Re: My doubts about OSPF

ciscolover · ‎03-10-2011

Hi all ¡¡¡

I'm studing ospf and i go to put all my easy questions about ospf in this post... Thanks all...

1)What is the difference about process ID and RID?? Its the same?? I know if you not etablish a RID, the router takes the more "big" router loopback interface up/up...But, if i do not make process id in the command router ospf <process id> the command its not valid...¿its the same the process id and the rid?

2º)When a router have interfaces in more than one area its an area border router or not?

3º)For example, i have 2 routers with a serial link in a network 10.0.0.0/24 . One of the routers have a interface in the network 192,168,1,0/24. I do not want to send hello messages or information related to ospf in the stub network 192.168.1.0 but i want one router annonce the other of that network. Its possible this without include the network 192,168,1,0 in the "network command of ospf?

Thanks all and sorry i speak bad english.

sujinair · ‎03-10-2011

Hi

Please find the answers for your queries inline:

1)What is the difference about process ID and RID?? Its the same?? I know if you not etablish a RID, the router takes the more "big" router loopback interface up/up...But, if i do not make process id in the command router ospf the command its not valid...¿its the same the process id and the rid?

Ans: OSPF RID and process are two different things. The OSPF proccess id which is configured using the command "router ospf x" (where x is the process id) this just an instance number for creating the ospf process and is local to the device. The Router-ID is a 32-bit number that uniquely identifies the router in the Autonomous System, this is used by OSPF to identify the routers in the topology, kind of number plate for OSPF routers in layman terms :-)

2º)When a router have interfaces in more than one area its an area border router or not?

Ans: Yes, an OSPF router with interfaces in different areas is an ABR (area border router)

3º)For example, i have 2 routers with a serial link in a network 10.0.0.0/24 . One of the routers have a interface in the network 192,168,1,0/24. I do not want to send hello messages or information related to ospf in the stub network 192.168.1.0 but i want one router annonce the other of that network. Its possible this without include the network 192,168,1,0 in the "network command of ospf?

Ans: You will have to include 192.168.1.0 in the "network" statement but disable OSPF from sending hellos on that interface you can enable "OSPF passive interface" using the following command:

router ospf 1

network 192.168.1.0 0.0.0.255 area 0

passive interface fa0/0

where fa0/0 is the interface on which you dont want to send hellos.

I hope the above answers help

Regards,

Sujit

Richard Burts · ‎03-10-2011

For question #2 about whether a router is an Area Border router when it has interfaces in 2 areas perhaps we need some additional clarification. For example, think about a router that has an interface in area 2 and in area 4. Is that router an Area Border router? Some people would say it is because it has interfaces in 2 areas. But the Cisco implementation of OSPF says that it is not an Area Border router. To be an Area Border router a router must have interfaces in more than one area and one of those areas must be the backbone area 0.

For question #3 I would say that yes there is a way to have OSPF advertise the network 192.168.1.0 without using a network statement in OSPF. If you redistribute connected then OSPF should advertise it without needing it in a network statement. For example:

router ospf 1

redistribute connected subnets

HTH

Rick

HTH

Rick

ciscolover · ‎03-11-2011

Wow ¡¡ Excelents answers, now i understend all better but... i have some doubts jejeje.

1º)Im not sure about the DR and BDR...This exists when 2 or more routers are connected... But, only in LAN links?? Never in wan?Im not sure about that because the book says"when a router receive a hello from other router it elects a DR and BDR? Only in LAN networks?? And finally, im not sure about the function of the DR.

2º)About the LSAs-->The lsas forms the topology of an area(i think). Some tipes of lsas exist...

Type 1-->identifyes the routers in the area etc...

The LSA tipe 2 is not easy for me... All the transit network in 2 routers with ospf is a LSA type 2?

The lsa type 3 is used with the ABR to transmit the routes between areas...

This is ok? Is the concept or not?

Thanks all, its not easy for me to explain me ¡¡¡

Richard Burts · ‎03-11-2011

The concept of DR can be confusing and I hope that this explanation will help.

First let us start with a basic concept of OSPF as a link state protocol. Every router must have the same information about the topology of the networks. This is accomplished by forming adjacencies and in forming adjacencies the routers exchange information from their topology data base.

With that concept in mind that a router forms adjacencies to share information with its neighbors let us consider the case of a point to point connection. It is simple, each router has a single neighbor, they establish the adjacency, exchange information and they are ready to go. There is no need for DR in this case.

Then let us consider the case of a multi access network such as Ethernet. Let us consider the case of an Ethernet network with 5 routers running OSPF. Each router has 4 neighbors and would have to establish 4 adjacencies, exchange information, etc. So 5 routers, each doing 4 sets of exchanges, would require 20 sets of exchanges ( 5* 4 = 20 ). That does not scale well - think about an Ethernet network with 10 routers. The number of routers doubles but the number of exchanges goes up to 90.

So the concept of DR was developed to help the multi access network scale better. If the DR is elected then each router must form the adjacency with the DR. There is an important principle that if rtrA is synced with DR and if rtrB is synced with DR, then rtrA is synced with rtrB. So the DR must form adjacencies with all neighbors but the other routers only need to form adjacency with the DR. So in the case of the 5 router network the number of exchanges for adjacency is ( 4 + 4 ) = 8 rather than 20 and in the 10 router network the number of exchanges is ( 9 + 9) = 18 rather than 80.

So the concept of DR helps OSPF scale in multi access networks such as Ethernet.

HTH

Rick

HTH

Rick

ciscolover · ‎03-12-2011

Thanks now i understand better the concept of Dr...

All the routers form the adjacencies with the dr and the network have a best perfomance...

Ok and about the RID and the process id...

Best regards.

ciscolover · ‎03-18-2011

Hi all ¡¡¡ I have study a lot and i have some questions ¡¡¡¡

1º)I have doubts about lsa type 3 and lsa type 5.... I think lsa type 3 its used for the ABR to inform about subnets of one area to another area. But im not sure, why his name is "summary"?

2º)LSA type 5 its used for ABR or ASBR to inform about "external routes". What is external routes?? Routes from another routing protocol, from other enterprise?

Thanks all ¡¡¡¡

Peter Paluch · ‎03-18-2011

Hello,

1º)I have doubts about lsa type 3 and lsa type 5.... I think lsa type 3 
its used for the ABR to inform about subnets of one area to another 
area. But im not sure, why his name is "summary"?

I believe there are two reasons for the name "summary LSA". First, the LSA-3 carries information only about network prefixes located in other areas but not the exact topology information. In a sense, it simplifies ("summarizes") the information about an area. Second, if area ranges are configured, the summarized ranges are advertised in LSA-3 only. No other LSAs actually carry summarized (or even unsummarized) networks from different areas.

2º)LSA type 5 its used for ABR or ASBR to inform about "external routes". What is external routes?? Routes from another routing protocol, from other enterprise?

LSA-5 is originated by ASBR only (not by ABR), and is used by all other routers in the OSPF domain to learn about networks which have been redistributed into OSPF. All networks brought into OSPF not by the network command but either by redistribution or by the default-information originate command (the default route) are carried as LSA-5. This way, it is easy to know which networks are "foreign" to our OSPF domain, and possibly filter them out if they are not needed (which is the main idea behind stubby areas).

Best regards,

Peter