OSPF - [Part 1] - QUICK REVIEW

OSPF (Open Shortest Path First)

Protocol Type -  Link State Protocol

Cisco Proprietary -  No (Open Standard)

IGP/EGP - IGP (Interior Gateway Protocol)

Administrative Distance -110

Routed Protocol Support -  IPv4 (IPv6 in OSPFv3)

Transport Protocol - IP protocol 89

Updates -  Only Changes (Triggered)

VLSM/CIDR Support - Support

Classlful/Classless Protocol  - Classless Protocol

Dis contiguous Network Support - Support

Hop Count Limit - None (Limited by Network)

Algorithm - Dijkstra Algorithm

Broadcast/Multicast Based -  Multicast Based. update multicast on change

Multicast Address - 

• 224.0.0.5 For all OSPF router
• 224.0.0.6 for all OSPF designated router

Authentication - 

• Simple password authentication
• And Message Digest authentication (MD-5).

Hierarchical Network Requirement -  We can also use concept of area and autonomous system in OSPF

Metrics - 

• Bandwidth - OSPF uses cost as its metric, which is computed based on the bandwidth of the link.
• Example :- Cost calculation for 100 Mbps reference bandwidth and interface bandwidth is 10 Mbps Ethernet.

Formula to calculate the cost is = [Reference Bandwidth / interface bandwidth]
OSPF Metric Cost value is 100 Mbps / 10 Mbps = 10.

Manual Summarization Support - Support

Auto Summarization Support - Does not support

Working Process -  OSPF follow below states-

• Down
• Init
• Two Way
• Ex-start
• Exchange
• Loading
• Full

Loop Avoidance Mechanism -  

• There is no loop avoidance mechanism in OSPF like distance-vector routing protocols.
• The loop avoidance is inherent in the SPF algorithm,which is used by OSPF as a link-state routing protocol to calculate routes.
• Via SPF, a tree is built for each router with itself as the root. Trees do not have loops.

Best Path Selection - 

• If multiple routes are available to the same network with different route types, then routers will be follow this order of preference from highest to lowest: 

1. Intra-area routes.
2. Inter-area routes.
3. External Type-1 routes.
4. External Type-2 routes.

• If multiple routes are available to the same network with same route types, then, OSPF will use cost as its metric to calculate the best route, which is computed based on the bandwidth of the link. The route with the lowest value for cost is chosen as the best route.
• If multiple routes are available to the same network with same route types and cost, then OSPF will choose all the routes which available in the routing table, and the router will perform equal cost load balancing across multiple paths.

Load Balancing - Load Balancing Across 4 Equal Cost Paths

Passive Interface -   In OSPF passive interface is used to stop both outgoing advertisements and incoming updates.

Packets - 

• Hello Packet
• Database Description (DBD) Packet 
• Link State Request (LSR) Packet
• Link State Update (LSU) Packet
• Link State Acknowledgement (LSAck) Packet.

States - 

• Down - OSPF neighbor will be detected at this moment
• Init - Hello packet received by OSPF neighbor
• Two-way - OSPF neighbor found own router ID in received hello packet
• Ex-start - Master slave roles will determine
• Exchange - Database Description Packets (DBD) will send
• Loading - Exchange of LSRs (Link State Request) and LSUs (Link State Update) packets
• Full - Now OSPF router have an adjacency

Convergence - Faster than RIP and slower than EIGRP.

Updates - Triggered updates

Timer -

• Hello interval in OSPF for broadcast and point-to-point networks or LAN - 10 Second.
• Hello interval in OSPF for non-broadcast multiple access (NBMA) networks - 30 Second.
• By default, dead interval is four times the default hello interval. 
• Dead interval in OSPF for broadcast and point-to-point networks or LAN - 40 Second.
• Dead interval in OSPF for NBMA networks - 120 Second.

Strength - 

• Fast converges compared to a distance vector protocol (RIP, IGRP).
• In OSPF Routing update packets are small, so the entire routing table is not sent as an update, only changes will sent in update. 
• Not prone to routing loops. 
• Compatible with large networks. 
• Supports VLSM or CIDR.

Weakness - 

• Complex to configure and understand than a distance vector protocol

How to enable 

Router#config t
Router(config)#router ospf 10

Router(config-router)#network 10.0.0.0 0.255.255.255 area 0

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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