No of Routers in OSPF Area

Hi,
could any one please tell me ideally how many routers should be there in a single OSPF area ?
regards
Neo

Hi,
The maximum number of routers per area depends on several factors, including the following:
What kind of area do you have?
What kind of CPU power do you have in that area?
What kind of media?
Will you be running OSPF in NBMA mode?
Is your NBMA network meshed?
Do you have a lot of external LSAs in the network?
Are other areas well summarized?
For this reason, it's difficult to specify a maximum number of routers per area. Consult your local sales or system engineer for specific network design help.
As per the Self-Study book, the optimal number of routers per area varies based on factors like network stability, but Cisco recommends no more than 50-100 routers per area.
Pls do rate all helpful posts.

Similar Messages

OSPF AREA 0

Hi,
I work for a company and noticed they having everything in area 0. They must have well over 350 routers in the core. As I was studying my CCNP I new there was a limitation to having some many routers in area 0. I have been told that they way this is designed is with loads of point to point interfaces so /30 so at max there is only 2 interface performing in area 0 at each time. Is this correct? is this a good OSPF design?

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As both Paul and Rick have already noted, the real advantage of OSPF areas is containing how much information a router must (internally) manage for the OSPF topology.
An old rule-of-thumb, was no more than 50 routers per OSPF area, but many variables are involved beyond just the number of routers.
Newer "routers" are often much "faster", and on Cisco devices, there's an optional incremental SPF feature which might reduce some of the CPU cycles needed to (internally) manage the topology. (NB: Cisco OSPF later OSPF implementations have other subtle ways of supporting OSPF that assist in supporting larger topologies.)
So, as Rick also noted, just knowing there's 350 routers doesn't mean it's actually a bad design.
PS:
BTW, for your /30 Ethernet links, is OSPF p2p defined on the interfaces?

Is there a single DR/BDR for the entire ospf area?

Hi guys, I was just wondering is there a single DR/BDR for an entire ospf area or does a dr/bdr election take place between every point to point Ethernet link between routers? Thanks

Hi,
It would be between every point to point Ethernet link if you don't configure ip ospf point-to-point under the interfaces.
HTH

Filtering External Type 1 routes between OSPF areas

Hello. I have a situation where I have two different telco's providing Metro Ethernet services to my site. Both providers will be sending me routes as OSPF external type 1's. I want to prevent the ISP's routers from peering with one another. One way I was thinking of accomplishing this is by making the connection to ISP1 under OSPF area 1 and to ISP2 area 2.
Is there a way I can filter the LSA's so no information is transferred between the two ISP routers in the two areas? Specifically, I don't want the type 5 LSA's generated by each ISP router to be sent to the other ISP router.
Thanks

Mike
I do not think that configuring two areas will do what you need. But I think that you could achieve this by configuring separate OSPF processes. When you have separate processes they do not share information automatically. You must redistribute routes from one process to another and you can control what routes are redistributed.
Finding the optimum solution would require more knowledge of your environment and your complete requirements than we have. But I syspect that something like this will work for you:
configure ospf 1 and put the connection from ISP 1 into it. configure ospf 2 and put the connection from ISP 2 into it. configure ospf 99 (or whatever number you like) and put the connections from your network into that process. Then you can redistribute from 1 and 2 into 99 but not redistribute between 1 and 2. And if you need to advertise your routes to the ISPs then you probably need to redistribute from 99 into 1 and 2.
HTH
Rick

OSPF Area Addition - Design Question

Hello,
I have a design question regarding OSPF. I am looking to add a new ospf area (1). The area will live on two Core routers and two Distribution routers. Can you please look at the attached Pics and tell me which design is better.
I would like to be able to connect Core-01 to Dist-01 and Core-02 to Dist-02 with a connection between Dist-01 and Dist-02, but this will result in a discontiguous area, correct?
Thanks,
Lee

I would say that the more common design is to have just backbone area links between the core routers. But there is no real issue with having an area 1 link between them...
If I were you, I would not make the area a totally NSSA. Here are my reasons for that:
- you will get sub-optimal routing out of the area since you have two ABRs and each distribution router will pick the closest one of them to get out to the backbone even though it may be more optimal to use the other one
- in an NSSA case, one of the two ABRs will be designated as the NSSA translator, which means that if you are doing summarisation on the ABRs, all traffic destined for these summarised routes will be drawn to the area through that one ABR.
Paresh

OSPF areas

Can anyone tell me why we have areas in ospf, and what routes do ABR,s exchange, its it about all routes in that area ?

Hi carl_townshend
Actually we use OSPF in a very large network (many routers) . OSPF is just used to divide the whole network to number of areas (we configure some routers in one area and some on different and so on). So if their is some network failure, the whole topology does not change but only the area in which that network fails suffer.
The Area Hierarchy is as follows for your info. there is one backbone (i.e. Area 0) and all the rest areas are connected to the backbone area.
The Router that connect the backbone and other area is ABR. and ABR exchange network of one area to another, but if you can configure to route summarized network (Interarea summarization) on ABR. to reduce routing table size.

Out-Of-Band Management over IPSec and OSPF Area Design

Hello,
i'm planning to implement Out of band management over OSPF over IPSec Tunnel and i have a question about the OSPF area design (please see attached figure).
As network administrator our NOC is sitting in OFFICE (OSPF Area 1). Internet access is guarenteed over our DCs (Multihomed BGP peering).
Additionally we have a second internet access at OFFICE where i want to use for IPSec Tunnel and building a OSPF neighboring to our Out-of-Band Firewall, which they too have theire own internet access.
I'm planning to declare this IPSec Tunnel as OSPF AREA 3 and AREA 4 respectively for DC1 and DC2. There are no subnet overlapping.
My Question is if I should connect both areas 3 and 4 at OFFICE to the backbone area over a virtual-link or not? Would be an disadvantage if i wont use the virtual-link?
Thx for any reply

1.The AUX port on Cisco routers is either RJ-45 or DB-25. If the AUX port is RJ-45, use a flat-satin rolled RJ-45--RJ-45 cable (part number CAB-500RJ= ), which is usually provided with every Cisco router for console connections. You also need an RJ-45 to DB-25 adapter marked "MODEM" (part number CAB-25AS-MMOD) to connect the rolled cable to the DB-25 port on the modem.
2. if your router has a DB-25 AUX port, use a straight-through DB-25Female - DB25Male RS-232 cable to connect the modem to the router.
Use this document.
http://www.cisco.com/en/US/tech/tk801/tk36/technologies_tech_note09186a0080094bbc.shtml

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Why in ospf , area 0 is made as back bone area ,

Hi all
i have a doubt regarding area 0 which is known to be backbone area and all other areas attached to backbone area by an ABR . Why can we make any other areas as backbone say area 1 is made as backbone and all other areas connected to this area 1 via an ABR . Why its not possible , why its made as a not possible topology ? Please let me know whats the reason for this . or is it possible

Hi,
The need for a backbone area (ignoring its specific number at this point) in OSPF is given by the requirement to provide loop-free routing between areas. Inside an area, OSPF operates on a link-state basis and uses the Dijkstra algorithm to compute the shortest paths. The Dijkstra algorithm by itself never causes a routing loop, and so the loop-free routing inside an area is guaranteed. However, between areas, OSPF resorts to distance-vector routing which is prone to transient routing loops. Therefore, to prevent routing loops in inter-area routing, OSPF by its internal design mandates and imposes a logical star topology between areas: the center of the star is the backbone area, the spokes are the non-backbone areas. Even if you configure a physical link between non-backbone areas, OSPF will not use it. As a result, the logical topology of areas in OSPF is always a loop-free star topology with the backbone area in the center and all other areas attaching to the backbone.
Notice that there cannot be multiple backbone areas as a star cannot have multiple centers.
Now because only a single area can ever act as a backbone area, OSPF designers decided that this area will be assigned the number 0 from beginning and that this number will not be configurable. In theory, they could adopt your approach: decide which single area is going to be a backbone, and then configure the routers to know that it really is the backbone. However, notice that this approach only adds to the complexity of the configuration and does not bring you any added flexibility: only a single area can ever be a backbone, and if the backbone area number was configurable, you would have an additional burden of configuring all routers with the information what area number should be interpreted as the backbone area ID. Simply put, there would be no added value in this: The backbone area would still be only a single area, and you would in addition need to configure extra commands to tell routers who the backbone area is. With the established rule in OSPF that the backbone area is always the area ID 0, the life is simpler :)
Best regards,
Peter

OSPF area plan

Dear All,
I have an existing network setup, ISP1 connecting to the MPLS cloud that connects to other branch offices. ISP2 to connect to the outside world. I only have area 50 in my network and area 0 is at ISP1 end that is the MPLS cloud.I do not have info about ISP 2 's area 0.
I would like to upgrade my network by using Nexus 5k's to connect to my servers . Also would like to run L3 between VSS Core and the N5k's.
My question is how do i plan my OSPF design and to which area do i assign the new server farm to?(N5k's to the core VSS)
Do i create a new area or use existing area 50?
Do i create a totally stub as i am trying to minimize the LSA updates.
Do i create new area 0 in my LAN network as it does not have Area 0 , but has got it only on ISP1's MPLS cloud.
Or else is there a new plan?
Please note that all my static routes from other branches in the MPLS cloud are redistributed into OSPF as this is the hub site and all the spokes will have to go through this Core VSS to reach outside.
I am attaching the topology for reference.
B1,B2,B3 are the branches where the MPLS clouds connect to.
Thanks in advance

If you are peering with ISP1 using OSPF which it sounds like you are then you definitely don't want to create another area 0 in your LAN.
And if you created a new area that area also would need connectivity to area 0 and not via area 50 unless you configured a virtual link between area 50 and your ISP router
However in general you should only use virtual links when you have to and the ISP would have to get involved if you wanted to configure it. So I would recommend against that as it complicates things significantly with very little gain.
All you are really doing is connecting a pair of switches to your existing infrastructure and i can't really see the need to do anything other than add them to your existing area.
I understand what you mean about the switches receiving all the LSAs for remote sites but i wouldn't have thought that would be a problem for Nexus switches.
If it really was a concern then you say you are proposing to connect the switches with L3 to the VSS switches. If that was the case you could always configure routed ports with IPs and if needed configure default routes on the Nexus switches pointing to the VSS switches and configure static routes for the server subnets on the VSS switches pointing to the Nexus switches.
Whichever you use i would make it as simple as possible so definitely no need for another area as far as i can see.
Jon

Ospf area

Hi experts..I have a bit confusion about NSSA. Stub area does not allow external routes but NSSA allow. So what is difference between configuring an area as NSSA and not configuring area as NSSA or any type of stub.
Pls make me clear.

Below link is a good explanation:
https://learningnetwork.cisco.com/docs/DOC-13814
If something is not clear, ask your question here.
HTH
Houtan

OSPF Intra-area Gost routes

Hello
I am currently facing issues while visualizing ospf routes from routers in all areas except within area 0 and ABR.
Attached the basic topology.
On R1 and R2, all routes (type IA, E2 and O) are on the ospf database and routing table.
However only ospf route type IA, E2 from the ABR seems to be announced to R3 (Area 100) and R4 (Area 102) "routes are in OSPF Database and routing table".
the ABR's route type "O" seems to to be anounced to R3 and R4 as they are not present to the OSPF database neither the routing table, EXEPT if i do a "sh ip route x.x.x.x" then i can see the announced route from the ABR.
It seems to me like a bug as i can not reproduce it from GNS3, but maybe i am missing something.
it is simple intra-area routing with no filtering implemented.
What do you think ?
thank you in advance for your help.
Regards
Philippe

Hello Rolf,
Bellow the output
1.1)
ABR-ROUTER#sh ip route 192.168.30.0
Routing entry for 192.168.30.0/24
Known via "ospf 10", distance 110, metric 52, type intra area
Last update from 10.100.4.5 on GigabitEthernet0/0, 3w0d ago
Routing Descriptor Blocks:
* 10.100.4.5, from 100.100.1.13, 3w0d ago, via GigabitEthernet0/0
Route metric is 52, traffic share count is 1
1.2)
ABR-ROUTER#sh ip ospf database summary 192.168.30.0
OSPF Router with ID (10.100.4.248) (Process ID 10)
1.3)
ABR-ROUTER#sh ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Lo0 10 0 10.100.4.248/32 1 LOOP 0/0
NV0 10 0 Unnumbered Lo0 1785 P2P 0/0
Gi0/0 10 0 10.100.4.1/29 1 BDR 1/1
Tu4 10 100 10.100.100.2/30 1000 P2P 1/1
Tu2 10 102 10.100.102.2/30 1000 P2P 1/1
2.1)
R3#sh ip route 192.168.30.0
Routing entry for 192.168.0.0/16, supernet
Known via "ospf 10", distance 110, metric 1011, type inter area
Last update from 10.100.100.2 on Tunnel1, 19:31:25 ago
Routing Descriptor Blocks:
* 10.100.100.2, from 10.100.4.248, 19:31:25 ago, via Tunnel1
Route metric is 1011, traffic share count is 1
R4#sh ip route 192.168.30.0
Routing entry for 192.168.0.0/16, supernet
Known via "ospf 10", distance 110, metric 1011, type inter area
Last update from 10.100.102.2 on Tunnel1, 01:33:27 ago
Routing Descriptor Blocks:
* 10.100.102.2, from 10.100.4.248, 01:33:27 ago, via Tunnel1
Route metric is 1011, traffic share count is 1
2.2)
R3#sh ip ospf database summary 192.168.30.0
OSPF Router with ID (10.100.100.255) (Process ID 10)
R4#sh ip ospf database summary 192.168.30.0
OSPF Router with ID (10.100.102.255) (Process ID 10)
Regards
Philippe

MPLS P-to-PE OSPF Inter-Area failover

Hi Guys.
I am simulating a MPLS core using OSPF for the control plane IGP.
Here's the setup:
Area 0 - backbone
Area 1 - PE routers in location A (PE-A)
Area 2 - PE routers in location B (PE-B)
Network is running MPLS/VPN
Here are the requirements:
1. There will be nxGE links between PE-A and PE-B for better latency requirement and bypass Area 0 for Location A<>B destined traffic
     - I can probably use a new direct route between PE-A and PE-B to establish MP-BGP.
2. When Area 1 to Area 0 links are down, Area1 should failover via Area 2.
    And when Area 2 to Area 0 links are down, Area2 should failover via Area 1.
    - I can probably use virtual-links here...But I dont want to complicate things.
Any recommendations on better design?
Thanks

A long time ago the rule of thump was that you can have up to 50 routers in one area. This was at the time that the routers and switches had low CUP speed and memory. Now days, the router and switches are powerful enough that can handle the database of more than 50 routers. I don't think this is going to be an issue with 24 routers, specially since you already have 20 routers in one area.
HTH

Are this OSPF LSA relate to each other ???

OSPF neighbor relationships progress
1) Down State
2) Init State
OSPF routers send Type 1 (hello)
3) Two-Way State
4) ExStart State (Type 2)
5) Exchange State
6) Loading State (type 3)
7) Full Adjacency
ALSO,
OSPF Area Types also use LSA exchange between routers or area
Type 1 generate by each router for each area it belongs to, flood only within particular area, describe the states of the routers link to the area.
Type 2 generate by DR in multi-access networks, flood only within the area that contain the network, describe the set of routes attched to a particular network.
Type 3 Orgin by ABR, flood throughout the backbone area to other ABRs, describe the links between ABR and the internal routes of a local area.
Type 4 Orgin by ABR, flood throughout the backbone area to other ABRs, describe routes to ASBRs.
Type 5 Orgin by ASBR, describe the routes to destinations external to the AS, flood throughout an OSPF AS.
The questions are this LSA relate to each other or it just happen like this ???

Hi Friend,
There is no relation between the type of LSA (1,2,3,4,5,7) exchanged between the routers within an area and between the areas to exchange the link state information and the packets which are used to form an ospf neigh and adjancy relationship.
Taking an example LSA 5 which is used to carry the external route information into an area is not at all related to type 5 LSA which you are talking for formaing a naighbor relationship.
I think the neighbor relationship is formed using hello packet, dd packet and LSU and LSR packets which you may name as type of LSA's. but these are not at all related to LSA's which are used to carry route information between the areas and within areas.
HTH
Ankur

Multi-areas in OSPF routing questions

Hi Guys,
I currently have 2 x L3 switches, one is for area 0 and the other one is for area 5 and they're connected to each other. Since there are only 2 VLANs on each switch, I'd like to replace both of them with a better switch but unsure about the areas routing on the same L3 switch. Please let me know what your thoughts are.
Switch A is connected to switch B thru interface g1/0/1 with ip address: 10.10.10.1/26
I'm currently using ospf and here is the configure of switch A:
router ospf 1
network 172.20.1.1 0.0.0.0 area 0
network 172.21.1.1 0.0.0.0 area 0
network 10.10.10.1 0.0.0.0 area 5
And this is OSPF routing configure of switch B:
router ospf 1
network 192.168.1.1 0.0.0.0 area 5
network 10.10.10.2 0.0.0.0 area 5
I'd assume the new switch C is the replacement of switch A & B. And here is my routing configuration of switch C:
router ospf 1
network 172.20.1.1 0.0.0.0 area 0
network 172.21.1.1 0.0.0.0 area 0
network 10.10.10.1 0.0.0.0 area 0
network 192.168.1.1 0.0.0.0 area 5
network 10.10.10.2 0.0.0.0 area 5
Is this going to work and/or best cisco practices? if not please advise. Thanks.

Hi,
Since you are replacing 2 switches with 1, now you don't need the transit subnet (10.10.10.1/26) between the 2 switches any more. What will the new switch connect to? Is the new switch going to peer with any other device? If it is a stand alone switch, why do you even need 2 OSPF areas or even need OSPF all together?
HTH

No of Routers in OSPF Area

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