OSPF: CONCEPTS: GENERAL PART 2


TOPICS

NETWORK TYPES, CONCEPT OF DR/BDR

HOW TO ENABLE OSPF

 


NETWORK TYPES, CONCEPT OF DR/BDR

 

GENERAL CONCEPT OF NETWORK TYPE

On every interface of router, OSPF network type has to be configured.

Example: R1 has four interfaces, depending on requirement, we can configure different network type on all four interfaces

 

How to check network type on interfaces

Router#sh ip ospf int | i int Vlan2
Vlan2 is up, line protocol is up
Internet Address 10.144.2.2/24, Area 0
Process ID 65225, Router ID 10.144.1.3, Network Type BROADCAST, Cost: 1

 

How to configure on devices

Router# conf t

Router(config)# int f0/0

Router(config-if)#ip ospf network  <point to point/broadcast/non-broadcast>

 

 

CONCEPT OF DR/BDR

PURPOSE OF DR/BDR ELECTION

In a Multi-access network, if we have 4 routers, they all will form adjacencies with each other

To avoid that much no. of adjacencies, we make one as DR(Designated router) & one as BDR(Backup Designated Router), remaining will stay in DROTHER state

So, every router will go through all the OSPF states to become fully adjacent with DR only

Note: DROTHERS routers stay in 2-way state with each other. 2-way state is normal, this is not a problem

So, ultimately number of adjacencies got reduced

 

HOW TO ELECT DR/BDR

Tie Breaker

  1. Highest Priority(configured over interface)

  2. Highest RID

 

Example:    R1 (int f0/0)———-(int f0/1)R2

Here, priority of int f0/0 at R1   and int f0/1 at R2 are compared.  Higher priority wins. Default priority is 1

Till now, this is the Theory

Practically, the first device that starts the OSPF process wins regardless of priorities/RIDs.

Reason: the device starts the wait timer (equal to dead timer) and it waits to see another OSPF enabled device  and finally declaring itself as DR after wait timer value.

 

NETWORK TYPES

Types of Network

Total: 5: Point to Point, Broadcast, Non-Broadcast, Point-to-Multipoint and Non-Broadcast Multi-Access(NBMA)

POINT TO POINT

  • Configured between directly connected devices
  • Default network type for Serial interfaces
  • Hello and Dead timer are 10 and 40 seconds
  • This network type does not maintain DR/BDR relationship

BROADCAST

  • Broadcast network type represent a segment
  • Default network type for Ethernet interfaces
  • Hello and Dead timer are 10 and 40 seconds
  • This network type maintains DR/BDR relationship
  • The Broadcast network type requires that a link support Layer 2 Broadcast capabilities

Regarding Hello Timer

Hello Packets are Broadcast (not Unicast)

 

NON-BROADCAST

  • Default network type for physical interfaces in Frame Relay technology
  • Hello and Dead timer are 30 and 120 seconds
  • This network type maintains DR/BDR relationship
  • Non-Broadcast networks requires the configuration of static neighbors

Regarding Hello Timer

Hello packets are sent to keep OSPF Neighborship up

In Non-Broadcast, Hello packets are sent as unicast

non-broadcast

Here, R1 will send Hello packet to R2, R3 and R4 individually(or, Unicast is done)

Note: we have the option of simulating Broadcast capability on Non-Broadcast network type also, so that Hello packets are sent using broadcast

 

POINT TO MULTIPOINT

  • OSPF treats Point-to-Multipoint networks as a collective of point-to-point links
  • Hello and Dead timer are 30 and 120 seconds
  • This network type does not maintain DR/BDR relationship

 

Non-Broadcast Multi-Access(NBMA)

An NBMA segment emulates the function of a broadcast network. Every router on the segment must be configured with the IP address of each of its neighbors. OSPF hello packets are then individually transmitted as unicast packets to each adjacent neighbor.

As in a true broadcast network, a DR and BDR are elected to limit the number of adjacencies formed


 


HOW TO ENABLE OSPF

 

EXAMPLE:     R1(int f0/0)————-(int f0/1)R2

R1:  int f0/0:  ip:   192.168.10.1  255.255.255.0

R2:  int f0/1:  ip:   192.168.10.2   255.255.255.0

 

Configuration With Three Methods:

R1(config)#router ospf   1

/router ospf  <process id>, process id is locally significant to router/

R1(config)#network 192.168.10.1   0.0.0.0  area 0       :      First Method

/Here, 0.0.0.0 represents wildcard mask, means R1 will search for exact interface to enable OSPF and will enable for all the connected interface. This is my recommendation to use this one, it is not confusing at all/

/Area 0 represents area number, in every multiple area network, there should be a one Backbone area or Area 0/

OR,

R1(config)#network 192.168.10.1  0.0.0.255  area 0       :     Second Method

/Here, wild card mask is calculated from Subnet mask. R1 will search for all the ip’s  192.168.10.X   and enable OSPF on all found ip’s

Earlier,method and this is same in terams of output/

OR,

Go into interface mode        :     Third Method

R1(config)#int f0/0

R1(config-if)#ip ospf   <process id>   <area number>

/This method is IOS specific. It may or may not work/

 

Recommendation: First Method, as this is easiest one, no confusion, will work on every router


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