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STP Questions 3

November 17th, 2017 Go to comments

Question 1


PortFast causes a switch or trunk port to enter the spanning tree forwarding state immediately, bypassing the listening and learning states.

Question 2

Question 3

Question 4


Answer A is not correct as we can choose which switch to become root bridge by configuring bridge priority. The switch with lowest bridge priority (value) would become the root bridge.

For answer B, this paragraph from Cisco confirms it is the correct answer:

“When you implement a root bridge in a switching network, you usually refer to the root bridge as the root switch. Each VLAN must have its own root bridge because each VLAN is a separate broadcast domain. The roots for the different VLANs can all reside in a single switch or in various switches.”

Reference: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

The meaning of answer C is not clear but maybe it means “every VLAN must use the same root bridge” which is not correct as Sw1 can be the root bridge for VLANs 1, 3, 5 but Sw2 can be the root bridge for VLAN 2, 4, 6…

From the quote above we can say answer D is not correct.

Question 5


PortFast BPDU guard prevents loops by moving a nontrunking port into an errdisable state when a BPDU is received on that port. When you enable BPDU guard on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs instead of putting them into the spanning tree blocking state. In a valid configuration, PortFast-configured interfaces do not receive BPDUs. If a PortFast-configured interface receives a BPDU, an invalid configuration exists. BPDU guard provides a secure response to invalid configurations because the administrator must manually put the interface back in service.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/stp_enha.html

Question 6

Question 7

Question 8


Only non-root bridge can have root port. Fa0/11 is the root port so we can confirm this switch is not the root bridge -> A is not correct.

From the output we learn this switch is running Rapid STP, not PVST -> B is not correct.

0017.596d.1580 is the MAC address of this switch, not of the root bridge. The MAC address of the root bridge is 0017.596d.2a00 -> C is not correct.

All of the interface roles of the root bridge are designated. SwitchA has one Root port and 1 Alternative port so it is not the root bridge -> D is correct.

Comments (8) Comments
  1. ten
    December 20th, 2017

    can someone explain Q7?

  2. silver-twist
    January 26th, 2018

    hello gys, somebody could explain me the last question Q7, im lost?

  3. potocki
    January 30th, 2018

    Hello guys, regarding to Question 7, I think that the answers are DHCP and DNS because only these two protocols are used by hosts (of course, DNS to get IP addresses from domain names like google.com or 9tut.com, DHCP to get IP adrress assigned to the host, very common practice).
    In well designed network hosts should not receive RIP or OSPF (neither EIGRP), as it is seen as, primo, unnecessary, as hosts don’t do routing, so they don’t need information about routes from dynamic routing protocols, secondly, this is seen as security vulnerability – and this is why the command “passive-interface” in routing protocol configuration mode exists – it disables routing protocol on interface which are connected towards hosts.
    CDP also isn’t used by hosts, I think so, and also, as with routing protocols descrived above, disabling CDP on interfaces towards hosts (command “no cdp enable” in interface mode, or “no cdp run” in global mode – disables CDP at all) is a good security practice.
    AND ok, we are coming back to the clue of question, so focus. PortFast is Cisco proprietary STP enhancement, which is enabled normally on switch on ports towards hosts (access ports) – and it makes these ports’ states are changing immediately to Forwading state, omitting Listening and Learning states in “classical” STP (IEEE 802.1d, also know as CST Common Spanning Tree). Without PortFast enabled, STP last for 50 seconds of delay (transition from Disabled > Listening > Learning > Forwarding state), which is a lot of wasted time in modern networks. In that period, any host traffic, including mentioned DHCP and DNS traffic, is not allowed on STP running switches. That’s exaclty why PortFast should be enabled on ports towards LAN, to speed up those host-used protocols DHCP and DNS.
    I am not sure answer about CDP (i.e. is CDP able to discover hosts? it works on layer 2 ISO/OSI, so it should see hosts which are working in 7. layer..), but for sure DHCP and DNS are best-fitting answers. Though, I would be grateful if anyone can confirm my thinking track :)

  4. DELES
    February 12th, 2018

    @Potocki > CDP is a CISCO proprietary protocol and it’s not used on hosts which in turns runs Operating systems. Thus the correct answers are DHCP and DNS, as Portfast may prevent delays in seeing that the host interface is up and running.

  5. Flows
    March 28th, 2018

    Which statement about spanning-tree root-bridge election is true?
    A. It is always performed automatically
    B. Each VLAN must have its own root bridge
    C. Each VLAN must use the same root bridge
    D. Each root bridge must reside on the same root switch

    But CST doesn’t support pervlan spanning tree
    I considered A?
    anybody can explain?
    correct me if I’m wrong


  6. Ronn
    May 23rd, 2018

    Q4 You’re wrong, answer A is not correct as we can choose which switch to become root bridge by configuring bridge priority. The switch with lowest bridge priority (value) would become the root bridge.

  7. Anonymous
    June 7th, 2018

    Flows answer B is correct

  8. Dany1
    June 8th, 2018

    Q7 explained: without portfast, port goes through all states from shutdown to forwarding, that means at least 30 seconds to wait until the port is ready to send/receive data frame.
    Let’s see:
    DNS: Client try ping http://www.cisco.com at least several times in 30 seconds interval and will think something is wrong because answer at pc screen will be “cannot resolved http://www.cisco.com“. Client is not STP expert and didn’t know about 30 seconds limitation
    DHCP: in that case, DHCP Process is automatically generated by command (let’s say ip address dhcp). In that time, switch port is in STP procedure. AFTER 4(four) DHCP Discovery messages, sended in 15-16 seconds, the process stop and somebody must run again the command. 15<30 so that are answers. The question is talking about "alleviate potential HOST startup issues", so is not referred to routers (OSPF, RIP)
    Flows: A is not corect (Ronn explain why). You have to check the truth of each separate sentence, do not try to link them to each other. Cisco has questions asking you to see otherwise the theory read. As with the martial arts movies where the hero identifies with nature, try to identify with Cisco thinking.
    And do not forget, when in doubt, uses GNS3 and Wireshark. These are fantastic tools that let you work from home as if you were in a Cisco lab.

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