OSPF (Open Shortest Path first)

OSPF

• It is works on Dijkstra Algorithm
•  It is purely a classless protocol
• It supports only VLSM
• It is a 2-layer hierarchy
• Minimizing the routing updates traffic
•  With in the Area only all the information will send to all routers in that area (LSA)
• The ABR will send Auto summary information to the backbone area ( So burden will reduced on the main system)
• The Multi-cast address are

• 224.0.0.5 - All other routers will communicate through this Multicast address.
•  224.0.0.6 - All routers with DR router will communicate through this.
•  NOTE: DR and BDR will Understand both packets from 224.0.0.5/6

• Its AD value is 110
• Its protocol number is 89
• Metric is calculated by using “ BANDWIDTH”
•  Cost of path =  Reference Bandwidth/My outgoing link BW
•  Note: Reference BW in OSPF is 100 Mbps ( We can also change the Reference BW)
•  EX: Serial – 1.54 Mbps = 64 cost, Fast Ethernet - 100Mbps = cost 1, Ethernet – 10Mbps = cost 10

Router types:

OSPF defines the following overlapping categories of routers:

Internal router (IR)

An internal router has all its interfaces belonging to the same area.

Area border router (ABR)

An area border router is a router that connects one or more areas to the main backbone network. It is considered a member of all areas it is connected to. An ABR keeps multiple copies of the link-state database in memory, one for each area to which that router is connected.

Backbone router (BR)

A backbone router has an interface to the backbone area. Backbone routers may be also area routers, but do not have to be.

Autonomous system boundary router (ASBR)

An autonomous system boundary router is a router that is connected by using more than one routing protocol and that exchanges routing information with routers autonomous systems. ASBRs typically also run an exterior routing protocol (e.g., BGP), or use static routes, or both. An ASBR is used to distribute routes received from other, external ASs throughout its own autonomous system. An ASBR creates External LSAs for external addresses and floods them to all areas via ABR. Routers in other areas use ABRs as next hops to access external addresses. Then ABRs forward packets to the ASBR that announces the external addresses.

The router type is an attribute of an OSPF process. A given physical router may have one or more OSPF processes. For example, a router that is connected to more than one area, and which receives routes from a BGP process connected to another AS, is both an area border router and an autonomous system boundary router.

Each router has an identifier, customarily written in the dotted decimal format (e.g., 1.2.3.4) of an IP address. This identifier must be established in every OSPF instance. If not explicitly configured, the highest logical IP address will be duplicated as the router identifier. However, since the router identifier is not an IP address, it does not have to be a part of any routable subnet in the network, and often isn't to avoid confusion.

Timmers:

• Hello – 10 sec
• Hold down timmer – 40 sec
• Dead, wait – 40 sec

For 2 multipoint links:

Hello – 30 sec

Holddown – 120 sec

Neighbourship requirements in OSPF:
In order to form neighbourship in OSPF with the adjacent routers they should follow these..

•   Same Subnet
•   Time interval also should match (hello time, dead time)
•   IP MTU must match (if MTU is not matched, it will form the neighbour but stuck in the exstart state and goes down then again down to init and init to exstart.)
•   The Router must be able to send/receive IP packets to one another
•   Interface primary IP address must be in same subnet
•   Must not be passive on the connected interfaces
•   Must pass neighbor authentication (if configured)
•   Must be in same area
•   Router ID must be unique

Router ID:

Router ID is a 32 bit number. It looks like an IP address but it is not a IP address, it don’t have Subnet Mask.

Modes of selection of Router ID:

1. The highest loopback address (If a router has a loopback address then it wont check for interface IP address)

2. If there is no loopback address then it will check for highest IP address for the up Interface.

We can also assign a Router ID manually:

#router ospf 10

#router-id 1.1.1.1

Types of Packets:

1. Hello

2. LSA (Link State Advertisment)

3. LSR (Link State Request)

4. LSU (Link State Update)

5. LSAck (Link State Acknowledgment)

OSPF databases / tables:

•      OSPF adjacency database = Neighbor table

•      OSPF link-state database = Topology table

•      OSPF forwarding database = Routing table

Hello Packet:

•  It contains à Router ID, Subnet (With prefix), Authentication, Stub Area, Timer, and Area ID.
•   Send as multicast to Multicast address 224.0.0.5
•   Reply will be in Unicast (to the particular router ID)