25

Not sure how much you know about switching and spanning tree but basically when starting out all switches claim that they are the root. All switches send BPDUs (Bridge Protocol Data Unit) which contain a priority and the BID (Bridge ID). The BID is 8 bytes long. 6 bytes is used for the MAC address of the bridge. 12 bits is used to indicate the VLAN, this ...


20

It depends. With IEEE 802.1Q specified STP, where you have one instance per trunk, STP will block ports. With Cisco's PVST+ (Per-VLAN STP), every VLAN has a corresponding instance of STP. This means that if there are no loops within the VLANs, STP will not block ports. MST, defined in IEEE 802.1s, works a bit differently. Instead of providing a one-to-one ...


18

UDLD is generally run on fibre media, it is not required on UTP due to the use of Fast Link Pulse which is already monitoring link status. This page has a very good explanation of the various L2 protections available. Here is an excerpt specifically regarding UDLD: UDLD is used to detect if a link is only available in one direction, for example half ...


17

In the spanning tree algorithm the process of determining the root bridge is based on the bridge priority (BID). When there were no VLANs (meaning that switches had to deal with only one broadcast domain) the BID was equal to: Bridge priority - MAC Address <2 bytes> <6 bytes> As Network administrators we can modify the bridge priority ...


16

You should run 'port-fast' (in standard terms edge port) in every port not part of your switch core. Even if it is switch. You should NOT have L2 loop through customer switches. You should run BPDUGuard and BUM policers all interfaces, customer facing interfaces should be 1/5th or less of core facing limits. Unfortunately limiting unknown unicast often is ...


14

You should not be using "on" for link aggregation as it can lead to problems. On the side with aggregation statically on, it will use the interfaces in the etherchannel, no matter what the configuration is on the other side. While storm control (from comments) can be very helpful with some of the problems that result, it does not resolve all of them. For ...


14

First, like the others have mentioned you have no bridging loop here due to running a Portchannel. That said, running STP is still fine. Let me clear some confusions on how these commands work on Cisco switches. spanning-tree portfast trunk This command is supposed to be run on trunk ports towards non bridging devices, such as a server with multiple VLANs ...


13

You're asking about STP theory: https://en.wikipedia.org/wiki/Spanning_Tree_Protocol But the basic idea is that each switch has a Bridge ID which is a combination of its priority and its MAC address. By default, all switches use 32768 as their priority so by default the switch with the lowest MAC will act as the Root Bridge. You can manipulate this by ...


13

In addition to spanning-tree portfast, you should also use spanning-tree bpduguard enable so that if someone creates a loop by plugging in things where they should not then the switch port will go into error disabled mode when it sees a BPDU rather than creating a loop and potentially bringing down the network. As well, if your goal is to track down the ...


13

Yes, configure it, just because your port channel is good doesn't mean your entire topology necessarily is - just connect another cable between the two switches on an active VLAN and watch the badness happen. I recommend MST (Multiple Spanning Tree) providing the switch supports it - interoperable, backwards compatible and of course, has the obvious benefit ...


12

You should be able to simply debug the TCNs. In my case I recently debugged them using debug spann mstp tc (as I run MSTP), but also using debug spanning-tree events you will see them: Jul 10 07:42:18 UTC: STP: VLAN0228 Topology Change rcvd on Gi1/0/9 <<< received Jul 10 07:42:18 UTC: STP: VLAN0228 sent Topology Change Notice on Po10 <&...


12

You really, really do not want to disable STP where you connect switches to other switches. That is the entire purpose of STP. If you disable STP, and there is a problem, it will really be too late because your entire network could crash when you notice it, and recovering from a broadcast storm is no fun at all. By the way, portfast doesn't actually disable ...


10

We have been using 2 Cisco 2960-S 48 Port switches which were connected together using a regular Cat5e cable and everything was working fine, however we have purchased a 3rd switch of the same model and need to connect it to the other switches. What would be the best way to connect all 3 switches? Generally, you connect all three switches in a "...


10

RSTP needs backwards compatibility with STP switches. Thus Discarding state merges Disabled, Blocking, Listening into one. Ideally if running a complete RSTP topology then discarding becomes practically unneeded due to the explanation below. In STP BPDU will only be sent from root bridge > down, therefore non-root bridges would only forward on BPDUs that ...


10

Layer 3 (mostly IP) generally relies on the underlying layer-2 network (mostly Ethernet or Wi-Fi) for delivery. Just like a layer-2 network uses layer-1 links to actually move the bits. The difference in moving data at layer 1, 2 or 3 is the complexity of the devices. Layer-1 devices (repeaters) just copied bits - simple, yet inefficient and long obsolete. ...


9

The way it works when connecting MST and PVST domain is that the MST switch at the edge of the domain will simulate PVST+ by sending same BPDU for all VLANs configured on the trunk towards the PVST+ switch. The CIST replicates BPDU for every VLAN. Because there is not one instance per VLAN for MST the sys ID is set to 0 on these BPDUs. When connecting MST ...


9

Yes, it's a good idea to run both as they protect you from different failures. If UDLD is not supported on both sides you can use LACP as a poor man's UDLD (between Cisco and Juniper for example) or you could even use Ethernet OAM LFM.


9

I am assuming several things since there are some points missing. You did not configure anything except for LACP ether-channel between the distro switches. NOTE: LACP is ether-channel protocol not trunking protocol. You need to check several for several things: Ports 47 and 48 are trunks (either statically configured or negotiated through DTP) check on ...


9

The bridge ID is an 8-Byte (64 bit) value composed of the following elements: The bridge priority value and the system ID extension together make up a 16 bit (2-byte) value. The bridge priority value, making up the left most bits, is a value of 0 to 61440. The extended system ID is a value of 1 to 4095 corresponding to the respective VLAN participating in ...


9

802.1D STP has configuration BPDUs sent by the root bridge. The designated bridges relay these BPDUs on their designated ports. All BPDUs flow from the root. With 802.1D, a port going up or down will generate a topology change, unless portfast has been configured on the port. When a switch detects a topology change it will generate a TCN BPDU, which is a ...


9

you need to understand the next Spanning-Tree Port Roles - Root Port (RP) - It is a port on a non-root switch, which is the shortest (the best) path towards the root bridge. (i.e. port 0/4 0/3 in SW3) - Designated Port (DP) - It is a port that is in the forwarding state. (i.e. port 0/1 0/2 SW2) - Non-Designated Port (NDP) - It is a port that is in a ...


9

Great question! This is because a layer-2 switching loop can form on the Consumer-grade switching equipment which could produce Broadcast Radiation and impact devices connected to the broadcast domain. STP was originally standardized as IEEE 802.1D. Its purpose is to build loop-free, layer 2 typologies. STP does not detect and then mitigate Broadcast ...


9

To add to Zac67's and JFL's answers: In case you decide to enable spanning-tree on the single switch, don't forget to configure the client's and server's switchports as Edge Ports (Cisco speak; spanning-tree portfast [trunk], spanning-tree portfast edge [trunk] or spanning-tree port type egde [trunk] , depending on platform and software generation). This ...


9

MSTP and RSTP converge equally fast, MSTP is the current protocol version. Given the topology in your diagram (ugh!) and its tree depth, no STP variant will converge quickly (if at all). xSTP default parameters are designed for a maximum tree depth or chain length of 20 hops. Building longer/deeper constructs requires parameter tuning or your network will ...


8

When dealing with a topology with a large number of dumb switches under desks and other undocumented locations do you really want to enable this [portfast] on all "supposedly" access switches? The official and pedantic answer is "no, do not enable portfast on switch to switch link"... There is a relevant discussion about this on Cisco's support forum. The ...


8

STP does not operate differently whether the network has three, four, or a dozen switches. Each switch applies the same logic locally, and don't even actually know how many switches exist within the network. Legacy and rapid STP behave differently (you'll need to research the differences) but both operate on the same basic principle: calculate the best path ...


8

Spanning tree only runs between switches, never to standard end hosts. Bear in mind that, under normal circumstances, a port supporting spanning tree will (when it first comes up) run through a sequence of first listening for BPDU's, then learning source addresses and then finally forwarding frames. This can take ~30 seconds, during which the connected ...


7

I would run the change from root outwards but I don't think there are any use cases for doing it each way (someone correct me if I'm wrong). Manual costs will stay exactly the same so you should at some point convert these to fall in line with your new costs once you have changed over. Be aware of the fact that during topology changes or before you change ...


7

It's important to note that PVST/PVST+ is Cisco proprietary. They do support one instance of STP per VLAN, and it is enabled by default. You can enable/disable STP on the following levels (taken from the NetIron config docs): Globally - Affects all VLANs Individual VLAN - Affects all ports within the specified VLAN. When you enable or disable STP within a ...


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