OSPF and understanding Point to Point (literally)

[SJ K]

Hi all,

I am learning about OSPF and have little or knowledge about WAN network and connectivity.

I came across this phrase

Adjacencies on Point-to-Point Interfaces
OSPF will always form an adjacency with the neighbor on the other side of a point-to-point interface such as point-to-point serial lines. There is no concept of DR or BDR. The state of the serial interfaces is point to point.

Q1) What is a point to point connection ?
- I have always thought point to point in the sense that 1 sender can only reach the same and only 1 receiver (and vice versa)

Q1a) if i am connecting host A to Host B through a broadcast network (ethernet) , through a switch. Is it still point to point ? Is there any differences between point to point network and point to point connection ?

•  Is point to point referring to the network (as in 2 nodes in the network can only get to each other only) or       
                 - is there any point to point network that involved more then 2 nodes ?
•  referring to the a particular connection (as in node A access node B in 1 connection, but node A can also access node C if wanted to (another connection)

Q2) Hence i have setup a router to router connection (R1<---> R2)  and setup OSPF on the routers. Doing a show ip ospf neighbor, I still see that DR and BDR are still being assigned to the 2 routers.   Isn't my connection between R1 and R2 point to point ? There is no switch in between.

Regards,
Confused Noob

[InayathUlla Sharieff · Accepted solution]

Hi,

Thanks for posting the query. Please find below the answer for the same:-

1- What is a point to point connection ?

Answer: Any link connecting directly between the two devices. (Example; Serial link between R1 connecting directly to R2. Or GE connecting directly between SW1--Sw2 and nothing in between this devices).

2-

Q1a) if i am connecting host A to Host B through a broadcast network (ethernet) , through a switch. Is it still point to point ? Is there any differences between point to point network and point to point connection ? -

Answer:- NO this is not a point to point. 

•  Is point to point referring to the network (as in 2 nodes in the network can only get to each other only) or       
                 - is there any point to point network that involved more then 2 nodes ?

Answer :Point to Point connection means anything directly connecting between them. (I.e direclty link between client to server, Client to client, SErver to server, Router to router, Switch to switch etc..).

•  referring to the a particular connection (as in node A access node B in 1 connection, but node A can also access node C if wanted to (another connection)

Answer:- Yes that can be possible if node A has dual nic and connected directly to node C.

Q2) Hence i have setup a router to router connection (R1<---> R2)  and setup OSPF on the routers. Doing a show ip ospf neighbor, I still see that DR and BDR are still being assigned to the 2 routers.   Isn't my connection between R1 and R2 point to point ? There is no switch in between. -

Answer:- Could you please share the config of your interface and ospf?

I just finish doing the same on my devices and it works fine:-

R1---serial link---R2

R2 config:-

Router(config)#int s0/3/0
Router(config-if)#ip address 1.1.1.2 255.255.255.0
Router(config-if)#router ospf 1
Router(config-router)#network 1.1.1.2 0.0.0.0 area 0
Router(config-router)#end
Router#

Router#sh ip ospf ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           0   FULL/  -        00:00:36    1.1.1.1         Serial0/3/0

Router#show ip ospf int s0/3/0 >>> This cmd will give you the output of ospf network type.
Serial0/3/0 is up, line protocol is up
  Internet address is 1.1.1.2/24, Area 0
  Process ID 1, Router ID 1.1.1.2, Network Type POINT-TO-POINT, Cost: 64  >>> you can see here point to point.

HTH

Regards

Inayath

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[SJ K]

Hi Inayath,

Thanks for your response!

Answer: Any link connecting directly between the two devices. (Example; Serial link between R1 connecting directly to R2. Or GE connecting directly between SW1--Sw2 and nothing in between this devices)

Answer :Point to Point connection means anything directly connecting between them. (I.e direclty link between client to server, Client to client, SErver to server, Router to router, Switch to switch etc..)..

q1) So i believe the keyword is nothing between the devices connecting to each other.
How about point to point connection over WAN ? Surely that will have go go through routers, wan devices.. etc. 
How is that consider point to point then ? Wouldn't it be the same  in comparison to "Device A <---> switch <---> Device B"

==============================

q2) For the OSPF configuration, i am using PT with 2 routers, connecting directly to each other using FastEthernet and not Serial.  Once i changed to using a serial interface, it become Point to Point.

Why has the physical connection got anything to do with the connectivity ?  Can't Ethernet connection be point to point ?

q3) How does OSPF determine if a connection is a point to point (like in the case above) or broadcast ?

Regards,
Noob

[InayathUlla Sharieff]

 Hi Noob,

Both Ethernet and IEEE 802.3 LANs are broadcast networks.  (More info:- http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html)

to change it to work as point to point you need to manullay configure under  interface:-
int f0/2
ip ospf network point-to-point.

To create a point-to-point connection between two routers, you can use other types of WAN encapsulations, such as HDLC, PPP, and SLIP. 

further question:-

q1) So i believe the keyword is nothing between the devices connecting to each other.
How about point to point connection over WAN ? Surely that will have go go through routers, wan devices.. etc. 
How is that consider point to point then ? Wouldn't it be the same  in comparison to "Device A <---> switch <---> Device B" -

Answer:- Yes because you will be connecting to the ISp router.

2) For the OSPF configuration, i am using PT with 2 routers, connecting directly to each other using FastEthernet and not Serial.  Once i changed to using a serial interface, it become Point to Point.

Why has the physical connection got anything to do with the connectivity ?  Can't Ethernet connection be point to point ?

-Answer: PT will not give all the commands which real routers give but close to 80-90 percent it provides. Some commands will not work here.

Ethernet connection can be point to point only when you configure to do so by command ip ospf network point-to-point else by default its a broadcast mediam.

Q3:- Answer already provided or read above.

HTH

Regards

Inayath

[SJ K]

Hi Inayath, Mark

Thanks for the prompt replies.

1- What is a point to point connection ?
Answer: Any link connecting directly between the two devices. (Example; Serial link between R1 connecting directly to R2. Or GE connecting directly between SW1--Sw2 and nothing in between this devices).

q1) So i believe the keyword is nothing between the devices connecting to each other. How about point to point connection over WAN ? Surely that will have go go through routers, wan devices.. etc. How is that consider point to point then ? Wouldn't it be the same in comparison to "Device A <---> switch <---> Device B" - Answer:- Yes because you will be connecting to the ISp router.

q1) But earlier you have mentioned that point to point connection means the connection/link between 2 devices connecting them directly and
A <--> Switch <--> B is not a point to point connection.   By the same logic, how come connection across a WAN, from device A through a ISP router to device B is a point to point connection then ?  

For q2 and q3) can i say that OSPF determine is a network is a broadcast or not, by checking its interface type (if it is ethernet -> broadcast, if it is serial -> point to point etc).  am i right ?

Regards,
Noob

[Mark Malone]

I have never seen ospf as an edge wan protocol used like that as its an IGP so i cannot comment on that , OSPF network types are dependent on layer 2 technology ppp/hdlc/ethernet  depending on the type in place that will set the network type for OSPF , they are for IGP

OSPF network types are used for maintaining the LSDB and identifying who is DR/BDR , on a p-p you dont have a BR/BDR , there is a DR/BDR for every Ethernet segment when a change is made the DR notifies the segment reducing unnecessary change information if he goes down BDR takes over , there is no DR/BDR on a p-p as its directly connected

[SJ K]

Hi Mark,

Thank for your reply.

Putting OSPF aside, are you able to share your understanding of point to point connection.

As per Inayath mentioned,  point to point connection is a connection/link between 2 devices without any intermediate devices between them.

Hence Node A <--> Switch <--> B is not consider a point to point connection.

But again, point to point conection across a WAN through switches, ISP router etc.. is consider as 1.

What is the difference ? 

Regards,
Noob

[Mark Malone]

point to point in ospf does not mean it has to be directly connected it means its the  type of network ospf will use to choose how it communicates the DR/BDR relationships to form keep the LSDB in sync

You can have a p-p across another switch once you use the ospf p-p command under the interface , i just labbed it with with 2 routers separated by switch and it works fine , both are showing there p-p with no DR/BDR when you check neighbor relationship

[SJ K]

Hi Mark,

Thanks for sharing on the lab result.

But as per my previous question - what is your understanding on a point to point connection ?

Putting OSPF aside, are you able to share your understanding of point to point connection.

As per Inayath mentioned,  point to point connection is a connection/link between 2 devices without any intermediate devices between them.

Hence Node A <--> Switch <--> B is not consider a point to point connection.

But again, point to point conection across a WAN through switches, ISP router etc.. is consider as 1.

Regards,

Noob

[Mark Malone · Accepted solution]

My understanding is its a direct connection between two nodes using serial interfaces,  this can be formed across several devices if required but the connection is limited to 2 devices only hence point to point , nowadays its not just serial though that can be point to point you have pppoa, pppoe etc

[SJ K]

Duly noted everyone.

Thank you  Inayath, Mark, Bilal for the wonderful inputs.

Regards,

Noob

[Mark Malone]

Hi

q1 Across  wan you would not be using ospf its an igp

q2,q3 on serial interface ppp and hdlc ospf runs point-point by default that's the way Cisco have it set, why broadcast on a straight p-p link

Fasthethernet is nbma by default you would need to tell it to be p-p if required by using command under interface opsf network point to point but only if its acting like a a standard p-p , it knows by the type of link whether its broadcast or not but it can be changed, This link explains them easily

http://packetlife.net/blog/2008/jun/19/ospf-network-types/

Router2# show ip ospf interface serial 0
   Serial0 is up, line protocol is up 
   Internet Address 1.1.1.1/24, Area 0 
   Process ID 1, Router ID 2.2.2.2, Network Type POINT_TO_POINT, Cost: 64

[mj8mj4]

Thanks for the explanation

[Bilal Nawaz]

Point to point is a term used to describe how devices are connected together. P2P networks such as T1 or subrate link, connect a single pair of routers. It could be argued that the definition can be two devices which share a segment to themselves, no matter which transport, via a service provider, via a EoMPLS network etc....

When the point-to-point network is active in OSPF configuration, the destination address of OSPF packets on these networks will always be the reserved class D address 224.0.0.5 known as ALLSPFROUTERS. The only exception to this rule is retransmitted LSA's which are always unicast on all network types.

Some engineers use point-to-point ospf command to manipulate OSPF advertisement, by changing the link type, especially in the case of loopback addresses.

Others use point-to-point to bypass the election process for DR BDR. Another reason is failure detection (with use of tools like BFD) failover/failback is lightning fast [subsecond failover times)