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Be on Guard This Spooking Spanning Tree Season


It’s Halloween — a time for an excessive amount of sweet, scary films, children in enjoyable costumes, and plenty of tips and treats. As I thought of what to put in writing for my weblog this month, I shortly went to one of many scariest issues for each community engineer: SPANNING TREE!!!! That’s proper… can something else carry the identical degree of dread and chilly sweats because the potential for a bridging loop?!

Worry not. With a bit of fine sensible design and configuration practices, spanning tree doesn’t need to be scary. Nevertheless, even the very best engineers (or reasonably respectable ones like myself) can overlook a greatest apply or two. Let me set the spooky scene for you…

It was a darkish and stormy night time…

The next anecdote passed off about three or 4 years in the past after I was a part of the DevNet Sandbox group. We had just lately stood up a brand new knowledge heart for internet hosting labs, and I had returned house from California after spending a number of weeks onsite, standing up the community and techniques on the knowledge heart. I used to be feeling fairly good about how properly issues had gone. Significantly, the velocity and effectivity we had been capable of carry issues on-line, because of a heavy quantity of automation and programmability. On reflection, I ought to have identified one thing was going to go incorrect…

I believe the primary signal there is perhaps an issue within the community was after I seen my distant connection into the brand new location began to get actually laggy. I even received disconnected from some servers. It will clear up pretty shortly. However when the problems repeated a number of instances, I began to marvel what is perhaps the trigger.

I checked different monitoring techniques. Intermittent community points had just lately began displaying up; sluggish response from techniques, occasional disconnects that may clear up pretty shortly, that kind of factor. Nothing overly drastic, however they actually had been signs that indicated one thing won’t be completely wholesome within the community. I started to poke round a bit extra. Finally, I stumbled throughout just a few issues that pointed to a doable challenge someplace within the layer 2 components of the community.

It was fairly some time in the past, so the main points are a bit fuzzy. I believe I used to be on one of many high of rack Nexus 9000 switches in a {hardware} internet hosting rack when syslog messages hit the terminal about MAC flapping occurring. Now, MACs will transfer round a community often. Nevertheless, a flapping MAC deal with occurs when a swap sees it altering backwards and forwards between two ports. This isn’t regular. It usually factors to a community loop — one thing spanning tree is meant to stop from occurring.

Right here is an instance syslog message associated to MAC Flapping:

*Apr 5 18:17:43.242 GMT: %SW_MATM-4-MACFLAP_NOTIF: Host d8e6.a5cd.3f41 in vlan 61 is flapping between port Ethernet1/23 and port Ethernet1/24

After a bit extra troubleshooting, I additionally seen that the community was reconverging spanning tree, altering the basis bridge over and over. This was undoubtedly an issue. Even “speedy” spanning tree convergence is noticeable to community customers who discover themselves ready for a port to transition to forwarding after ports change state.

Discover how Loop Detection Guard prevents community loops on Catalyst 9000 switches. Learn “Stopping Community Loops! A Function You Have to be Conscious of” now.

Sufficient of the trick already, Hank… the place’s the deal with?

Lengthy story quick, the basis of the issue (pun TOTALLY supposed) was a brand new bodily swap that was being added to the community for one of many {hardware} labs we had been establishing.

The brand new swap hadn’t been totally configured for its new position but, and the upstream switches it was linked to already had the ports enabled in preparation for the brand new lab gear being added. The lab topology had a number of ports linked between this new swap and the info heart material for various functions and networks, however not one of the ultimate configuration had been utilized but. There have been really some remnants of previous configuration utilized to the swap, which resulted within the bridging loop and MACFLAP log messages.

Moreover, this swap had beforehand served because the spanning tree root in a earlier community and had a decrease (i.e., higher) precedence than the precise spanning-tree root in our knowledge heart. Between connections being made/eliminated, ports getting errdisabled for various causes, and different instabilities, the basis was bouncing between this new swap and the principle distribution switches within the knowledge heart each couple of minutes.

I used to be capable of shortly cease the issues from occurring by shutting down the ports linked to this new swap till it was accurately configured and able to be made an lively a part of the community. So, downside solved… kinda.  

The larger downside was that I had missed the important spanning tree design and greatest practices for the configuration step in bringing the brand new knowledge heart community up and on-line. Had I remembered my fundamentals, this downside wouldn’t have occurred: The community would have mechanically blocked ports that had been behaving in surprising methods.

You might be NOT root: Stopping surprising root bridges with root guard

Think about this quite simple triangle of switches as a fast assessment of the significance of the basis bridge in a spanning-tree community. 

Switches linked along with layer 2 hyperlinks use BPDUs (bridge protocol knowledge items) to find out about one another and decide the place the “root” of the spanning tree might be positioned. The swap that has the very best (i.e., lowest) precedence turns into root. With the basis bridge recognized, switches start the method of breaking loops within the community by blocking ports that spanning tree identifies as having the worst precedence on redundant hyperlinks.

A full dialogue on the spanning-tree course of for constructing the tree is out of scope for this weblog publish. It’s a vital matter for community engineers to grasp, so I’d return to spanning tree in future weblog posts. If you happen to’d prefer to dive deeper into the subject now, try our CCNA and ENCOR programs.

The method of electing the basis bridge and converging on a loop-free community can take tens of seconds to even a minute (or extra) in massive networks, relying on which model of spanning tree is used and the way properly the community is designed. Through the strategy of convergence, the community prevents bridging loops by defaulting to blocking visitors on ports. This may lead to important disruption to any customers and functions which might be actively utilizing the community. Bear in mind in my instance above, how my community entry had gotten “laggy” and my connections had even turn out to be disconnected? So long as the basis bridge stays secure and does NOT change, including a brand new swap to a community is a non-disruptive exercise.

So, how does a community engineer forestall the basis bridge from altering within the community? I’m glad you requested.

Figuring out the basis bridge for the community

Step one is to have a look at the community design and establish which swap makes essentially the most logical sense to be the basis, explicitly configuring it to have the very best (i.e., lowest) precedence. Right here, I configure my root swap to run speedy per-vlan spanning tree (rapid-pvst) and set the precedence to 16384.

root#present run | sec spanning

spanning-tree mode rapid-pvst
spanning-tree prolong system-id
spanning-tree vlan 1-4094 precedence 16384


root#present span

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Precedence    16385
             Handle     5254.000e.dde8
             This bridge is the basis
             Good day Time   2 sec  Max Age 20 sec  Ahead Delay 15 sec

  Bridge ID  Precedence    16385  (precedence 16384 sys-id-ext 1)
             Handle     5254.000e.dde8
             Good day Time   2 sec  Max Age 20 sec  Ahead Delay 15 sec
             Ageing Time  300 sec

Interface           Function Sts Price      Prio.Nbr Kind
------------------- ---- --- --------- -------- --------------------------------
Gi0/1               Desg FWD 4         128.2    P2p 
Gi0/2               Desg FWD 4         128.3    P2p 
Gi0/3               Desg FWD 4         128.4    P2p 

Be aware: With “per-vlan spanning-tree” each VLAN could have its personal spanning-tree constructed. The precedence of every bridge is the configured precedence plus the VLAN quantity. So for VLAN 1, the precedence is 16384+1 or 16385.

If we take a look at the spanning-tree state on one of many different switches within the community, we are able to affirm the basis bridge and the creation of a loop-free community.

switch-1#present span

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Precedence    16385
             Handle     5254.000e.dde8
             Price        4
             Port        2 (GigabitEthernet0/1)
             Good day Time   2 sec  Max Age 20 sec  Ahead Delay 15 sec

  Bridge ID  Precedence    32769  (precedence 32768 sys-id-ext 1)
             Handle     5254.0017.ae37
             Good day Time   2 sec  Max Age 20 sec  Ahead Delay 15 sec
             Ageing Time  300 sec

Interface           Function Sts Price      Prio.Nbr Kind
------------------- ---- --- --------- -------- --------------------------------
Gi0/1               Root FWD 4         128.2    P2p 
Gi0/2               Desg FWD 4         128.3    P2p 
Gi0/3               Altn BLK 4         128.4    P2p 

switch-1#present cdp neighbors gigabitEthernet 0/1

Machine ID        Native Intrfce     Holdtme    Functionality  Platform  Port ID
root             Gig 0/1           146             R S I            Gig 0/1

If you happen to examine the deal with of the basis bridge proven on switch-1 to the output above from root, you will note that the Handle and Precedence for the basis bridge match. Additionally, discover that interface G0/1 has the position of “Root” — that is the interface on the swap that has the very best path again to the basis bridge. And because the output from CDP reveals, it’s really instantly linked to the basis.

Stopping a brand new root on the block… err, community

Figuring out an supposed root bridge in your community is nice, but it surely doesn’t forestall a newly added swap from inflicting hassle.

Think about again to my instance from my anecdote the place a brand new swap was being added to the community that had beforehand been configured as the basis in one other community. Whereas it may very well be argued that it’s best apply and necessary to clear previous configuration from a swap earlier than including it to the community, the truth is… issues like this occur. You will need to engineer a community to deal with occasions like this.

First, let’s see what occurs to the spanning-tree community when bad-root is cabled into the community with none further configuration defending the spanning-tree community.

switch-1#present span

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Precedence    4097
             Handle     5254.001e.82a2
             Price        4
             Port        1 (GigabitEthernet0/0)
             Good day Time   2 sec  Max Age 20 sec  Ahead Delay 15 sec

  Bridge ID  Precedence    32769  (precedence 32768 sys-id-ext 1)
             Handle     5254.0017.ae37
             Good day Time   2 sec  Max Age 20 sec  Ahead Delay 15 sec
             Ageing Time  300 sec

Interface           Function Sts Price      Prio.Nbr Kind
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    P2p 
Gi0/1               Desg FWD 4         128.2    P2p 
Gi0/2               Desg FWD 4         128.3    P2p 
Gi0/3               Altn BLK 4         128.4    P2p 


switch-1#present cdp neighbors gigabitEthernet 0/0

Machine ID        Native Intrfce     Holdtme    Functionality  Platform  Port ID
bad-root         Gig 0/0           154             R S I            Gig 0/1

Whole cdp entries displayed : 1

Discover how the deal with and precedence for the basis bridge have modified, and that port Gi0/0 is now the “Root” port for switch-1. That is undoubtedly not what we’d wish to occur if a bad-root had been linked to the community.

Bringing out the Guard… root guard, that’s

We will leverage root guard to stop this from taking place. Root guard is among the “elective spanning-tree options” that basically shouldn’t be thought of “elective” in most community designs.

As a community engineer, it’s best to be capable of take a look at your community and know which ports “must be” the basis port on every swap. Then think about the redundancy that you just’ve constructed into the community and establish which port ought to turn out to be the basis port if the first port had been to have issues. Each different port on every swap ought to by no means turn out to be the basis port. These are the ports that must be configured with root guard.

Be aware: The foundation bridge in a community has NO root ports as it’s the root of the tree. Subsequently ALL PORTS of the basis bridge ought to have root guard enabled.

Now we’ll go forward and allow root guard on interface Gig0/0 on each switch-1 and switch-2.

switch-1(config)#interface gigabitEthernet 0/0
switch-1(config-if)#spanning-tree guard root 

*Oct 13 15:06:28.893: %SPANTREE-2-ROOTGUARD_CONFIG_CHANGE: Root guard enabled on port GigabitEthernet0/0.
*Oct 13 15:06:28.909: %SPANTREE-2-ROOTGUARD_BLOCK: Root guard blocking port GigabitEthernet0/0 on VLAN0001. 

And take a look at that. As quickly as it’s enabled, we see syslog messages indicating that root guard has begun blocking the port. If we examine the standing of spanning tree on switch-1 we are able to confirm that the basis of the spanning tree has returned to the right root swap.

switch-1#present span

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Precedence    16385
             Handle     5254.000e.dde8
             Price        4
             Port        2 (GigabitEthernet0/1)
             Good day Time   2 sec  Max Age 20 sec  Ahead Delay 15 sec

  Bridge ID  Precedence    32769  (precedence 32768 sys-id-ext 1)
             Handle     5254.0017.ae37
             Good day Time   2 sec  Max Age 20 sec  Ahead Delay 15 sec
             Ageing Time  300 sec

Interface           Function Sts Price      Prio.Nbr Kind
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Desg BKN*4         128.1    P2p *ROOT_Inc 
Gi0/1               Root FWD 4         128.2    P2p 
Gi0/2               Desg LRN 4         128.3    P2p 
Gi0/3               Altn BLK 4         128.4    P2p  

There’s one different command that’s helpful to know when troubleshooting spanning-tree ports that aren’t behaving as anticipated:

switch-1#present spanning-tree inconsistentports 

Identify                 Interface                Inconsistency
-------------------- ------------------------ ------------------
VLAN0001             GigabitEthernet0/0       Root Inconsistent

Variety of inconsistent ports (segments) within the system : 1  

Take the scare out of spooky spanning tree with data

Hopefully, this publish helps to decrease your coronary heart price a bit the subsequent time you consider making adjustments to the community that may influence your spanning-tree community. However I additionally hope it reveals you, as a community engineer, the significance of recalling the basic abilities and data you could have realized as you progress onward to extra specialised areas of networking. I used to be undoubtedly kicking myself after I realized that I had fully missed making certain that our spanning-tree community was well-designed and shielded from surprising or unintended adjustments.

Whereas nobody desires to have a community outage or perhaps a minor disruption, they’ll occur. What’s necessary, is that we be taught from them. And we turn out to be higher community engineers for them.

Do you could have a spooky community ghost story from your individual work as a community engineer? Ever had a scary encounter with a community outage or downside that helped you be taught a lesson you’ll always remember? Share them within the feedback. Trick or deal with!

Some helpful hyperlinks for digging deeper into spanning tree:

If you happen to’d prefer to dive deeper into this matter, I pulled just a few hyperlinks collectively for you.

 

 

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