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
Terminology
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 224.0.0.5.
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.
Before we start BGP configuration check out some important of BGP CCIE exam topics.
Here are the lists of all OSPF labs and theories CCNA to CCIE.
Topology
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 10.1.1.1 YES manual up up
Loopback0 192.168.100.50 YES manual up up
MARK#show ip interface brief
Interface IP-Address OK? Method Status Protocol
Serial3/0 10.1.1.2 YES manual up up
Loopback0 192.168.150.75 YES manual up up
LUKE(config)#router ospf 1
LUKE(config-router)#network 10.0.0.0 0.255.255.255 area 0
LUKE(config-router)#network 192.168.100.0 0.0.0.255 area 0
MARK(config)#router ospf 1
MARK(config-router)#network 10.0.0.0 0.255.255.255 area 0
MARK(config-router)#network 192.168.150.0 0.0.0.255 area 0
LUKE#show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Lo0 1 0 192.168.100.50/24 1 LOOP 0/0
Se3/0 1 0 10.1.1.1/8 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 10.1.1.2/8 64 P2P 1/1
LUKE#ping 192.168.150.75
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.150.75, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/27/68 ms
LUKE#traceroute 192.168.150.75
Type escape sequence to abort.
Tracing the route to 192.168.150.75
VRF info: (vrf in name/id, vrf out name/id)
1 10.1.1.2 52 msec 88 msec 24 msec
MARK#ping 192.168.100.50
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.100.50, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/24/36 ms
MARK#traceroute 192.168.150.75
Type escape sequence to abort.
Tracing the route to 192.168.150.75
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.150.75 8 msec 8 msec 8 msec
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