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Showing posts with the label OSPF

OSPF Options Field

OSPF Options Field DN = Down bit : used in MPLS Layer 3 VPNs. If a route learnt from a customer network via OSPF is advertised across a BGP / MPLS VPN using Multiprotocol BGP, and is advertised back to a customer network via OSPF, there is a possibility of a loop to occur in which the OSPF route is redistributed back to the VPN service provider network via BGP. The DN bit prevents this type of routing loop. O = The “O” bit is set when the originating router supports Type 9,10 and 11 opaque LSAs. Not used in normal OSPF implementations DC = The “DC” bit is set when the originating router supports OSPF over Demand Circuits. Not used in normal OSPF implementations L = Indicates whether the OSPF packet contains a LLS (Link-Local signalling) data block. This bit is set only in Hello and DBD packets. N = The N bit is used only in Hello packets. The N bit is set when the originating router supports Type-7 NSSA-External-LSAs. Neighboring routers with mismatched N bit value will not form

OSPF Interview Questions - Part 4

How would you speed up OSPF convergence? Configure lower hello and dead interval timers Configure ISPF - incremental SPF --> The principle of ISPF is to update the affected nodes only without rebuilding the whole tree, thereby, resulting in a faster convergence and saving CPU intensive activity of processing. Configure OSPF's SPF throttling - The SPF throttling feature makes it possible to configure SPF scheduling in millisecond intervals and to potentially delay SPF calculations during network instability. Command : "timers throttle spf [spf-start] [spf-hold] [spf-max-wait] How do you modify the cost of a default route propogated into an area? Use the command "default-information originate [always] [metric value] [metric-type value] [route-map name]" where: metric value = numerical cost metric-type value = Type 1 / Type 2 etc. route-map name = name of the route-map Command to create a stub area area xx stub Command to create a totally stub area area xx stub no

OSPF Interview Questions - Part 3

What are the tables maintained by OSPF? Neighbor Table : This table stores information about all the OSPF neighbors. Can be displayed using the command : show ip ospf neighbor Database / Topology Table : This table stores link state database of all the OSPF learned routes. Can be displayed using the command : show ip ospf database Routing table : After running the SPF algorithm on all the routes in the database table, the best route is selected and promoted to the Routing table. Can be displayed using the command : show ip route ospf What are the benefits of dividing an OSPF domain into multiple areas? Decrease routing overhead per router (this especially makes sense for internal non-backbone routers i.e. routers inside a single non-backbone area Ensure speedy convergence.. less routes implies faster processing Confine network instability into single area.. the inter-area summary routes and the routes from special areas such as stub, totally stub, NSSA, totally NSSA are usually &quo

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