Top Most CCNP Switching Interview Questions Part – 2

1.What is Hub?

hub is the simplest of these devices. Any data packet coming from one port is sent to all other ports. It is then up to the receiving computer to decide if the packet is for it. Imagine packets going through a hub as messages going into a mailing list. The mail is sent out to everyone and it is up to the receiving party to decide if it is of interest. The biggest problem with hubs is their simplicity.

Since every packet is sent out to every computer on the network,there is a lot of wasted transmission. This means that the network can easily become bogged down. Hubs are typically used on small networks where the amount of data going across the network is never very high.

2.What is Bridge?

A bridge goes one step up on a hub in that it looks at the destination of the packet before sending. If the destination address is not on the other side of the bridge it will not transmit the data. A bridge only has one incoming and one outgoing port. To build on the email analogy above,the bridge is allowed to decide if the message should continue on.

It reads the address [email protected] and decides if there is a [email protected] on the other side. If there isn’t,the message will not be transmitted. Bridges are typically used to separate parts of a network that do not need to communicate regularly,but still need to be connected.

3.Explain about Switch?

A switch steps up on a bridge in that it has multiple ports. When a packet comes through a switch it is read to determine which computer to send the data to. This leads to increased efficiency in that packets are not going to computers that do not require them.

Now the email analogy has multiple people able to send email to multiple users. The switch can decide where to send the mail based on the address. Most large networks use switches rather than hubs to connect computers within the same subnet.

4.Router :

A router is similar in a switch in that it forwards packets based on address. But,instead of the MAC address that a switch uses,a router can use the IP address. This allows the network to go across different protocols. The most common home use for routers is to share a broadband internet connection.

The router has a public IP address and that address is shared with the network. When data comes through the router it is forwarded to the correct computer. This comparison to email gets a little off base. This would be similar to the router being able to receive a packet as email and sending it to the user as a fax.

5.What is mac address and why it is required?

MAC address is a number created by the manufacturer to identify the computer on the network.

 6.Define Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP) is a required TCP/IP standard defined in RFC 826,”Address Resolution Protocol (ARP).” ARP resolves IP addresses used by TCP/IP-based software to media access control addresses used by LAN hardware.

ARP provides the following protocol services to hosts located on the same physical network:

* Media access control addresses are obtained by using a network broadcast request in the form of the question “What is the media access control address for a device that is configured with the enclosed IP address?”
* When an ARP request is answered,both the sender of the ARP reply and the original ARP requester record each other’s IP address and media access control address as an entry in a local table called the ARP cache for future reference.

7.What is the use of Spanning Tree Protocol (STP)?

The function of Spanning Tree Protocol (STP) is to prevent Layer 2 switching loop and broadcast storms in a Local Area Network (LAN). The Spanning Tree Protocol (STP) allows redundant links in a network to prevent complete network failure if an active link fails,without the danger of Layer 2 Switching loops.

8.What is the difference between STP and RSTP?

The main difference between Rapid Spanning Tree Protocol (RSTP IEEE 80
2.1W) and Spanning Tree Protocol (STP IEEE 80
2.1D) is that Rapid Spanning Tree Protocol (RSTP) assumes the three Spanning Tree Protocol (STP) ports states Listening,Blocking,and Disabled are same (these states do not forward frames and they do not learn MAC addresses). Hence RSTP places them all into a new called Discarding state. Learning and forwarding ports remain more or less the same.

* In Spanning Tree Protocol (STP IEEE 80
2.1D),bridges would only send out a BPDU when they received one on their root port. They only forward BPDUs that are generated by the Root Bridge. Rapid Spanning Tree Protocol (RSTP IEEE 80
2.1W) enabled switches send out BPDUs every hello time,containing current information.
* Spanning Tree Protocol (STP IEEE 80
2.1D) includes two port types; Root Port and Designated Port. Rapid Spanning Tree Protocol (RSTP IEEE 80
2.1W) includes two additional port types called as alternate ports and backup ports. Analternate port is a port that has an alternative path or paths to the root but is currently in a discarding state (can be considered as an additional unused root port). A backup port is a port on a network segment that could be used to reach the root switch,but there is already an active designated port for the segment (can be considered as an additional unused designated port).

9.What is the difference between broadcast and collision domain?

Broadcast Domain:

* A logical set of computer systems those are reachable without using a router.
* Broadcast Domain is limited to the specific IP sub network for all the IP broadcasts.
* Broadcast domain uses a switched environment to broadcast.
* The network systems will have 255 at the end of the IP addresses. Collision Domain:
* It refers a set of devices in which packet collision could occur.
* The devices can include the devices of another IP sub networks.
* There is a potential packet collision as multiple devices transmit data on one wire / link.
* A collision can occur on cable ,hub or repeater.

10.What is path cost?

It is an arbitrary value,based on hop count,bandwidth,or another calculation,that is typically assigned by a network administrator and used by the routing protocol to compare different routes through an internetwork. Routing protocols use cost values to select the best path to a certain destination. The lowest cost identifies the best path. Also known as path cost.

11.Types of Bridge Protocol data units (BPDU).

Three types of Bridge Protocol Data Units (BPDUs) are Configuration BPDU (CBPDU),Topology Change Notification(TCN) BPDU and Topology Change Notification Acknowledgement (TCA).

12.What is the purpose of STA and BPDU?

The basic purpose of the Bridge Protocol Data Units (BPDUs) and the Spanning Tree Algorithm (STA) is to avoid Layer 2 Switching loops and Broadcast storms.

13.Define selection criteria of STP root bridge.

If a Switch has multiple paths to reach the Root Bridge (Switch),it must select one path and the associated port as theRoot Port. Following are the different steps for selecting the Root Bridge (Switch).

* If there are two or more paths to reach the Root Bridge (Switch) select the path with the lowest accumulated path cost.
* Select the neighbouring switch with the lowest Switch ID value to reach the Root Bridge (Switch).
* Select the port with the lowest Port Priority value,if you have multiple paths to reach the Root Bridge (Switch) via same neighbour switch.
* If all the ports are configured with same priority number (32 by default),select the lowest port number on the switch.

14.what are the Port states of spanning tree protocol?

* Blocking
* listening
* learning
* Forwarding
* disabled.

15.How to enable or disable spanning tree protocol?

Spanning Tree Protocol (STP) is enabled by default on modern switches. It is possible to disable or enable theSpanning Tree Protocol (STP) when required. To enable Spanning Tree Protocol (STP) on an IOS based switch,use the “spanning-tree vlan vlan_number” command from global configuration mode as shown below. switch01> switch01>enable switch01#configure terminal Enter configuration commands,one per line.

End with CNTL/Z. switch01(config)#spanning-tree vlan 1 switch01(config)#exit switch01# To disable Spanning Tree Protocol (STP) on an IOS based switch use “no spanning-tree vlan vlan_number” commandfrom global configuration mode as shown below. switch01>enable switch01#configure terminal Enter configuration commands,one per line. End with CNTL/Z. switch01(config)#no spanning-tree vlan 1 switch01(config)#exit switch01#

16.What is UDLD and why it is required?

UDLD is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link exists. All connected devices must support UDLD for the protocol to successfully identify and disable unidirectional links. When UDLD detects a unidirectional link,it administratively shuts down the affected port and alerts you. Unidirectional links can cause a variety of problems,including spanning-tree topology loops.

17.Explain about three functions of a Switch .

There are three distinct functions of layer 2 switching : address learning,forward/filter decisions,and loop avoidance.

Address learning : Layer 2 switches and bridges remember the source hardware address of each frame received on an interface,and they enter this information into a MAC database called a forward/filter table.

Forward/filter decisions : When a frame is received on an interface,the switch looks at the destination hardware address and finds the exit interface in the MAC database. The frame is only forwarded out the specified destination port. Loop avoidance : If multiple connections between switches are created for redundancy purposes,network loops can occur. Spanning Tree Protocol is used to stop network loops while still permitting redundancy.

18.Define Root bridge.

The root bridge is the bridge with the best bridge id. With STP,the key is for all the switches in the network to elect a root bridge that becomes the focal point in the network. All other decisions in the network – such as which port is to be blocked and which port is to be put in forwarding mode – are made from the perspective of this root bridge.

19.Define BPDU.

BPDU : All the switches exchange information to use in the selection of the root switch as well as in subsequent configuration of the network. Each switch compares the parameters in the Bridge protocol Data Unit that it sends to one neighbour with the one that it receives from another neighbour.

20.Define Bridge ID.

The bridge ID is how STP keeps track of all the switches in the network. It is determined by a combination of the bridge priority (by default 32,768) and the base MAC address. The bridge with the lowest bridge ID becomes the root bridge in the network.

21.Define Port cost :

Port cost determines the best path when multiple links are used between two switches and none of the links is a root port. The cost of a link is determined by the bandwidth of a link.

22.What is Root Port?

The root port is always the link directly connected to the root bridge,or the shortest path to the root bridge. If more than one link connects to the root bridge,then a port cost is determined by checking the bandwidth of each of link. The lowest-cost port becomes the root port. If multiple links have the same cost,the bridge with the lower advertising bridge ID is used.

23.What is Designated Port?

A designated port is one that has been determined as having the best cost. A designated port will be marked as a forwarding port.

24.What is Rapid Spanning Tree Protocol?

The convergence time for legacy Spanning Tree Protocol (STP) IEEE 80
2.1D standard is 30 to 50 seconds. When the network is converging on a topology change,no traffic is forwarded to or from any of the network bridges and switches. In modern networks this convergence time gap is not acceptable and Cisco enhanced the original Spanning Tree Protocol (STP) 80

2.1D specification with features such as PortFast,UplinkFast and BackboneFast to speed up the convergence time. But these were proprietary enhancements. The Rapid Spanning Tree Protocol (RSTP) IEEE 80
2.1W standard is available to address the convergence time gap issue. Rapid Spanning Tree Protocol (RSTP) enables root ports and designated ports to change from the blocking to forwarding port state in a few seconds.

25.Per-VLAN Spanning Tree (PVST) and Per-VLAN Spanning Tree Plus (PVST+)

The type of a single instance Spanning Tree Protocol (STP) is known as Common Spanning Tree (CST). Delay in receiving BPDUs is common in large switched Common Spanning Tree (CST) networks. The delay in receiving BPDUs can cause problems like convergence time problems. Per-VLAN Spanning Tree (PVST) is a solution for these problems. Per-VLAN Spanning Tree (PVST) is a Cisco proprietary Spanning Tree Protocol (STP) which operates a separate instance of Spanning Tree Protocol (STP) for each individual VLAN.

A separate instance of Spanning Tree Protocol (STP) for each VLAN helps VLAN to be configured independently and also can perform better. Per-VLAN Spanning Tree (PVST) requires Inter-Switch Link (ISL). Per-VLAN Spanning Tree+ (PVST+) is an extension of the PVST standard. Per-VLAN Spanning Tree+ (PVST+) allows interoperability between CST and PVST in Cisco switches and support the IEEE 80
2.1Q standard.

26.Spanning Tree Protocol Convergence?

Spanning Tree Protocol (STP) convergence (Layer 2 convergence) happens when bridges and switches have transitioned to either the forwarding or blocking state. When layer 2 is converged,Root Switch is elected and Root,Designated and Non-Designated ports in all switches are selected. At Converged condition,the Root and Designated are in forwarding state,and all other ports are in blocking state.

The time it usually takes to change from blocking to forwarding state is 50 seconds (Blocking to listening is 20 seconds; Listening to learning is 15 seconds; Learning to forwarding is 15 seconds),when Spanning Tree Protocol (STP) timers are in their default values. Until the port reaches forwarding state,no user traffic is forwarded through the port and the end user may feel that there is no network connectivity.

27.what is Spanning Tree Protocol Port Fast?

Spanning Tree Protocol (STP) convergence (Layer 2 convergence) happens when bridges and switches have transitioned to either the forwarding or blocking state. Normal Spanning Tree Protocol (STP) convergence (Layer 2 convergence) time is 50 seconds and the end user traffic is blocked until Root Port and/or Designated Port reach the forwarding state. We can use the feature called PortFast to speed up convergence on ports which are connected to a workstation or a server (which will not cause layer 2 loops).

PortFast feature should be used only to connect a single workstation to a switch port to avoid layer 2 switching loop. Spanning-tree PortFast feature causes a port to enter the forwarding state immediately,bypassing the listening and learning states. When Spanning Tree Protocol (STP) is running,PortFast ports on the same switch can forward traffic between each other,but need to wait for Spanning Tree Convergence to communicate with a port on which the PortFast feature is disabled (normally a port connected to another switch). .

28.what is non-designated port

A non-designated port is one with a higher cost than the designated port. Non designated ports are put in blocking mode – they are not forwarding ports.

29.SPT Blocking Port states :

A blocked port won’t forward frames; it just listens to BPDUs. The purpose of the blocking state is to prevent the use of looped paths. All ports are in blocking state by default when the switch is powered up.