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OSPF Interview Questions - Part 2

What are types of OSPF packets? Hello Database Description (DBD) Link-State Requests (LSR) Link-State Updates (LSU) Link-State Acknowledgements (LSAck) What is OSPF finite state machine? What are the states in OSPF neighborship? DOWN - No OSPF packets have been received on the interface ATTEMPT - Applies only to non-broadcast multi-access networks (NBMA) where the neighbors must be configured manually using the "neighbor x.x.x.x" command INIT - Router has seen a Hello message from an OSPF router TWO-WAY - A Hello message has been sent to the neighbor and the neighbor has replied with its Hello message. In Broadcast networks, the DR/BDR election takes place after this state EXSTART - DR and BDR establish OSPF adjacencies with each of the routers in the network. Master / Slave election takes place at this stage. The Master sends its DBD (Database Descriptor) first EXCHANGE - Routing information may be exchanged via DBD, Link State Requests (LSR) and Link State Updates (LS

OSPF Interview questions - Part 1

What is the protocol number of OSPF? 89 What is the multicast address of OSPF messages? for OSPF All-routers multicast for OSPF All-Designated routers multicast What are the OSPF hello packet timers? Hello packets are sent every 10 seconds with the Dead timer being 40 seconds Is it mandatory to have OSPF process ID the same on adjacent routers to form OSPF neighborship? No What is the impact of interface MTU mismatch on adjacent OSPF routers? MTU is not checked during the formation of neighbor adjacency. However, mismatched MTU values impede in the successful exchange of Database Description (DD) packets and thereby prevent the neighbors from reaching the "FULL" neighborship state. What are the types of OSPF LSAs? Type 1 - Router LSA Type 2 - Network LSA Type 3 - Summary LSA Type 4 - ASBR Summary LSA Type 5 - External LSA Type 6 - Multicast OSPF (MOSPF) LSA Type 7 - Not-So-Stubby-Area (NSSA)LSA Type 8 - External Attribute LSA for BGP Who generates Typ

Routing Information Protocol - Basics

Dynamic routing protocol Administrative distance - 120 Metric for best path selection - Hop count (Route with lowest hop count preferred) Maximum hop count allowed - 15 Route is considered unreachable for hop count - 16 RIP versions - RIP version 1, RIP version 2, RIP-NG RIP version 1 Classful routing protocol i.e. doesn't send subnet mask information in the update messages Sends periodic updates as broadcast Destination IP address for broadcast - Doesn't support authentication of update messages RIP version 2 Classless routing protocol but does support classful Sends periodic updates as multicast Destination IP address for multicast - Supports authentication of update messages RIP-NG Classless routing protocol (of course, this is IPv6 we are talking about) Sends periodic updates as multicast Destination IPv6 address for multicast - FF02::9

Common Routing protocol AD values

The AD values of the protocols are generally different across different vendors and care must be taken to take this fact into account in case of multi-vendor environments. Cisco Directly Connected Interface = 0 Static route = 1 EIGRP summary route = 5 External Border Gateway Protocol (eBGP) = 20 Enhanced Interior Gateway Routing Protocol (EIGRP) - Internal = 90 Open Shortest Path First (OSPF) = 110 Intermediate System to Intermediate System = 115 Routing Information Protocol (RIP) = 120 Enhanced Interior Gateway Routing Protocol (EIGRP) - External = 170 Internal Border Gateway Protocol (iBGP) = 200 Juniper Directly Connected Interface = 0 Static route = 5 Intermediate System to Intermediate System - Level-1 Internal = 15 Intermediate System to Intermediate System - Level-2 Internal = 18 Routing Information Protocol (RIP) = 100 Summary Route (Aggregate route) = 130 Open Shortest Path First (OSPF) = 150 Intermediate System to Intermediate System - Level-1 External = 160 Intermediate Syst

Routing - Administrative Distance

Most routing protocols have metric structure that are not comparable with each other, in case multiple protocols are being used in a network. For eg. RIP uses hop count and OSPF uses cost for best path selection to a particular route. In order to have a common parameter which can be used for computing the best path, the concept of Administrative Distance (AD) was introduced. AD is simply a number which helps identity a particular route learned from a particular protocol. For eg, if the route is learned from Routing Information Protocol (RIP), then it would have a default AD value of 120 (unless it has been explicitly modified). Similary, a route learned from OSPF would have the AD value of 110. Rule: A route with lower Administrative Distance (AD) value would always be preferred over the one with higher AD value. In the above scenario, if the route is being learned from both RIP as well as OSPF, then the route learned via OSPF would be preferred since OSPF has a lower AD value of 110 t

Cisco SDA - Components

Cisco Software Defined Access is a new paradigm towards building Enterprise Networks. Primarily built upon the Cisco's DNA Center software, it leverages several next-generation components to design, provision and apply policy to create an intelligent wired and wireless network infrastructure. Cisco DNA Assurance which turns the network devices into sensors, giving visibility to everything on the network; guided remediation which automates resolution to keep the network at its optimal performance and thereby improve the end user experience, are some of Cisco's USPs. Cisco SDA accomplishes this intent based networking via the following components: Fabric Control Plane nodes: Based on LISP map-server and map-resolver functionality combined together on the same node, control plane node (database) tracks the endpoints in the fabric site and associates the endpoints to fabric nodes. Border nodes and edge nodes register with the control plane nodes. Control Plane node can be dedicate

Cisco ACI - Forwarding inside the Fabric

One of the most intriguing (of course, if you get the hang of it) or depressing concepts of Cisco ACI is how the traffic forwarding takes place inside Cisco ACI. Let's start with an endpoint sending the frame to the connected leaf: The leaf checks the destination MAC address of the frame. The leaf will do a layer 2 lookup to find the destination MAC. If the leaf knows the location of the destination MAC (either local to the leaf or some other leaf), it will determine the destination's EPG. Depending on the EPG, it would determine if a contract is required to allow the frame to forward.. If yes, it would look into the L3 and L4 contents of the packet to determine if the contract exists. If it does, allow the traffic, if not drop. If the frame has the destination MAC address of that of the leaf, it will be routed. This will be the standard destination IP based routing. If a route exists for the destination in the VRF of the source, it is routed. If not, it will be dropped. With r