IP ROUTING - INTERVIEW QUESTIONS (Part 3)

Question- 1 What is AD ?

• The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. 
• An administrative distance is an integer from 0 to 255. 
• Where 0 is the most trusted and 255 means no traffic will be passed via this route.

Table 1 shows the default administrative distances that a Cisco router uses to decide which route to take to a remote network.

                    Route Source                                      Default AD

                    Connected interface                                                                                     0
                    Static route to exit interface                                                                         0
                    Static route to next-hop address                                                             1
                    RIP (Routing Information Protocol)                                                          120
                    IGRP (Interior Gateway Routing Protocol)                                               100
                    Internal EIGRP (Enhance Interior Gateway Routing Protocol)                 90
                    External EIGRP                                                                                         170
                    EIGRP with summary route                                                                       5
                    OSPF (Open Shortest Path First)                                                              110
                    EBGP (External Border Gateway Protocol)                                            20
                    Internal BGP                                                                                             200
                    IS-IS (Intermediate System-to-Intermediate System)  115
                    EGP (Exterior Gateway Protocol) 140
                    ODR (On Demand Routing) 160
                    DMNR (Dynamic Mobile Network Routing) 3
                    NHRP (Next Hop Resolution Protocol)                                                    250
                    ex. DHCP-learned (Floating Static Route) 254
                    Unknown 255

Unknown 255 (This route will never be used.) If a network is directly connected, the router will always use the interface connected to the network. If you configure a static route, the router will

then believe that route over any other learned routes. You can change the administrative distance of static routes, but by default, they have an AD of 1

Question - 2 Which route will be prefer by router, if you have a static route, a RIP-advertised route, and an EIGRP-advertised route listing the same network ?

If you have a static route, a RIP-advertised route, and an EIGRP-advertised route listing the same network, then by default, the router will always use the static route unless you change the AD of the static route

Question- 3 What is AS ?

An AS is a collection of networks under a common administrative domain.

Question- 4 Types of routing ?

Question- 5 What is IGP and EGP ?

• Two types of routing protocols are used in internetworks: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs). 
• IGPs are used to exchange routing information with routers in the same autonomous system (AS). An AS is a collection of networks under a common administrative domain, which basically means that all routers sharing the same routing table information are in the same AS.
• EGPs are used to communicate between ASes. An example of an EGP is Border Gateway Protocol (BGP).

Question- 6 How routing protocol select the best path ?

• If a router receives two updates listing the same remote network, the first thing the router checks is the AD (Administrative Distance). If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.
• If both advertised routes to the same network have the same AD, then routing protocol metrics (such as hop count or bandwidth of the lines) will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table. 
• But if both advertised routes have the same AD as well as the same metrics, then the routing protocol will load-balance to the remote network (which means that it sends packets down each link).

Question- 7 What is hop count ?

Each time a packet goes through a router, that’s called a hop.

Question- 8 What is Distance Vector Routing Protocol ?

• Distance vector The distance-vector protocols in use today find the best path to a remote network by judging distance. For example, in the case of RIP routing,The route with the least number of hops to the network is determined to be the best route. The vector indicates the direction to the remote network. Both RIP and IGRP are distance-vector routing protocols. They periodically send the entire routing table to directly connected neighbors.
• The distance vector routing algorithm periodically passes complete routing table content to neighboring router, which combined the recieved routing table entries with their own routing table to complete the router's routing table. 

Question- 9 Why  Distance Vector Routing Protocol is also known as routing by rumor ?

Because a router receiving an update from a neighbor router believes the information about remote networks without actually finding out for itself.

Question - 10 What is Link State Routing Protocol ?

• Link state In link-state protocols, also called shortest-path-first protocols.
• They know more about the whole inter-network than any distance-vector routing protocol. OSPF is an IP routing protocol that is completely link state.

Question- 11 Explain three separate tale of Link State Routing Protocol ?

Three separate table are created on each link state routing enable route. 

• One table is used to hold details about directly connected neighbors.
• One is used to hold the topology of entire inter-network
• And last one is used to hold the actual routing table 

Question- 12 Explain the process of link state routing protocol ?

• Link state routing protocol send the information about directly connected links to all the routers in the network. 
• Example of link state routing protocols include OSPF - Open Shortest Path First and IS-IS - Intermediate System to Intermediate System. 

Question- 13 Explain the link state routing operation ?

Link State Routing operation follows four simple steps, each link state enable router must perform the following.

• Discover its neighbors and builds its neighbor table (Building and distributing Link State Packet) - Each link state enabled router periodically sends a HELLO message on each of its link. Neighbor routers respond to these HELLO messages to identify themselves, within the replies, network address of the routers are attached and are used by the HELLO initiator to build up its neighbor table.
• Measure the cost (Delay, bandwidth etc) to each of its neighbor - A set of tests is preformed on each router to measure the cost to each of its neighbor. The cost could be measure of the end-to-end delay, throughput, or a combination of these metric.
• Construct and send a routing update telling all it has learned to all router in the network. - Each router builds a packet that containing its neighbors and the corresponding link cost to these neighbors. At the beginning of the packet, each router add its identity along with the sequence number and an age parameter. After the construction process, the packet is flooded in the network.
• Apply Dijkstra algorithm to construct the shortest path to all possible destination - Using all the details from its link state table, a router is able to compute, using the Dijkstra Algorithm, the shortest path to any given destination.

Question- 14 What is use of "Hello message" in link state routing protocol ?

Link State Protocol tend to use HELLO messages to establish neighbor-ship because they normally do not send out periodically route update and there has to be some mechanism to help neighbors realize when a new peer has moved in or an old on has left or goes down.

Question- 15 What is LSP ?

They are used to efficiently determine

• Link State Packet that containing its neighbors and the corresponding link cost to these neighbors. At the beginning of the packet, each router add its identity along with the sequence number and an age parameter. 
• What the new neighbor is, if a link failure occurs and the cost of changing a link if the need arises

Question- 16 What is hybrid routing protocol ?

Hybrid Hybrid protocols use aspects of both distance vector and link state—for example, EIGRP.

Question- 17 What is difference between distance vector and link state routing protocol ?

Distance Vector Protocol

• Entire routing table is sent as an update 
• Distance vector protocol send periodic update at every 30 or 90 second
• Update are broadcast.
• Updates are sent to directly connected neighbor only.
• Routers do not have end to end visibility of entire network.
• Distance vector routing protocol network may have path in network carrying wrong information.
• It is proned to routing loops.
• Routing loop avoidance mechanism used as Maximum hop count, Split Horizon, Route Poisoning and Hold Down Timer.
• Distance vector routing protocol has slow convergence due to periodic update.
• Example RIP, IGRP

Link State Protocol

• Update are incremental and entire routing table is not sent as an update.
• Update are triggered not periodically.
• Update are sent to entire network and to just directly connected neighbor.
• Updates are carry SPF tree information and SPF cost calculation information and information of entire topology.
• Routers have visibility of entire network of that area only.
• No routing loops.
• Convergence is fast because of triggered updates.

Question - 18 Explain loop avoidance mechanism.

Maximum Hop Count

RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable. In other words, after a loop of 15 hops, Network  will be considered down. Thus, the maximum hop count will control how long it takes for a routing table entry to become invalid or questionable.

Split Horizon

This reduces incorrect routing information and routing overhead in a distance vector network by enforcing the rule that routing information cannot be sent back in the direction from which it was received.

Route Poisoning

When Network goes down, Router initiates route poisoning by advertising Network with a hop count of 16, or unreachable (sometimes referred to as infinite).

Holddowns

A hold-down prevents regular update messages from reinstating a route that is going up and down (called flapping). Typically, this happens on a serial link that’s losing connectivity and then coming back up.

Question- 19 Types of cabling ?

     Cable                                                                        Connection

Straight Through Cable                                                   Host to Switch or Hub
                                                                                            Router to Switch or Hub

Cross Over Cable                                                              Router to Router via FastEthernet
                                                                                            Switch to Switch
                                                                                            Host to Host
                                                                                            Hub to Hub
                                                                                            Router to Direct Host

Rolled Cable                                                                      Console Port to PC

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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