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OSPF

husycisco
Level 7
Level 7

Hi all

I have a few questions about the OSPF topology attached. Please comment/suggest on questions in seperate comments so that I can rate them individually.

1) Dont see R5, R2 and R4 in attached topology diagram, this is about R1 and R3. R1's loopback interface has an ip of 172.16.8.193/27. I did not issue bandwidth command in interfaces (for testing purposes), so all serial and frame-relay connections between R1 and R3 has a cost of 64.

Under these circumstances, R3 does not do equal cost load sharing. Only 1 entry for 172.16.8.193/29 appears in route table, which is via frame-relay. But if I establish a virtual-link and Area 1 becomes transit area, equal cost balancing occurs. But in fact, I have a valid OSPF relation over Point-To-Point link as described behaviour in below question. Can you please explain this and normal behaviour in this scenario?

2) Dont see R5, R2 and R4 in attached topology diagram, this is about R1 and R3. R3's loopback interface has an ip of 172.16.8.193/29. I did not issue bandwidth command in interfaces (for testing purposes), so all serial and frame-relay connections between R1 and R3 has a cost of 64. There is no virtual link configured

Under these circumstances, R3 has an interface in Area 0 to operate in OSPF correctly. For testing purposes, I removed the loopback 172.16.8.193/27, from R1 in posted diagram and added it to R3 (So L0/0 of R3 belongs to area 0 temporarily, unlike the diagram). Then I started pinging loopback interface of R3 from R1. Then I disabled ser1/0 interface of R3, which is the frame-relay interface that connects R3 to backbone area. Ping was not successfull, untill the dead time expired. After a lot of lost pings and reached dead time, pings came back, pings are successfull. How? R3 neither has an interface in Area 0 nor Area 1 is a transit area! When I issue “sh ip ospf” in R3, I see “Area BACKBONE(0) (Inactive)”. And in route table of R3, I see 172.16.8.193/27 marked as IA (Inter Area)? Is it treating Area 1 as Backbone area when its interface in Area 0 is failed? Can you please explain this and normal behaviour in this scenario?

3) Dont see R5, R2 and R1 in attached topology diagram, this is about R3 and R4. F0/0 of R3 has an ip of 172.16.10.1/25, and L0 of R3 has an ip of 172.16.10.129/25. R4's loopback interfaces, which belongs to Area 3 are 172.16.11.1/25 and 172.16.11.129/25

Under these circumstances, I apply “area 2 range 172.16.10.0 255.255.255.0” command in R3, then R1 gets the summarized one single route, thats fine. As you can see, R4 does not have an interface in Area 0, and we have to configure a virtual-link for area 3 connectivity. When I apply virtaul link and Area 2 becomes transit area, R1 gets Area 3 routes fine, but Area 2 routes, which is now a transit area, are all displayed without being summarized. Both summarized and not-summarized routes of Area 2 appear in route table of R1. Can you please explain this and normal behaviour in this scenario?

Any comments appreciated!

Regards

23 Replies 23

Alright, this is bringing back memories of IE preparation days. Let's see if I can take a stab at your question(s).

""this is about R1 and R3. R1's loopback interface has an ip of 172.16.8.193/27. I did not issue bandwidth command in interfaces (for testing purposes), so all serial and frame-relay connections between R1 and R3 has a cost of 64.

Under these circumstances, R3 does not do equal cost load sharing. Only 1 entry for 172.16.8.193/29 appears in route table, which is via frame-relay.""

This is normal OSPF behavior. OSPF would always choose an intra-area route over an inter-area route. Even if you make the cost lower via the p-t-p link it would still choose the frame relay.

""But if I establish a virtual-link and Area 1 becomes transit area, equal cost balancing occurs.""

Virtual link extends area 0 scope via the transit area 1 and thus R1's loopback appears as intra-area route in R3 and hence, R3 picks both routes for load balancing (equal-cost).

I am going to have step away for a few minutes. Shall respond to your other questions a little later if no one had responded by then.

HTH

Sundar

Hi,

I believe that Sundar has answered the first one perfectly, as for the second one, i believe this RFC has all your answers, you need to understand the ABR behavior and operation, specially in Cisco's implementation with all the recent modifications, in your case you have no active backbone connection, but the router will consider all the summary LSAs from all the attached areas, thats why the route will be received however no active backbone connection exists.

http://www.ietf.org/rfc/rfc3509.txt

BR,

Mohammed Mahmoud.

Hi Sundar and Mohammed,

Thanks for your participation guys, really revealed a lot in my mind.

Now by Sundar's comment, I modified my "Inter Area" understanding as "Any route !including routes belong to backbone area!, learned from an ABR". I thought R3 would know that the route learned over Area 1, belongs to Backbone area since it has an interface in Backbone Area, and would treat it as Intra Area.

By Mohammed's comment, I learned that an ABR does not have to have an interfcae in Backbone Area as described in RFC. So if I remove Area 2 from R3, R3 will still be an ABR since it has an interface in Area 0 and in Area 1. And when s1/0 int of R3 goes down, its connection to Area 0 will be lost and R3 will lose its ABR title since it has interface in only one Area, and will not be able to learn any routes correct?

Waiting comments on Q3 also. Thanks!

Hi Huseyin,

You are very welcomed, and to make my point clear, here you are my labing for OSPF ABR behavior:

(Area6) R6 <--(VLAN56-Area56)--> R5 <--(VLAN45-Area45)--> R4 (Area4)

. Without any interface configured in Area0 on any router, no interArea LSAs are exchanged.

. With loopback0 configured in Area0 on all the routers (but still no Area0 Adjacency - Partitioned backbone - no active backbone connection), each router will advertise its direct connceted LSAs to its neighbors over any attached area, but won't relay any LSAs from a neighbor to the other neighbor - Area0 LSAs will be also exchanged as InterArea routes since there is no adjacency in Area0 between all the routers.

. With virtual-link configured between the three routers (the backbone is not Partitioned anymore - all routers have active backbone connection) the LSAs will be exchanged normally between all the routers (and now the Area0 LSAs are exchanged as IntraArea routes).

BR,

Mohammed Mahmoud.

Hi,

Sorry forget to attach the file.

BR,

Mohammed Mahmoud.

Huseyin,

Mohammed's response is accurate about ABR status of R3 even when no active backbone connection exists. R3 continues to remain an ABR even when it has no connection to the backbone. You can verify this by issuing the command 'show ip ospf border-routers' in R1. Area 0 becomes partioned after R3 loses it connection to the backbone.

You would need a virtual link through area 1 for only one of two reasons. Load balance traffic between R1 & R3 for any networks behind those routers. When area 0 becomes partitioned any area other than area 1 wouldn't be reachable if no virtual link exists via area 1.

HTH

Sundar

Sundar,

Please allow me to welcome you back, i haven't noticed that you have left the forum for a while, as i my self disappeared for a while for completing my CCIE. Welcome back.

BR,

Mohammed Mahmoud.

Mohammmed,

Thanks.

Congratulations on your CCIE!! Has life changed a lot since getting your #?

Regards,

Sundar

Hi Sundar,

You are very welcomed.

Thanks, after struggling for around a year and a half to accomplish my CCIE i thought life would get easier, but after getting my number, people around me treats me with the concept of with great power comes great responsibilities :)

BR,

Mohammed Mahmoud.

Hi Guys

Thanks for the insight, especially for your every second on the lab you prepared and attached Mohammed. I will return to my lab asap and work on ABRs by help of your directions, then will again steal your valuable time with some questions.

Also any comments of you guys is greatly appreciated in the following question of mine.

http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&type=Subscriptions&loc=.2cc004ff/1&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching

Regards

Hi Huseyin,

You are very welcomed, enjoy your lab, and please never hesitate to come back with any question.

BR,

Mohammed Mahmoud.

Lab is OK now, but one thing is still a mystery.

In router 3, I have the following command for area 2 summarization

area 2 range 172.16.10.0 255.255.255.0

And R1 has the following route in its table

O IA 172.16.10.0/24 [110/65] via 172.16.8.251, 00:00:53, Serial1/0

But as I issue the following in R3 to make Area 2 a transit area and let R4 advertise Area 3 routes to OSPF domain,

area 2 virtual-link routeridofR4

and in R4

area 2 virtual-link routeridofR3

Area 3 is advertised fine but route table of R1 changes as following

O IA 172.16.10.0/25 [110/65] via 172.16.8.251, 00:00:53, Serial1/

O IA 172.16.10.128/25 [110/65] via 172.16.8.251, 00:00:53, Serial1/0

O IA 172.16.10.0/24 [110/65] via 172.16.8.251, 00:00:53, Serial1/0

It gets both summary address and not summarized address. What is the problem here? Is this the default behaviour? Transit areas can not be summarized?

I don't have access to my lab gear at this time to test. However, I believe you would have to summarize the area 2 routes in R4 as well to suppress the specific routes. Use the same 'area 2 range 172.16.10.0 255.255.255.0' and in R4 and see if that takes care of the specific routes.

BTW, these are very good questions.

HTH

Sundar

Hi,

Agree with Sundar, this is indeed a very good lab. And also agree with him regarding the area range issue, after you've configured the virtual-link between R3 and R4, R4 is now an active backbone ABR router (since it now has an adjacency in Area0), adding the area range command on R4 should address you issue.

BR,

Mohammed Mahmoud.

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