OSPF 1 - Basics

Open Shortest Path First

Introduction

For the CCNA we need to only configure and verify a SINGLE area OSFPv2

Link State Routing Protocols

Every router creates a "connectivity" map of the network
To do this, each router advertises information about its interfaces (connected networks) to its neighbours.These advertisements are passed along to other routers, until all routers in the network develop the SAME map of the network
THEN, each router independently uses this map to calculate the best routes to each destination
Link state protocols are more CPU intensive as more information is shared
BUT... they are faster in reacting to changes in the network than DV (distance vector) protocols

OSPF

Open Shortest Path First - uses the SHORTEST PATH FIRST algorithm invented by Dutch computer scientist Edsger Dijkstra, commonly called Dijkstra's Algorithm
OSPF has 3 versions
- OSPF v1 (1989) - OLD and not in use any more
- OSPF v2 (1998) - Used for IPv4 only
- OSPF v3 (2008) - Used for IPv6 (can be used for IPv4, but usually OSPF v2 is used)
Routers store information ABOUT the network in LSA's (link-state-advertisements) which are organised in a structure called the LSDB (link-state-database)
Routers will flood LSA's until all routers in the OSPF AREA develop the same map of the network (LSDB)

LSA Flooding

Become neighbours with other routers connected to the same segment (ie R4 became neighbours with R2 and R3
Exchange LSA's with neighbour Routers
Each router independently calculates the BEST ROUTES to each destination and inserts them into the routing table

OSPF AREAS

OSPF uses areas to divide up the network.however small areas can be single-area without negative effects on network performance. For example, using a single area in a large network can produce poor performance due to the SPF algorithm taking a longer time to calculate routes, thus using up more CPU resources and also the increase in size of the LSDB takes up more memory resources.

By dividing a large OSPF network into smaller ones we can avoid the above negative effects

We define an AREA as:

a set of routers and links that share the same LSDB
the BACKBONE area (area 0) is an area that all others MUST connect to
Routers with all interfaces in the SAME area are called INTERNAL ROUTERS
Routers with interfaces in MULTIPLE AREAS are called Area Border Router (ABR's)
- ABR's maintain a SEPARATE LSDB for EACH area they are connected to. Its recommended that you connect an ABR to a MAXIMUM of 2 areas as connecting more that 2 can overburden the router.
Routers connected to Area 0 are called BACKBONE ROUTERS (This also includes ABR's)
An intra-area route is a route to a destination inside the SAME OSPF area
An inter-area route is a route to a destination in a DIFFERENT OSPF area

OSPF areas MUST be contiguous - ie cant have Area1, Area2 and then Area1 again - wont work, It would need to be Area 3
All OSPF areas MUST have at least ONE ABR connected to the BACKBONE area
OSPF interfaces in the same SUBNET must be in the same AREA

Basic OSPF Configuration - Single Area OSPF

With the CCNA - only need to know about one area - Area 0 - the backbone Area. Here all interfaces and Routers are in Area 0 and Routers 2, 3 and 4 have already been configured. We will configure Router 1 (R1)

The configuration is very similar to EIGRP with a few differences:

The process ID - OSPF can run multiple OSPF processes at once and this process ID is LOCALLY SIGNIFICANT and is used to identify which instance of OSPF is running (not like EIGRP where it uses AS numbers which have to be the same across other EIGRP routers for them to achieve adjacencies)
Network uses wildcard masks just like EIGRP and requires an AREA number to be specified. Again the network command ACTIVATES OSPF on the SPECIFIC interfaces in the specified AREA and the routers will then try to form adjacencies with neighbouring OSPF activated routers
The passive-interface command - stops sending 'HELLO' messages out of those interfaces
Added in default route to internet and then used the R1(config-router)#default-information originate to advertise the default route into OSPF
- R1(config-router)# passive-interface gi2/0 - stops hello messages R1(config)# ip route 0.0.0.0 0.0.0 203.0.113.2 R1(config-router)# default-information originate NB!! Using this command the router interface connects to the Internet so it AUTOMATICALLY BECOMES AN ASBR (autonomous system border router - an ASBR) !!!!! R1#show ip protocols **Remember Router ID is same as EIGRP. Manual configuration of the router-id takes precedence, then the highest IP address on a loopback interface and lastly if no loopback interface then the highest IP address on a PHYSICAL inetrface R1(config-router)# router-id 1.1.1.1 ** Router ID same as EIGRP but with OSPF - its just router-id not "ospf router-id" like in EIGRP where its configured "eigrp router-id 1.1.1.1" etc. R1(config-router)# clear ip ospf process ** Unlike EIGRP we have to issue the above command for the router-id to take affect R1(config-router)# maximum-paths 8 ---- Same as EIGRP except OSPF cannot do unequal-cost load-balancing but does support ECMP load balancing. 4 default ECMP routes configured. Changed to 8 R1(config-router)# distance 85 ----- changes OSPF defauly AD from 110 to 85 so OSPF routes are now preferred over EIGRP routes on this router..