to say they have not been deemed part of the spanning tree, will be set to blocking. Consider the below picture. A----B | | | | C----D Assume that A is root (wins the election). First off, C, B and D will need to pick their port that is closest to the root bridge and set it as the root port. C will choose the north port, B the west, and D will have two choices. Based on comparison of priorities & such, D will chose one of the two, say B in this case, and thus D sets its north port to root. Now, each segment needs to elect a designated bridge per segment. In this case, A is not considered given it is the root bridge. In the C--D link, C will win by virtue of being closer to A and likewise B will win on the B--D link. Thus, C sets port east to Designated and B sets port South to Designated. At this time, the necessary ports have been selected to build the tree. The only port left is D's east port which is neither root, nor designated for the segment and thus does not belong in the tree. It is set to blocking. If you follow how this works (assuming I did a halfway decent job of describing it) you will see that on any given point to point link, one bridge will have to win the designated bridge election and therefore become part of the tree. However, it only takes the other bridge to block the port to effectively prune the link from the tree, thereby accomplishing the goal of the algorithm. Hope that helps! Pete On Thu, 2002-11-14 at 12:41, [EMAIL PROTECTED] wrote: > Could you explain? > > > > > > Peter van Oene em 14/11/2002 10:28:04 > > Para: "[EMAIL PROTECTED]" > cc: [EMAIL PROTECTED] > > Assunto: Re: Strange Spanning-Tree Behavior [7:57449] > > > Only one side of a given link will block and thus what you are seeing is > normal behavior. > > On Thu, 2002-11-14 at 11:00, [EMAIL PROTECTED] wrote: > > Does anybody know what is this? On interface fa0/24 of SW2, the state of > > spannnig-tree for Vlan1 was forwarding and for Vlan11 was blocking . > All > > the values, bridge priority - port priority - port cost, were the default > > values. I expected both Vlans as blocking. > > > > The network is: > > > > SW1(fa0/22) -------(fa0/22) SW2(fa0/24) > -------(fa0/24) > > SW3 > > (fa0/23)|__________________________________________|(fa0/24) > > > > After almost get crazy, I figured out that maybe the switch was crazy, so > I > > erased the configuration, reload the switch and typed the configuration > > back. And it works fine. > > > > Before that, I just reload the switches, and there was no changes. > > > > > > SW2#sh spann int fa 0/24 > > Interface Fa0/24 (port 38) in Spanning tree 1 is FORWARDING > > Port path cost 19, Port priority 128 > > Designated root has priority 32768, address 0002.fd34.9180 > > Designated bridge has priority 32768, address 0005.326d.65c0 > > Designated port is 38, path cost 19 > > Timers: message age 0, forward delay 0, hold 0 > > BPDU: sent 31481, received 604975 > > > > SW3#sh spann int fa 0/24 > > Interface Fa0/24 (port 38) in Spanning tree 1 is BLOCKING > > Port path cost 19, Port priority 128 > > Designated root has priority 32768, address 0002.fd34.9180 > > Designated bridge has priority 32768, address 0005.326d.65c0 > > Designated port is 38, path cost 19 > > Timers: message age 2, forward delay 0, hold 0 > > BPDU: sent 605190, received 32565 > > > > > > > > SW 1 > > > > > > host SW1 > > ! > > interface FastEthernet0/1 > > duplex half > > speed 10 > > switchport access vlan 11 > > ! > > interface FastEthernet0/2 > > duplex half > > speed 10 > > ! > > interface FastEthernet0/3 > > duplex half > > speed 10 > > ! > > interface FastEthernet0/4 > > duplex half > > speed 10 > > ! > > interface FastEthernet0/5 > > duplex half > > speed 100 > > ! > > interface FastEthernet0/22 > > description *** connection to SW2 *** > > switchport mode trunk > > ! > > interface FastEthernet0/23 > > description *** connection to SW3 *** > > switchport mode trunk > > ! > > Int vlan 1 > > ip ad 1.1.1.254 255.255.255.0 > > > > > > SW 2 > > > > host SW2 > > ! > > interface FastEthernet0/22 > > description *** connection to SW1 *** > > switchport mode trunk > > ! > > interface FastEthernet0/24 > > description *** connection to SW3 *** > > switchport mode trunk > > ! > > Int vlan 1 > > ip ad 2.2.2.254 255.255.255.0 > > > > SW 3 > > > > host SW3 > > ! > > interface FastEthernet0/23 > > description *** connection to SW1 *** > > switchport mode trunk > > ! > > interface FastEthernet0/24 > > description *** connection to SW2 *** > > switchport mode trunk > > ! > > Int vlan 1 > > ip ad 3.3.3.254 255.255.255.0 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=57463&t=57449 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
