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
jskfanAsked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

JustInCaseCommented:
All environments should implement portfast on non interswitch links. If portfast is not configured on end user ports there it causes unicast flooding in environment.

STP:
When TCN is received on switches configured with STP - value of Aging_time (MAC address table) is reduced to value of Forward_Time on all ports (any current state aging_time 15-300 seconds is reduced to 15 seconds). TCN is relevant only to switches, hosts do not change aging time for hosts in their MAC address table.
Issue is, that, if some end host A did not forward any traffic in 15 seconds after BPDU was received on switches its MAC address will be removed from all switches. If, later, some host B start to forward traffic to host A - MAC address of end host is not present in any switch MAC address table and all switches that are part of broadcast domain will do unicast flooding until host A itself start to transmit (all switches relearn switch A MAC address).
RSTP:
When TCN is received on switches with configured with RSTP - all MAC addresses will be flushed from switches. And has even worse effect on creating unicast storms, since all traffic, after MAC addresses on switches are flushed, will be flooded to all ports in domain until MAC addresses are relearned.
1

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
jskfanAuthor Commented:
I thought when you configure a Port as Access Port it does not generate Neither BPDUs nor TCN as long as a Computer is connected to it, but if you plug a Switch to it then it will negotiate a Trunk and participate in STP.
0
atlas_shudderedSr. Network EngineerCommented:
A port set to mode access will still go through STP listening/negotiation process when a device is connected.  When portfast is enabled, the listening phase is bypassed and the device will go straight to forwarding.  In both cases, BPDUs will still be generated.  This is why cisco recommends all ports configured access be configured with bpdu guard as well, or bpdu filter and root guard.  See output below for STP output from both config types:

Access port only:

Switch#show run int gig 1/0/34
Building configuration...

Current configuration : 174 bytes
!
interface GigabitEthernet1/0/34
 switchport access vlan 203
 switchport mode access
 spanning-tree portfast
end


Switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#int gig 1/0/34
Switch(config-if)#int gig 1/0/34
Switch(config-if)#shut
Switch(config-if)#no spann portf
Switch(config-if)#no shut
Switch(config-if)#do show spann | inc 1/0/34
:::
Switch(config-if)#do show spann | inc 1/0/34
Gi1/0/34            Desg LIS 4         128.34   P2p 
Switch(config-if)#do show spann | inc 1/0/34
Gi1/0/34            Desg LIS 4         128.34   P2p 
:::
Switch(config-if)#do show spann | inc 1/0/34
Gi1/0/34            Desg LRN 4         128.34   P2p 
Switch(config-if)#do show spann | inc 1/0/34
Gi1/0/34            Desg LRN 4         128.34   P2p 
:::
Switch(config-if)#do show spann | inc 1/0/34
Gi1/0/34            Desg FWD 4         128.34   P2p 

Open in new window



Access + Portfast:

Switch(config-if)#shut
Switch(config-if)#spann portf
%Warning: portfast should only be enabled on ports connected to a single
 host. Connecting hubs, concentrators, switches, bridges, etc... to this
 interface  when portfast is enabled, can cause temporary bridging loops.
 Use with CAUTION

%Portfast has been configured on GigabitEthernet1/0/34 but will only
 have effect when the interface is in a non-trunking mode.
Switch(config-if)#no shut                   
Switch(config-if)#do show spann | inc 1/0/34
Gi1/0/34            Desg FWD 4         128.34   P2p Edge 

Open in new window

0
Ultimate Tool Kit for Technology Solution Provider

Broken down into practical pointers and step-by-step instructions, the IT Service Excellence Tool Kit delivers expert advice for technology solution providers. Get your free copy now.

JustInCaseCommented:
Portfast is marking that end host is connect to prevent sending TCN. BPDUs are sent from every switch port not only on tunks.
0
jskfanAuthor Commented:
Mmmm..
So Cisco should enable Portfast By Default Right after you type Switch Port Mode Access  command, then press enter
0
atlas_shudderedSr. Network EngineerCommented:
no, you have to enter the command:

spanning-tree portfast
0
jskfanAuthor Commented:
sw
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 ?
0
JustInCaseCommented:
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.
0
jskfanAuthor Commented:
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)
0
JustInCaseCommented:
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). :(
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Switches / Hubs

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.