OSPF LSA type 4

aueddonline

the book has confused me on this one. In the chapter it has

AS external ASBR summary link LSA (Type 4)—ASBRs produce this LSA to advertise their
presence. Types 3 and 4 are called inter-area LSAs because they are passed between areas.


and in the Q&A it has

AS external ASBR summary link—Sent from an ABR to a router that connects to the
outside world (ASBR). It contains the metric cost from the ABR to the ASBR. This
update is identified by the type code Type 4.


so which one is it cisco

EdTheLad

The type 4 lsa advertises the cost to reach the ASBR, it will be received on an ABR and flooded into an area(unless totally stubby) to intra-area routers.
If your unsure read the ospf rfc and work through the examples given,then you will understand 90% of ospf.

aueddonline

so is the type 4 produced by ASBR and ABR to advertise the cost to the ASBR

i.e. first advertised by the ASBR (advertising a cost of 0?) and then the ABR advertising the cost to reach the ASBR to the areas they are connected to unless totally stubby?

tech-airman

aueddonline,

aueddonline wrote:

the book has confused me on this one. In the chapter it has

You mention "the book." Which book are you talking about?

aueddonline wrote:

AS external ASBR summary link LSA (Type 4)—ASBRs produce this LSA to advertise their
presence. Types 3 and 4 are called inter-area LSAs because they are passed between areas.


and in the Q&A it has

AS external ASBR summary link—Sent from an ABR to a router that connects to the
outside world (ASBR). It contains the metric cost from the ABR to the ASBR. This
update is identified by the type code Type 4.


so which one is it cisco

aueddonline

it's the BSCI cert guide 4th edition

tech-airman

aueddonline wrote:

it's the BSCI cert guide 4th edition

Since I do not have that book, I will try to help with online sources.

As a CCNA, you should understand that with OSPF in a single area, that single area belongs to Area 0. Let's say for the sake of specifics, you've got two routers, RouterA and RouterB connected to each other as listed below...

[RouterA]----------[RouterB]

By definition, that area between RouterA and RouterB is Area 0. So let's add that to the diagram.

[RouterA]--------{Area 0}------[RouterB]

Now, since both RouterA and RouterB are OSPF routers, each will send out LSA Type 1 to each other. Now, let's say the DR election process selected RouterA to be the DR for Area 0. Now, that means that RouterA will be issuing both LSA type 1 and LSA type 2. RouterB on the other hand will be issuing only LSA Type 1.

Now, let's create another area to the right of RouterB called Area 1. Since OSPF areas are between OSPF routers, we need another router, so let's call that RouterC. So let's add that to the diagram.

[RouterA]-----{Area 0}----[RouterB]----{Area 1}-----[RouterC]

Now, let's give OSPF name terms to each of the routers:

1. RouterA = Internal Router to Area 0 or Backbone Router
   [list=1:ebd774d575]
2. Since RouterA is an OSPF router, it sends out LSA Type 1 to Area 0
3. Since RouterA is the DR for Area 0, it sends out LSA Type 2 to Area 0

[*]RouterB = Area Border Router

1. Since RouterB is an OSPF router, it sends out LSA Type 1 to Area 0
2. Since RouterB is an OSPF router, it sends out LSA Type 1 to Area 1
3. Since RouterB is the DR for Area 1, it sends out LSA Type 2 to Area 1
4. Since RouterB is an ABR, it sends out LSA Type 3 to RouterA, the DR for Area 0

[*]RouterC = Internal Router to Area 1

1. Since RouterC is an OSPF router, it sends out LSA Type 1 to Area 1

[/list:o:ebd774d575]

Now, let's convert RouterC from an Internal Router in Area 1 to being the Autonomous System Border Router or ASBR for short. Let's say the external Autonomous System uses RIP for it's routing protocol. We'll need to add another router, let's call it RouterD.

[RouterA]----{Area 0}----[RouterB]----{Area 1}----[RouterC]----{RIP}----[RouterD]

Let's review the roles of each router:

1. RouterA = OSPF Backbone Router
2. RouterB = Area Border Router
3. RouterC = Autonomous System Border Router
4. RouterD = RIP Router

Now, let's expand the above to include the types of LSAs that each one would be expected to send.

1. RouterA = OSPF Backbone Router
   [list=1:ebd774d575]
2. Since RouterA is an OSPF router, it sends LSA Type 1 to Area 0
3. Since RouterA is a DR router for Area 0, it sends LSA Type 2 to Area 0

[*]RouterB = Area Border Router

1. Since RouterB is an OSPF router, it sends out LSA Type 1 to Area 0
2. Since RouterB is an OSPF router, it sends out LSA Type 1 to Area 1
3. Since RouterB is the DR for Area 1, it sends out LSA Type 2 to Area 1
4. Since RouterB is an ABR, it sends out LSA Type 3 to RouterA, the DR for Area 0
5. Since RouterB is an ABR, it sends out LSA Type 4 to RouterA, the DR for Area 0

[*]RouterC = Autonomous System Border Router

1. Since RouterC is an OSPF router, it sends out LSA Type 1 to area 1
2. Since RouterC is an ASBR, it sends out LSA type 5 to RouterC, the DR for Area 1

[*]RouterD = RIP Router

1. Since RouterD is a RIP router, unless RouterC is configured for redistribution, does not participate in the OSPF process

[/list:o:ebd774d575]

So in summary, pun intended, according to the OSPF Design Guide, here's a summary of LSA Type, OSPF router type that sends them, and what is contained in the LSA:

1. LSA Type 1 = Generated by each router for each area it belongs to. Router Link advertisements.
2. LSA Type 2 = Generated by Designated Routers. Network Link advertisements.
3. LSA Type 3 = Generated by Area Border routers. Summary Link advertisements. Type 3 describes routes to networks, also used for aggregating routes
4. LSA Type 4 = Generated by Area Border routers. Summary Link advertisements. Type 4 describes routes to ASBR.
5. LSA Type 5 = Originated by ASBR. AS external link advertisements.

I hope this helps.

Source:

1. OSPF Design Guide - http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml

EdTheLad

tech-airman wrote:

[RouterA]----------[RouterB]


[RouterA]--------{Area 0}------[RouterB]

Now, since both RouterA and RouterB are OSPF routers, each will send out LSA Type 1 to each other. Now, let's say the DR election process selected RouterA to be the DR for Area 0. Now, that means that RouterA will be issuing both LSA type 1 and LSA type 2. RouterB on the other hand will be issuing only LSA Type 1.

The DR is elected per multiaccess network not per area.

aueddonline

that look like it took you a lot of time so thanks, but my question wasn't

'what is OSPF and how does it work'

it was really just asking about the two conflicting statements and if both true how they fit together

EdTheLad

aueddonline wrote:

so is the type 4 produced by ASBR and ABR to advertise the cost to the ASBR

i.e. first advertised by the ASBR (advertising a cost of 0?) and then the ABR advertising the cost to reach the ASBR to the areas they are connected to unless totally stubby?

Read the ospf rfc, if you examine how the ospf database is created you'll see that external routes are advertised by the ASBR, these routes are put in the ospf database with a next-hop equal to the router-id of the ASBR, the problem is the routers might not have a route to this next-hop address.So to overcome this issue ospf creates a type 4 lsa that is used to flood the routes to all ASBRs throughout the ospf network.
Regarding the cost, i'm not sure and i dont have the time to check,possibly the routes are advertised with a cost related to the interface where the routre-id address resides.For example normally we make lo0 the router-id int, so the 100mb/(bw of lo0) = ospf cost of lo0 route.
My advise is to lab it up and see for yourself.

tech-airman

EdTheLad wrote:

tech-airman wrote:

[RouterA]----------[RouterB]


[RouterA]--------{Area 0}------[RouterB]

Now, since both RouterA and RouterB are OSPF routers, each will send out LSA Type 1 to each other. Now, let's say the DR election process selected RouterA to be the DR for Area 0. Now, that means that RouterA will be issuing both LSA type 1 and LSA type 2. RouterB on the other hand will be issuing only LSA Type 1.

The DR is elected per multiaccess network not per area.

RFC 2328 - OSPF Version 2 wrote:

9.4. Electing the Designated Router

This section describes the algorithm used for calculating a
network's Designated Router and Backup Designated Router. This
algorithm is invoked by the Interface state machine. The
initial time a router runs the election algorithm for a network,
the network's Designated Router and Backup Designated Router are
initialized to 0.0.0.0. This indicates the lack of both a
Designated Router and a Backup Designated Router.

The Designated Router election algorithm proceeds as follows:
Call the router doing the calculation Router X. The list of
neighbors attached to the network and having established
bidirectional communication with Router X is examined. This
list is precisely the collection of Router X's neighbors (on
this network) whose state is greater than or equal to 2-Way (see
Section 10.1). Router X itself is also considered to be on the
list. Discard all routers from the list that are ineligible to
become Designated Router. (Routers having Router Priority of 0
are ineligible to become Designated Router.) The following
steps are then executed, considering only those routers that
remain on the list:

(1) Note the current values for the network's Designated Router
and Backup Designated Router. This is used later for
comparison purposes.

(2) Calculate the new Backup Designated Router for the network
as follows. Only those routers on the list that have not
declared themselves to be Designated Router are eligible to
become Backup Designated Router. If one or more of these
routers have declared themselves Backup Designated Router
(i.e., they are currently listing themselves as Backup
Designated Router, but not as Designated Router, in their
Hello Packets) the one having highest Router Priority is
declared to be Backup Designated Router. In case of a tie,
the one having the highest Router ID is chosen. If no
routers have declared themselves Backup Designated Router,



Moy Standards Track [Page 75]

RFC 2328 OSPF Version 2 April 1998


choose the router having highest Router Priority, (again
excluding those routers who have declared themselves
Designated Router), and again use the Router ID to break
ties.

(3) Calculate the new Designated Router for the network as
follows. If one or more of the routers have declared
themselves Designated Router (i.e., they are currently
listing themselves as Designated Router in their Hello
Packets) the one having highest Router Priority is declared
to be Designated Router. In case of a tie, the one having
the highest Router ID is chosen. If no routers have
declared themselves Designated Router, assign the Designated
Router to be the same as the newly elected Backup Designated
Router.

(4) If Router X is now newly the Designated Router or newly the
Backup Designated Router, or is now no longer the Designated
Router or no longer the Backup Designated Router, repeat
steps 2 and 3, and then proceed to step 5. For example, if
Router X is now the Designated Router, when step 2 is
repeated X will no longer be eligible for Backup Designated
Router election. Among other things, this will ensure that
no router will declare itself both Backup Designated Router
and Designated Router.[5]

(5) As a result of these calculations, the router itself may now
be Designated Router or Backup Designated Router. See
Sections 7.3 and 7.4 for the additional duties this would
entail. The router's interface state should be set
accordingly. If the router itself is now Designated Router,
the new interface state is DR. If the router itself is now
Backup Designated Router, the new interface state is Backup.
Otherwise, the new interface state is DR Other.

(6) If the attached network is an NBMA network, and the router
itself has just become either Designated Router or Backup
Designated Router, it must start sending Hello Packets to
those neighbors that are not eligible to become Designated
Router (see Section 9.5.1). This is done by invoking the
neighbor event Start for each neighbor having a Router
Priority of 0.



Moy Standards Track [Page 76]

RFC 2328 OSPF Version 2 April 1998


(7) If the above calculations have caused the identity of either
the Designated Router or Backup Designated Router to change,
the set of adjacencies associated with this interface will
need to be modified. Some adjacencies may need to be
formed, and others may need to be broken. To accomplish
this, invoke the event AdjOK? on all neighbors whose state
is at least 2-Way. This will cause their eligibility for
adjacency to be reexamined (see Sections 10.3 and 10.4).


The reason behind the election algorithm's complexity is the
desire for an orderly transition from Backup Designated Router
to Designated Router, when the current Designated Router fails.
This orderly transition is ensured through the introduction of
hysteresis: no new Backup Designated Router can be chosen until
the old Backup accepts its new Designated Router
responsibilities.

The above procedure may elect the same router to be both
Designated Router and Backup Designated Router, although that
router will never be the calculating router (Router X) itself.
The elected Designated Router may not be the router having the
highest Router Priority, nor will the Backup Designated Router
necessarily have the second highest Router Priority. If Router
X is not itself eligible to become Designated Router, it is
possible that neither a Backup Designated Router nor a
Designated Router will be selected in the above procedure. Note
also that if Router X is the only attached router that is
eligible to become Designated Router, it will select itself as
Designated Router and there will be no Backup Designated Router
for the network.

Where does it say that in RFC 2328?

Source:

1. RFC 2328 - OSPF Version 2 - http://www.ietf.org/rfc/rfc2328.txt

aueddonline

thanks Ed, good advise I should probably lab this sucker up

tech-airman

aueddonline wrote:

that look like it took you a lot of time so thanks, but my question wasn't

'what is OSPF and how does it work'

it was really just asking about the two conflicting statements and if both true how they fit together

aueddonline,

If you truly understood "what is OSPF and how does it work" then you should know which OSPF router would be sending out LSAs of Type 4 then come to a conclusion of the true/falsenees of the two statements. According to the OSPF Design Guide, it clearly states that only the Area Border Routers send out LSAs of Type 4 because it says "Generated by Area Border routers."

kryolla

tech-airman ask yourself what is the purpose of DR and you will find your answer. Hint it is not to represent an area that is what an ABR is.

aueddonline

tech-airman wrote:

aueddonline wrote:

that look like it took you a lot of time so thanks, but my question wasn't

'what is OSPF and how does it work'

it was really just asking about the two conflicting statements and if both true how they fit together

aueddonline,

If you truly understood "what is OSPF and how does it work" then you should know which OSPF router would be sending out LSAs of Type 4 then come to a conclusion of the true/falsenees of the two statements. According to the OSPF Design Guide, it clearly states that only the Area Border Routers send out LSAs of Type 4 because it says "Generated by Area Border routers."

well then you'll understand why the two statements I have in my question seem to conflict, mr DR

tech-airman

kryolla wrote:

tech-airman ask yourself what is the purpose of DR and you will find your answer. Hint it is not to represent an area that is what an ABR is.

kyrolla,

What about the case of a single OSPF area?

EdTheLad

tech-airman wrote:

What about the case of a single OSPF area?

A DR is elected per multiaccess network.Why dont you lab it up and check.Connect two routers via serial links, serial links are ptp links in ospf and will exchange type 1 lsa's.Issue the command "show ip ospf database" check which lsa's you have.
Next connect 3 routers in a chain via ethernet, all 3 routers are in area 0, issue the "show ip ospf neighbor" command on all three, check how many DR&BDRs you have in the area.

gojericho0

kryolla wrote:

tech-airman ask yourself what is the purpose of DR

Isn't the whole point of the DR to reduce the amount of OSPF traffic sent across a multi-access or broadcast network?

kryolla

gojericho0 wrote:

kryolla wrote:

tech-airman ask yourself what is the purpose of DR

Isn't the whole point of the DR to reduce the amount of OSPF traffic sent across a multi-access or broadcast network?

yes and it does that by representing the segment if not than every node in the segment will send out messaging to the whole area.

tech-airman

EdTheLad wrote:

tech-airman wrote:

What about the case of a single OSPF area?

A DR is elected per multiaccess network.Why dont you lab it up and check.Connect two routers via serial links, serial links are ptp links in ospf and will exchange type 1 lsa's.Issue the command "show ip ospf database" check which lsa's you have.
Next connect 3 routers in a chain via ethernet, all 3 routers are in area 0, issue the "show ip ospf neighbor" command on all three, check how many DR&BDRs you have in the area.

EdTheLad,

I cited my source, so you cite your's.

kryolla

tech-airman maybe you need to read the RFC again specifically the different type of routers and what type of LSA each router sends and how these LSA propagate across the network (intra and inter area). Maybe you will have a better understanding of OSPF

EdTheLad

tech-airman wrote:

EdTheLad wrote:

tech-airman wrote:

What about the case of a single OSPF area?

A DR is elected per multiaccess network.Why dont you lab it up and check.Connect two routers via serial links, serial links are ptp links in ospf and will exchange type 1 lsa's.Issue the command "show ip ospf database" check which lsa's you have.
Next connect 3 routers in a chain via ethernet, all 3 routers are in area 0, issue the "show ip ospf neighbor" command on all three, check how many DR&BDRs you have in the area.

EdTheLad,

I cited my source, so you cite your's.

You can lead your horse to water but you cant make him drink.
Look at page 18.

Note that the LSA for Network N6 is
actually generated by one of the network's attached routers:
the router that has been elected Designated Router for the
network.

Now checkout the topology on page 19, all the routers are in area 0.N3,N9 and N6 are all multiaccess networks,they all have DRs, check out the resultant directed graph.If you dont understand go back to the beginning of the document and study until you understand how directed graphs are created.Then look at the topology again and create your own directed graph,then try create a spt diagram.Now when you succeed with this move to the multi-area ospf area, this will also show how ext routes are injected.Then if your interested read the additional ospf rfc's, they are not so long.Now after all that you will understand how ospf really works and you might stop breaking my balls .