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Questions tagged [spanning-tree]

For questions about Spanning Tree Protocol (STP), a common protocol to prevent bridge loops at OSI layer-2.

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What Does Exactly Happen When A Switch Is Connected To A Portfast Port?

I am trying to get a thorough understanding of what does happen when a switch is connected to a portfast port on another switch that is part of a stable STP network in case of: 1- the port has no ...
1 vote
2 answers
160 views

STP TCN at root port role change

I am facing a situation in our work topology, and I will try to simplify the question as much as possible. I have this non-root switch (SW1) that has two ports connected to separate switches in a ...
6 votes
2 answers
370 views

Why FastEthernet STP Link is Selected over EtherChannel?

What are the possible reasons why STP would choose to use a FastEthernet link over an EtherChannel? Below is a network diagram where this is happening.
0 votes
0 answers
54 views

Dell VLT (MLAG) and Root Bridge Priority

I'm not a network guy... We'll be deploying some Dell switches configured for VLT soon. From what I understand, this is just Dell's version of MLAG. Correct me if I'm wrong, but I'm pretty sure that I ...
2 votes
2 answers
80 views

STP related question

Hi this is a portion of my current set up.. All vlan ports are untagged. Everything works and communicates fine until a connection is made between the 2 switches it takes down the network and the pc ...
1 vote
1 answer
59 views

Will implementing a MSTP stack allow me to use that interchangeably between STP and RSTP

I've to provide support for STP/RSTP/MSTP, while going through the specification I see the Force Protocol Version majorly ensures compatibility between STP/RSTP bridges. Mostly, I see the transmission ...
1 vote
0 answers
50 views

Need for additional Master Port Role in MSTP

MSTP IEEE specification, it says the Root Port for CIST will be the Master Port for all MSTIs. I'm wondering what is the need for this additional port role, is it just to isolate the region boundaries ...
-1 votes
1 answer
1k views

HP 6120XG switch LACP is blocked - error condition

Folks, I am dealing with very strange issue related LACP and not sure if its bug of 6120XG switch or something else. so let me explain my issue here. Network Diagram: I have two Cisco Nexus 9396PX ...
0 votes
1 answer
68 views

In STP, how does a switch know that a peer port is blocked?

Let's consider the below image: Let's assume the following: Cat1 is the root bridge fa0/23 on cat2 is blocked after stp has converged When a frame arrives at cat1 to be forwarded to cat2, how ...
0 votes
0 answers
13 views

Fast port connection and ability to daisy chain unmanaged switches on Cisco Catalyst [duplicate]

I have a Cisco Catalyst 2960-S that I use in a home environment. In the configuration I currently have (very basic, basically just switchport mode trunk on upstream and switchport mode access on reset ...
3 votes
1 answer
1k views

RSTP / MSTP use cases and best practices

Hi, what would be the best practice to achieve separate tree topologies for each of the rings? Two options come to my mind, although I do not completely understand IF RSTP can be made VLAN-aware ...
1 vote
3 answers
1k views

STP Priority field and 4 bits in multiples of 4096

For the STP Priority Field, how do you get "multiples of 4096" out of 4 bits?
1 vote
2 answers
119 views

Spanning Tree Protocol - Redundant Links

There are two things about STP that confuse me. Consider the following network: There is a designated port from S1 to S3. There is also one from S1 to S2, and then S2 to S3. If S1 sends an unknown ...
0 votes
2 answers
96 views

RSTP convergence of p2p links

Ramping up on RSTP here. One thing that is unclear to me is how RSTP links converge initially on P2P links. Consider the classic 3 switch topo as below: For RSTP, won't all the links move to ...
2 votes
2 answers
300 views

Juniper EX bpdu-timeout-action not blocking interface

In short I'm trying to configure bpdu-timeout-action block on trunk interfaces to other switches, but it doesn't go into blocking mode when there are no BPDU's received (eg, when connecting my laptop ...
1 vote
2 answers
3k views

Finding the optimum root bridge for a spanning tree

The spanning tree protocol uses the bridge ID and the MAC address of the switch to elect the root bridge. After the election process, a spanning tree is created, preventing switching loops by ...
0 votes
1 answer
437 views

How does STP technically generate the path cost for an interface?

How path cost is being calculated in STP is still not clear to me. Let's say we got this architecture, where all switches were just turned on for the first time: During BPDUs exchange, every switch ...
0 votes
1 answer
74 views

how can a STP switch time out MAC addresses from the table in Listening state?

I am looking over some materials describing STP's port states (the original version from 802.1d) and they say that a port in Listening state can not modify the MAC address table (it doesn't learn new ...
5 votes
2 answers
1k views

Two switches, double connection, no LACP: how does it work?

We have a network with an HP 1910-24 and a V1410-16 (unmanaged). The two are connected via two network cables, but there's no Link Aggregation configured on the 1910, and all the connections are ...
0 votes
2 answers
776 views

designated ports and root ports in STP

I am just a bit confused about designated ports in STP, i know how to determine them and the confusion is about the say " designated port is the only port in charge to forward traffic on a segment ". ...
3 votes
1 answer
121 views

Spanning-Tree vlan priority on a pass through switch

I am still understanding a bit about this priority for spanning-tree on VLANs. I have a SWITCH A that is the root of a VLAN. It trunks through SWITCH B with no ports on that vlan. SWITCH B trunks to ...
3 votes
2 answers
369 views

Loops in an STP-enabled network - How?

I've got a question about STP/RSTP (it applies to both, but I'll just say STP). If I connect several switches and the STP algorithm eventually creates a loop-free environment, then how can a single ...
2 votes
2 answers
174 views

MST - multiple instance design in single region

I am preparing for migration of this rather simple network layout from RST to MST: From what I have read so far, it looks like the best approach is to avoid interoperability issues and simply rollout ...
1 vote
1 answer
90 views

Is non-blocking per-VLAN (rapid) spanning tree configuration possible?

Imagine a pair of switches connected with a couple of trunk links created in the following fashion: Assuming the numbers on lines correspond to different VLANs, how would you arrange a working ...
1 vote
1 answer
35 views

Do STP 802.1D support Uplink Fast, Backbone Fast and Port Fast?

Do STP 802.1D support Uplink Fast, Backbone Fast and Port Fast ? Aren't these features cisco proprietary ? Is IEEE later add these feature into 802.1D ? If so, Why are they not called Alternate Port, ...
1 vote
0 answers
116 views

HSRP does not work on two switches. Cisco

The problem is this: clients receive IP addresses from the L31 DHCP server, but they cannot ping the gateway, although HSRP seems to be working, there are no problems, I suspect that this is a bug in ...
1 vote
0 answers
119 views

Cisco device chooses link with higher cost as the root

The problem is that psw11 (SG350-28P) is choosing the path to sw10 (C2960L-48TS-LL) with a cost of 20000 as the root instead of the path to sw56 (C2960L-48TS-LL) with a cost of 10000, even though the ...
3 votes
2 answers
993 views

Does SPB really replace STP?

I have been reading IEEE 802.1Q 2014 about SPB. I also have watched a few videos about SPB on youtube. However I don't understand why no one mentioned that BPDUs are still transmitted in the network, ...
0 votes
1 answer
618 views

Palo Alto 440 and FS 5860 Switches -- L2 Loop/Spanning-Tree issue

I have identified what seems to be a looping issue, due to STP not blocking an alternate path. As you can see in the diagram, ports Eth7 and Eth8 on the switches connect to the Active/Passive PA-440 ...
-2 votes
4 answers
15k views

How Layer-2 data will be fragmented?

Got a doubt while studying about LLC layer. From Network layer I came to know that IP fragmentation will be done, For L4 TCP will segment the data. Here, the question is Protocols starting from Layer-...
2 votes
2 answers
570 views

Spanning tree loop prevention

Can I get some advice on what STP features to enable as best practice please, mainly to mitigate loops? In the past a user mistakenly connected two floor ports together causing a loop. Below are the ...
1 vote
2 answers
72 views

Alternative Solution to Avoid Loop aside STP

SITUATION: I have two switches SW1 and SW2. These two switches are connected with two trunk ports. While SW1 is connected to PC1, SW2 is connected to PC2.  If PC1 sends a broadcast frame to PC2 via ...
0 votes
1 answer
58 views

How would a switch know that it should forward a frame through a different interface in a STP topology?

Imagine the following STP topology If PC0 was to ping PC1, all of the switches would create entries in their MAC table according to the source MAC address of the PCs associated with the corresponding ...
0 votes
1 answer
82 views

Is it risky to change hardcoded STP mLAG port cost on Cisco Nexus LACP port

When two Cisco Nexus are connected in VPC, the stp cost is hardcoded to 200 for the mLAG LACP ports. I would like to manually change the cost on my devices because my 2G and 200G links have the same ...
4 votes
6 answers
1k views

Why does STP choose the lowest bridge ID as the root bridge?

The switch which has the lowest bridge ID can be the root bridge,and the bridge ID is made of priority and MAC address,that is , bridge ID = priority + MAC address So we will choose the switch which ...
4 votes
3 answers
3k views

How do I stop %C4K_EBM-4-HOSTFLAPPING message for wireless ports, but leave on for STP issues?

I do not have a spanning tree loop. I work in a manufacturing environment and roaming is prevalent. I don't want to see mac flaps or host flap messages on the wireless ports, I only want them for STP. ...
1 vote
3 answers
648 views

Why switch uses Mac address during implementation of spanning tree protocol (STP)?

We know that layer2 switch doesn't use Mac address ,uses only certain port number to accept the frame. But I have been read on book as well as internet that uses Mac address to find spanning tree for ...
0 votes
1 answer
163 views

Which interface will be the root port in this topology?

I am currently studying for my CCNA and the current topic is STP. I ran into this topology and it caused me some confusion. I was trying to identify the root port for SW2. Here was my logic for that. ...
2 votes
3 answers
733 views

what is switch device mac address?

I'm studying STP protocol and how to configure STP on switches. I know each switch has several ports and each port has one MAC address. but when I read the below article I noticed that it allocate one ...
2 votes
2 answers
142 views

Why in STP is the switch that is the "center" of the network is called the "Root Bridge," not "Root Switch"?

I am interested in the terminology of the term "root bridge." Why was this word chosen instead of the word switch, which would have been more intuitive?
2 votes
3 answers
665 views

What is the behavior of two managed switches connected to one unmanaged switch?

Image the followign scenario Switch 1 Managed (root) Switch 2 Managed Switch 3 Unmanaged They are connected as follows: Switch 1 <-> Switch 3 Switch 2 <-> Switch 3 Switch 1 <-> ...
2 votes
1 answer
340 views

Using PT and EtherChannels along with STP

I'm very new to all of this but I am trying to create a network topology as practice that uses three etherchannels across three switches and then rely on STP to block one. So far it was working well. ...
4 votes
3 answers
640 views

I'm confused on how spanning tree works

Like take this example: Also not mentioned in image, lets take: Segment 1 is between switch 1 and switch 2 Segment 2 is between switch 2 and switch 3 Segment 3 is between switch 3 and switch 1 All ...
5 votes
2 answers
363 views

STP convergence process needs detailed explanation, please

So, I understand how STP convergence works on high-level. However, when I am trying to look into the process deeper, I run across some doubts I would appreciate someone could clarify to me. Here we go:...
4 votes
2 answers
977 views

MSTP with different VLANs blocking port

I am trying to use MSTP with HP Procurve switches. The edge switch (D in diagram) has half of the ports set to VLAN10 and half set to VLAN20, all untagged. The problem is that the switch D to forward ...
5 votes
1 answer
368 views

STP frames over VPLS/VPWS

Do VPLS and VPWS services transfer STP frames? How about other L2 control frames? Can two geographically dispersed sites have one STP region over any L2VPN?
-1 votes
2 answers
345 views

RSTP Network Switches Port Base MAC Address

How to ethernet managable network switch manage the dynamic table entries although receive "port base MAC address frame" like RSTP,STP protocole ? I know the dynamic mac table save the ...
0 votes
2 answers
526 views

what is the main function of root bridge and ring master?

When we learn the STP or RSTP,we all learn the election of root bridge, the lowest priority number or MAC address of switch will become the root bridge.So here is my first question: 1.So what can the ...
1 vote
1 answer
481 views

Why does only the root bridge send RSTP BPDUs after sync?

From a recent packet capture, after the sync process in RSTP, only the root bridge sends RSTP BPDUs every 'hello' interval. Why is it so? Shouldn't all switches send BPDUs every 'hello' interval since ...
4 votes
0 answers
189 views

Why does RSTP send proposal bit and agreement bit as '1' initially?

From a recent Wireshark capture of a RSTP BPDU on a switch, both proposal and agreement bits were set to '1'. In my understanding, initially both switches 'propose' themselves as root. The better root ...

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