Understanding Topology Change Notification on Switches
Understanding Topology Change Notification on Switches
Reading this article:https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html#anc12
If I am getting it right TCN can also be caused by the Hosts that are connected to Access Switches, the article states that this can be avoided by configuring Portfast on interfaces to which the Hosts are connected to .
As far as I know, not all environments configure Portfast on the Interfaces where Hosts connect to, at least the environment where I work. However there is no issue reported even if there was TCN .
Any Switch Expert to shade some lights on this topic.
Thank you
Reading this article:https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html#anc12
If I am getting it right TCN can also be caused by the Hosts that are connected to Access Switches, the article states that this can be avoided by configuring Portfast on interfaces to which the Hosts are connected to .
As far as I know, not all environments configure Portfast on the Interfaces where Hosts connect to, at least the environment where I work. However there is no issue reported even if there was TCN .
Any Switch Expert to shade some lights on this topic.
Thank you
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Portfast is marking that end host is connect to prevent sending TCN. BPDUs are sent from every switch port not only on tunks.
ASKER
Mmmm..
So Cisco should enable Portfast By Default Right after you type Switch Port Mode Access command, then press enter
So Cisco should enable Portfast By Default Right after you type Switch Port Mode Access command, then press enter
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I included the diagram above, so that I will have clear understanding. No Portfast is configured yet on the Switch ports where PCs are plugged in :
TCN, if I am not wrong, can cause other switches to reduce the aging time of a specific Mac address ( Mac address of the port where the TCN originated)
in our example above if we turn off PC1 , this will generate TCN that will travel to the Root Switch. it will get acknowledged by the Root Switch and this latter will generate TCA, and will notify every switch , downstream, to reduce the Mac address of the port on which PC1 is connected ,from 5 minutes to 15 seconds.
Well, if that s the case... Will this TCN/TCA story have any palpable impact on other users. Or it will go seamless ?
It will cause aging tie reduce only in the case of STP, in the case of RSTP all MAC addresses are flushed without delay.
Effect depends on how many end devices are there in network. In larger networks in can be huge problem, that's why it is recommendation that VLANs should be kept local. In some design recommendations VLANs should not be going out of rack and routing is moved to access level.
Effect depends on how many end devices are there in network. In larger networks in can be huge problem, that's why it is recommendation that VLANs should be kept local. In some design recommendations VLANs should not be going out of rack and routing is moved to access level.
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Predrag,
With RSTP
Looking at the Diagram above, if the Blocking Port is located on SW4 (port that is linked to SW3).
then If you unplug PC1 or PC2 from the Switch ports, it will not have any impact on the topology, because the same port on SW4 will stay Blocked and others will stay Forwarding.
Even if you take PC1 and PC2 out of the picture and replace them with switches , for instance (SW5 and SW6), assuming SW1 was Manually configured as a Root Switch, then SW5 and SW6 will not have any impact on the topology and that port on SW4 will still stay as Blocked.
Users will have no disruption.
With the Design on the Diagram above. I believe there will be no disruption even if you configure Portfast on the ports where PC1 and PC2 were connected to and replace them with Switches(SW5 and SW6)
With RSTP
Looking at the Diagram above, if the Blocking Port is located on SW4 (port that is linked to SW3).
then If you unplug PC1 or PC2 from the Switch ports, it will not have any impact on the topology, because the same port on SW4 will stay Blocked and others will stay Forwarding.
Even if you take PC1 and PC2 out of the picture and replace them with switches , for instance (SW5 and SW6), assuming SW1 was Manually configured as a Root Switch, then SW5 and SW6 will not have any impact on the topology and that port on SW4 will still stay as Blocked.
Users will have no disruption.
With the Design on the Diagram above. I believe there will be no disruption even if you configure Portfast on the ports where PC1 and PC2 were connected to and replace them with Switches(SW5 and SW6)
If I remember correctly that's not the case (I will have to double check it since those details are easily forgotten (and not easily found in documentation + many documentation has some wrong details)).
For example if PC1 port is going down and port itself is marked as edge port TCN will not be sent. However, if port is going into forwarding state and portfast is not configured, even if there is no real impact on topology TCN need to be sent to neighboring switches, minimum 2 neighboring switch's trunks would be affected (if TCN would stop there, depending on root bridge placement) would be impacted. So, neighboring switches would still have to flush all MAC addresses.
When I found time I will try to find documentation (since I don't have currently devices that support rstp in my home). :(
For example if PC1 port is going down and port itself is marked as edge port TCN will not be sent. However, if port is going into forwarding state and portfast is not configured, even if there is no real impact on topology TCN need to be sent to neighboring switches, minimum 2 neighboring switch's trunks would be affected (if TCN would stop there, depending on root bridge placement) would be impacted. So, neighboring switches would still have to flush all MAC addresses.
When I found time I will try to find documentation (since I don't have currently devices that support rstp in my home). :(
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