Is Cisco Root Switch the only one that sends Hello Messages

jskfan
jskfan used Ask the Experts™
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Is Cisco Root Switch the only one that sends Hello Messages

in a network with hierarchical switches , do all switches send Hello messages to their neighbors or  only the Root switch will do that ?

Thank you
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atlas_shudderedSr. Network Engineer

Commented:
Do you mean bpdu's?
atlas_shudderedSr. Network Engineer

Commented:
If so then no, all switches will send bpdus out as this is how the STP topology is maintained.

Author

Commented:
I believe BPDUs  are sent by the Root switch only , other switches will just relay them to other switches downstream

Regrading Hello messages every 2 seconds, I do not know if each switch is supposed to send hello message to its neighbor...

There is  kind of confusion when it comes to timers.

Max age timer :20 sec by default
Message age : the downstream switches from the root will decrement 1 off the Max age. example : 20-1 = 19 sec

Hello message :ever 2 sec
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atlas_shudderedSr. Network Engineer

Commented:
Negative - All bridges will continue to send bpdu's.  This is how the topology tables are maintained and how STP detects topology changes.

There are changes to timers inherent to different flavors of STP and they are able to be manipulated within the protocol config itself.

Regarding the notes you post up on timer age out.  This is accurate and it underscores the need to be aware of and sensitive to, BPDU traffic and activity on a network.  There have been recorded instances where timeouts have dropped and caused a physical topology to actually have two seperate logical topologys to form and then cause problems toward the middle of the domain, resulting in symptoms similar to looping.

Check the following two links for more information:

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html

https://www.dummies.com/programming/networking/cisco/spanning-tree-protocol-stp-introduction/

Author

Commented:
Let's say SW1 is the Root SW2  contected to SW1 and SW3, SW3 connected to SW1 and SW2

SW4 connected to SW5 and both connected to SW2
SW6 connected to SW7 and both connected to SW3

then  link between SW2 and SW1 fails

how are the timers going to interact  on the  Downstream Switches  ?
atlas_shudderedSr. Network Engineer

Commented:
Draw it out jsk.  Include all links.

Author

Commented:
I will make a diagram later.

By the way , in regards to timers , this Thread explains it clearly:

https://learningnetwork.cisco.com/thread/50291


It talks about BPDUs, and it says Downstream Switches relay BPDUs  received from Upstream..That was my initial understanding.
atlas_shudderedSr. Network Engineer

Commented:
They do, but all switches will send out their own bpdus as well.  Check the documents that I sent links to.

Author

Commented:
OK
what about the Max age, 20 Sec
if a switch  port  misses 3 Hellos (6 Sec) then it will  declare itself as Root.  So it will not wait 20 Sec... I mean what 's the usage of Max age (20 Sec)
atlas_shudderedSr. Network Engineer

Commented:
Hello timer is the period of time between bpdu's being sent from a switch.  Translate - how often bpdus are sent.

Max age is the maximum amount of time that a given switchport will store its bpdu information configuration.  Translation - how long to wait before declaring information stale and potential for a topology change.   This does not mean the switch will declare itself root, just that a new election is possible.

I need to go back to my prior comment as well.  Sorry, I've been answering on the fly and not paying close attention to detail.

STP operation.

1.  A switch is brought online, switch floods bpdus out all ports and listens to all ports for incoming bpdus.
2.  Election process initiates (if lower priority in new switch than existing root) - Listening mode
3.  Topology update occurs (election of local root and designated ports) and any necessary shifts of traffic due to update
4.  Root continues to generate BPDUs (Hello Timer 2 seconds default) - all other hosts propogate root BPDUs but cease send their own (this goes back to your question prior I think - sorry for the misinfo).
5.  As each lower switch receives BPDUs, increment age of BPDU by 1.  This increment tells switches how many hops away from root they are.
6.  Far switch hits max age timeout on Root port (20 second max age timeout).  Floods network with bpdus - go to 2

I think that about covers everything you have asked.

Author

Commented:
atlas_shuddered

Thanks for the Detailed info..
I just wanted to know if  the 3 Missed Hellos that will initiate the new Root election  ?

if So, then when the Max-age of 20 Seconds is reached then what happens when everything is fine and what happens when one of the switches as we stated has missed 3 Hellos...I mean  in either  case switches still  flush their BPDU information every 20 Seconds ?
Sr. Network Engineer
Commented:
Couple of things.  First, you have to remember that the 3 missed hellos are a function introduced for RSTP.  Original STP is 10.

Also, this link may help to get you some more clarity:

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html

That said, you also need to remember that the two functions are for two different event types when considered functionally.

3 missed hellos is for compensation of an upstream link loss to root to transition local blocking port to *new root port.  Remember that blocking ports are other ports that root bpdus have been received on previously but with a higher path cost.

20 second hold over timer is for complete loss of root in topology and transition/election of new root.

Does that help any?

Author

Commented:
OK..let 's see if I got it.

Let's say we have 3 Switches, connected in Triangle:

Switch1 is the Root
When a Root Port on Switch2 that is connected to Switch1 misses 3 Hellos ,  then Switch2  will announce itselsf as the Root, but if it hears a better BPDU from another Switch3 that has also connection to the Root switch, then Switch2 will just transition its connecting port to Switch3 to be Root Port.


For Max-Age 20 seconds, if I understand your Explanation, Switch2 and Switch3 both have to stop Receiving BPDUs from SWITCH1(Root) for 20 seconds, before they enter the Root election..in this case they flush their BPDU configuration  before they enter elections.
atlas_shudderedSr. Network Engineer

Commented:
In the first scenario, switch 2 will bring its blocked port to sw3 as a root port.  This does not necessarily lead to an election process.  Only if it hits the 20 second timer globally.  The 3 missed hellos is port specific.  20 second age out is global.

In scenario 2, either can stop receiving bpdu's but this loss has to be in total - no bpdus from anywhere.  The affected switch will then begin the election process and attempt to re-establish topology.

Author

Commented:
Thank You so much for Clarifications

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