Important Networking interview Questions Part-2

VLAN:-VLAN is layer 2 entity. It is logical grouping of switch port. Which segregate the broadcast domain that means one broadcast domain into multiple broadcast domains.

It’s main purpose to stop unnecessary broadcast.

Ether channel: - It is logical grouping of multiple physical port. It is called link aggregation. Through this

1. Increase the bandwidth

2. Load sharing

3. Redundancy

Native VLAN:- A native vlan is the untagged vlan on an 802.1q trunked switch port.

Inter VLAN Routing :-Inter VLAN routing, which is used to allow routing & communication between VLAN networks.                                                 OR

VLANs divide broadcast domains in a LAN environment. Whenever hosts in one VLAN need to communicate with hosts in another VLAN, the traffic must be routed between them.

This is known as inter-VLAN routing. On Catalyst switches it is accomplished by creating Layer 3 interfaces (Switch virtual interfaces (SVI) ).

Port Security :-  Using Port Security, We can configure each switch port with a unique list of the MAC addresses of devices that are authorized to access the network through that port.

Private VLAN:- Private VLAN, also known as port isolation, Private VLAN is an extension to the VLAN standard, Private VLANs provide a mechanism to control which devices can communicate within a single subnet. The private VLAN uses isolated and community secondary VLANs to control how devices communicate.

802.1s Multiple Spanning Tree Protocol:-

Multiple Spanning Tree Protocol (MSTP) is defined in IEEE 802.1s allows you to configure multiple STP instances. This will allow several VLANs to be mapped to a reduced number of spanning-tree instances. This ensures loop-free topology for 1 or more VLANs that have the same Layer 2 topology.

VTP:-  Vtp is a protocol which works at layer 2. It is management protocol. We can manage all vlan information from centralized location. We can add, delete and modify the vlan over the trunk links.

VLAN Access-List (VACL)

VLAN access-lists (VACL) are very useful if you want to filter traffic within the VLAN.

Flex Links

When you have a redundant switched topology, spanning-tree will block some of the interfaces to create a loop-free topology. Another method to deal with redundant topologies is by using flexlinks. When you configure Flex Links you’ll have an active and standby interface

Dynamic Trunking Protocol: - DTP is protocol which works on layer 2.The Dynamic Trunking Protocol (DTP) is used to negotiate forming a trunk between two Cisco devices.

By default DTP is enabled and the interfaces of your switches will be in “dynamic auto” or “dynamic desirable” mode. DTP frames are transmitted every 30 seconds.

RSTP: - One big disadvantage of STP is the low convergence which is very important in switched network. To overcome this problem, then RSTP introduced.

While STP can take 30 to 50 seconds to transit from a blocking state to a forwarding state, RSTP is typically able to respond less than 10 seconds of a physical link failure.

Virtual Router Redundancy Protocol (VRRP):-VRRP is an open standard that can be used in environments where equipment from multiple vendors exists. Its operation is similar to HSRP.

 a group is configured that contains a number of routers (gateways); one will be selected by the network engineer to be the master.

Gateway Load Balancing Protocol (GLBP):-GLBP is another Cisco proprietary protocol that can be used for first hop redundancy. GLBP offers something dynamic load balancing. When a GLBP group is configured, one of the routers within the group will be elected as the Active Virtual Gateway (AVG); each of the other routers will back up the AVG,

The AVG is responsible for assigning virtual MAC addresses to each of the members of the GLBP group; each of these members is referred to as an Active Virtual Forwarder (AVF).

The AVG is responsible for responding to ARP request by subnet devices, and selecting which group’s router will handle the traffic.

VLAN:-VLAN is layer 2 entity. It is logical grouping of switch port. which segregate the broadcast domain that means one broadcast domain into multiple broadcast domain.

It’s main purpose to stop unnecessary broadcast.

RIP:-  RIP is distance-vector routing protocol. It works on application layer. Router exchange routing table information every 30 sec. RIP uses hop count to determine the best path between two locations. it has a maximum allowable hop count of 15 by default, meaning that 16 is deemed unreachable. It ha s AD value 120. 

EIGRP:-EIGRP (Enhanced Interior Gateway Routing Protocol) is hybrid routing protocol developed for IP networks .It works on network layer. It uses the Diffusing-Update Algorithm (DUAL) to find the best path. It supports for discontiguous networks and it uses composite metric by default bandwidth & delay. It has AD value 90.

OSPF:- OSPF is a link state routing protocol developed for IP networks. It works on network layer. Each router maintains an identical database with in area. It uses SPF algorithm and provide loop free path. It supports for discontiguous networks and it's metric is cost. It has AD value 110.

BGP:-BGP is path vector routing protocol developed for IP networks. It works on application layer. BGP exchange routing information between Autonomous Systems (ASs).It’s metric is path attribute. It has AD value 200. BGP uses TCP with port no 179. Current BGP version is 4.

===Distance vector/Link state/path vector

===layer

===metric

===AD value

===multicast  address

Do I need BGP?

Ans:- Not unless you are connected to more than one Internet Provider. Static routing is acceptable for most applications, including load balancing.

AS:-An Autonomous System (AS) is a group of networks under a single administrative control

MPLS:-Mpls stands for multiprotocol level switching. It is neither layer 3 nor layer 2.Its works on two and half layer. Its works on the basis of lable. Lebles are generated by ldp.

1. What is a router? Or define the basic requirements of a router?

Answer:  A router is a layer 3 network device used to establish communication between different networks. Basic roles performed by a router are:

·         * Inter-network communication

·         * Best path selection

·         * Packet forwarding

·         * Packet filtering

2. What is the use of routing? Or why we use routing?

Answer:  By default, a router provides inter-network communication only for directly connected networks. To establish communication between indirectly connected networks, we require ROUTING. We can use static or dynamic (IGP or EGP) routing, according to topology requirement.

3. Define the criteria for best path selection of a router?

Answer: A router’s routing table contains only best route. To select a route as best, a router considers the following parameters;

·         * Longest prefix match

·         * Minimum AD (administrative distance)

·         * Lowest metric value

If all listed parameters are the same, then it would perform equal cost load balancing.

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4. Define “stuck in active.”

Answer:  If a successor route (best route) fails, then the router sends a query message to its neighbor demanding a feasible successor (back-up route) and a query received by the router may be forwarded to other neighbors that could lead to a loop, as well. The wait for the response of query message is called “stuck in active” (SIA).

5. Can we use OSPF without backbone area?

Answer: Yes, but it will be limited to intra-area (same area) communication. By default, Inter-area communication is not possible without backbone area.

6. What do you mean by OSPF transit area?

Answer: A transit area is the area that has a virtual link connecting two or more ABRs attached to this area.

7. What is the difference between an OPPF neighbor and an adjacent neighbor?

Answer:  Neighbors are the routers that are in the same area and exchange hello packets, but not LSA information. Adjacent routers are routers that have fully exchanged their LSA information and are stable.

If OSPF state is in 2WAY/DROTHER, it means a neighbor relationship and, if the state is FULL/DR or FULL/BDR, it means that the adjacency is formed.

8. BGP neighborship is not coming up. Please define the various steps to troubleshoot it.

Answer: To troubleshoot BGP, first we need to check neighbor state using “show ip bgp summary.” If the state is Idle, it means that the peer address or AS is not defined properly; if the state is Active, it means that TCP port 179 is not open, the peer is not reachable, network congestion, or BGP  misconfiguration.

Common neighbor stability problems of BGP

·         * Misconfigured neighbor’s IP address and AS number

·         * Reachability issues when interfaces other than directly connected interfaces are used                     while peering (update-source issue).

·         * Authentication must be properly implemented (if configured)

·         * Router-ID must be unique

You can get more information from my BGP Tshoot article.

9. What is route reflector and why it is required?

Answer:    Route reflector is a solution for BGP split horizon. The rule says “prefix learned from an IBGP neighbour will not be advertised to another IBGP neighbour.”
To overcome this situation, we have multiple options:

1. Make your network a full mesh
2. Route confederation
3. Confederation

Route reflector is something like a central point acting as a route reflector server: Rather than peering with every IBGP router in a full mesh, it makes IBGP neighbors as route reflector clients to overcome the split horizon issue.

10. What is the difference between standard and extended ACL?

Answer: Standard ACLs are source-based, whereas extended ACLs are source- and destination-based. Standard ACLs can only filter layer 3 network traffic, while extended ACLs can be used to filter layer 3 and layer 4, as well.

11. What is the use of distribute-list?

Answer: To filter a routing database, we use distribute-list, which can be applied over most routing protocols. This means that, If you don’t want any specific network in your routing table, then you can use distribute-list.

12. MPLS works on which layer?

Answer: MPLS operates between layer 2 and layer 3, so it is sometimes called layer 2.5.

13. What is penultimate hop popping (PHP) and what is its use?

Answer: PHP is the technique for removing the (POP) MPLS label before the egress router. The MPLS label on a switched packet is popped by either the egress router or the penultimate router, depending on your configuration. If you decide to use penultimate hop popping, you essentially terminate the LSP one hop earlier. The MPLS labels are popped by the routers that connect to the egress router, rather than all of them being popped by the same egress router.

1. Does EIGRP support secondary addresses?

EIGRP and OSPF

Answer: EIGRP does support secondary address. Since EIGRP sources data packets from the primary addresses, Cisco recommends that we configure all routers on a particular subnet with primary addresses that belong to the similar subnet. Routers don’t form EIGRP neighbors over secondary network. For which,  if all of the primary IP addresses of routers don’t agree, problems arises with neighbor adjacency.

2. What does the EIGRP stuck in active message mean?

Answer: When EIGRP returns a stuck in active message, this means that it has not received a reply from a query. It sends a query when a route is lost and another feasible route does not exist inside the topology table. The SIA is caused by 2 sequential events:

• The reported route by the SIA has gone away.
• An EIGRP neighbor (or neighbors) have not replied to the query for that route.

When the SIA occurs, the router clears the neighbor that did not reply for the query. When it happens, determine which neighbor has been cleared. We should Keep in mind that this router can be many hops away.

3. What does the neighbor statement in the EIGRP configuration section do?

Answer: The neighbor command is used in EIGRP in order to define a neighboring router with which to exchange  information of routing. Because of the current behavior of the command, EIGRP exchanges information of routing with the neighbors in the form of unicast packets whenever the neighbor command is configured for interface. EIGRP holds on processing all multicast packets that come inbound on that interface. EIGRP also mutes sending multicast packets on that interface.

The ideal behavior of this command is for EIGRP to start sending EIGRP packets as unicast packets to the specified neighbor, but  don’t stop sending and receiving multicast packets on that interface. As  the command does not behave as intended, the neighbor command must used carefully.

4. Why does the EIGRP passive-interface command remove all neighbors for an interface?

Answer: The passive interface command disables the transmission and receipt of EIGRP hello packets on interface. Not similarly IGRP or RIP, EIGRP sends hello packets in order to form and sustain neighbor adjacency. Not having a neighbor adjacency, EIGRP doesn’t exchange routes with a neighbor. For which the passive interface command prevents the exchange of routes on the interface. Although EIGRP does not  receive or send routing updates on an interface configured with the passive interface command, it includes the address of the interface in routing updates sent out of other non-passive interfaces.

5. What is the difference in metric calculation between EIGRP and IGRP?

Answer: The EIGRP metric is obtained when you multiply the IGRP metric of 256. The IGRP uses only 24 bits in its update packet for the metric field, but 32 bit is used by EIGRP  in its update packet for the metric field. E.g the IGRP metric to a destination network is 8586, but the EIGRP metric will be 8586 x 256 = 2,198,016. Integer division is used when we divide 10^7 by lowest BW, so the calculation involves division of integer, which may leads to a variation from manual calculation.

6. How does EIGRP behave over a GRE tunnel compared to a directly connected network?

Answer: EIGRP will use the same administrative distance and metric calculation to the GRE tunnel. The calculation of cost is based on delay and bandwidth. The bandwidth and delay of the GRE tunnel must be taken from the tunnel interface configured on the router. The tunnel will be treated like a directly connected network.

If there are 2 paths to reach a n/w either through a tunnel interface or VLAN interface, EIGRP prefers the Virtual-Access Interface VLAN interface because the VLAN interface has greater bandwidth than the tunnel interface. To influence the routing through the tunnel interface, It increases the bandwidth parameter of the tunnel interface, or increases the delay parameter of the VLAN interface.

7. Can an OSPF default be originated into the system based on external information on a router that does not itself have a default?

OSPF

Answer: OSPF generates a default only if it is configured using the command default-information originate and if there is a default network in the box from a different process. In OSPF the default route is 0.0.0.0. If we want an OSPF enabled router to generate a default route even if it does not have a default route, use the command default-information originate always.

8. How does Cisco accommodate OSPF routing on partial-mesh Frame Relay networks?

Answer: We can configure OSPF to understand whether it should attempt to use multicast facilities on a multi-access interface. If multicast is available, OSPF usually uses it for its normal multicasts

Cisco IOS Software Release 10.0 includes a feature called sub-interfaces. We can use sub interfaces with Frame Relay to tie together a set of virtual circuits to form a virtual interface, it acts as a single IP subnet. All systems inside the sub-net should be fully meshed.

9. Which address-wild-mask pair should I use for assigning an unnumbered interface to an area?

Answer: When we configured an unnumbered interface, it references  an another interface on the router. When we enable OSPF on the unnumbered interface,  we should use the address-wild-mask pair of interfaces to which the unnumbered interface is pointing.

10. What is the link-state re-transmit interval, and what is the command to set it?

Answer: OSPF sends acknowledgment of every newly received link-state advertisement. It does this by sending LSA packets. After that  LSAs are re-transmitted until they are acknowledged. The link state when re-transmit the interval it defines the time between the re-transmissions. We  can use the command IP OSPF re-transmit interval to set the re-transmit interval. 5 seconds is the default value.

1. What is Spanning tree ?

Spanning Tree Protocol

Answer: It is the protocol which is used in switching network to create a loop free topology. STP is enabled by default on every VLANs on Catalyst switches. STP switches send BPDUs to each other to form their topology databases. BPDUs are sent out every ports every two seconds, which are forwarded to a specific MAC multicast address.

2. What is non-designated port?

Answer: A non-designated port is the port with a higher cost rather than the designated port. In blocking mode non designated ports are put. These are not used as forwarding ports.

3. SPT Learning Port States:

Answer: The switch port hears to BPDUs and learns all the paths in the network of switched. A port in learning state is also populates the MAC address table but it doesn’t forward data frames. Forward delay takes the time it takes to transition a port from listening to learning mode, which is set to by default 15 seconds and can be seen in the show spanning p-tree output.

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

Answer: Spanning Tree Protocol has the protocol is to prevent Layer 2 switching loop and to broadcast storms in a Local Area Network. The STP allows redundant links in a network for preventing complete network failure if an active link fails, without the danger of Layer two Switching loops.

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

Answer: Three types of Bridge Protocol Data Units are Configuration BPDU, Topology Change Notification BPDU and Topology Change Notification Acknowledgment.

6. What is the purpose of STA and BPDU?

Answer: The main purpose of the BPDU and the Spanning Tree Algorithm is to avoid Layer 2 Switching loops and Broadcast storms.

7. What is path cost?

Answer: It is an arbitrary value, based on bandwidth,  hop count, or another calculation, that is totally assigned by a network administrator and used by the routing protocol to compare different routes by an inter network. Protocols of Routing use cost values to select the best path to a destination. The minimum cost identifies the best path. It is also known as path cost.

8. What is UDLD and why it is required?

Answer: 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 an unidirectional link exists. All connected devices supports UDLD for the protocol to successfully identify and disable the unidirectional links. When UDLD detects an unidirectional link, it shuts down the affected port and alerts us. Unidirectional links causes a variety of problems, including ST topology loops.

9. Explain about three functions of a Switch.

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

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

Forward/filter decisions :
When a frame is received on the interface, the switch looks at the destination hardware address and finds the exit interface in the MAC database. The frame is forwarded out the specified destination port.

Loop avoidance :
If multiple connections between switches are created only for redundancy purposes, network loops occurs. SPT is used to stop network loops while still permitting redundancy.

10. Define Root Bridge.

Answer: The root bridge is the bridge having the best bridge id. With spanning tree protocol, 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 i.e. 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.

2. What is PPP?

Answer: The Point-to-Point Protocol is designed for users who need a trusted point-to-point connection to the Internet. PPP operates at the data link and physical layers of the OSI model.

10. Explain the three types of addresses in TCP/IP?

Answer: There are three types of addresses are used by systems using the TCP/IP protocol: the internet work address, the physical address, and the port address. The physical address, it  is  also known as the link address, is the address of a node as defined by its WAN or LAN. The IP address defines uniquely a host on the Internet. The port address recognizes a process on a host.