Tuesday, 30 October 2018

What is Open Shortest Path First (OSPF) complete?

Open Shortest Path First (OSPF) 

Open shortest path first is an open standard routing protocol that’s been implemented by a wide variety of network venders, include Cisco. And it’s that open standard characteristic that’s the key to OSPF flexibility and popularity. OSPF use the Dijkstra algorithm to initially construct a shortest path tree and follows that by populating the routing table with the resulting best route. Its quick convergence is another reason it’s a favorite. Another two great advantages OSPF offers are that it supports multiple, equal-cost routes to the same destination and it also supports both IPv4 and IPv6 routed protocols.

Here’s a list that summarizes some of OSPF features:

·         Link-state routing protocol.
·         Open standard (IETF)
·         Allow for the creation of areas and autonomous system
·         Minimize the routing update traffic
·         It’s highly flexible, versatile, and scalable
·         Support VLSM/CIDR
·         Offers an unlimited hop count


Link a link is a network or router interface assigned to any given network. When an interface is added to the OSPF process, it’s considered to be a link.

Router ID the router ID (RID) is an IP address used to identify the router. Cisco routers choose the router ID by using the highest IP address of all configured loopback interface. To router ID is basically the “name” of each router.

Neighbor neighbor are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link. OSPF neighbors must have a numbers of common configuration options to be able to successfully establishing a neighborship, and all of these options must be configured exactly the same way:
·         Area ID
·         Stub area flag
·         Authentication (if using one)
·         Hello and Dead intervals

Adjacency an adjacency is a relationship between two OSPF routers that permits the direct exchange of routes updates. OSPF directly share routes only with neighbors that have also established adjacencies. Not all the routers neighbors will become adjacent – this depends upon both the type of network and configuration of the routers.

Designated router a designated router(DR) is elected when OSPF routers connected to the same broadcast network to minimize the number of adjacencies formed and to publicize received routing information to and from the remaining routers on the broadcast network or link. Election are won based Upon a router’s priority level, with the one having the highest priority becoming the winner. If there’s a tie, the router ID will be used break it.

Backup designated router a backup designated router is hot standby for the DR on broadcast, or multi-access, links. The BDR receives all routing update from OSPF adjacent routers but does not disperse LSA updates.

Hello protocol the hello protocol provide dynamic neighbor discovery and maintain neighbor relationship. Hello packets are sent to multicast address

Neighbor database the neighbor database is a list of all OSPF routers for which hello packets have been seen.

Topological database the topological database contains information from all of the link state advertisement packets that have been received from an area.

Link state advertisement a link state advertisement (LSA) is an OSPF data packet containing link-state and routing information that’s shared among OSPF routers. LAS packets are used to update and maintain the topological database. There are different types of LSA packets.

OSPF area an ospf area is a grouping of contagious networks and routers.

Broadcast (multi-access) broadcast multi-access networks such as Ethernet allow multiple devices to connect to or access the same network, enabling a broadcast ability in which a single packet is delivered to all nodes on the network.

Nonbroadcast multi-access (NBMA) nonbroadcast multi-access network are networks such as frame relay, X.25, and Asynchronous Transfer Mode (ATM). These kinds of networks allow for multi-access without broadcast ability like Ethernet.

Point-To-Point   Point-To-Point   refers to a type of network topology made up of a direct connection between two routers that provides a single communication path.
Point-to-multipoint Point-to-multipoint refers to type of network topology made up of a series of connections between a single interface on one router and multiple destination routers

 OSPF Metric

OSPF uses a metric referred to as cost. A cost is associated with every outgoing interface include in an SPF tree. Ospf metric is not define in standards, every vendor uses different formula to calculate metric. cisco  uses a simple equation of 108/bandwidth.


Goal: verifying reachability between routers LUKE and MARK with basic OSPF  single area,

LUKE#show ip interface brief

Interface              IP-Address             OK? Method         Status             Protocol
Serial3/0                      YES manual           up                    up
Loopback0       YES manual          up                    up

MARK#show ip interface brief
Interface              IP-Address      OK? Method        Status                Protocol

Serial3/0            YES manual              up                    up
Loopback0  YES manual           up                    up

LUKE(config)#router ospf 1
LUKE(config-router)#network area 0
LUKE(config-router)#network area 0

MARK(config)#router ospf 1
MARK(config-router)#network area 0
MARK(config-router)#network area 0

LUKE#show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Lo0          1     0       1     LOOP  0/0
Se3/0        1     0              64    P2P   1/1

MARK#show ip ospf interface br
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Se3/0        1     0              64    P2P   1/1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/27/68 ms

Type escape sequence to abort.
Tracing the route to
VRF info: (vrf in name/id, vrf out name/id)
  1 52 msec 88 msec 24 msec

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/24/36 ms

Type escape sequence to abort.
Tracing the route to
VRF info: (vrf in name/id, vrf out name/id)
  1 8 msec 8 msec 8 msec

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