Hello NetworkLessons Team
Thanks a lot for clear explanation.
Help me please. i couldn’t choose right answers for question below:
Q: Which two areas does OSPF send a summary route by default?
Possible answers:
Is Normal area = Standard area or there is no “normal” area type?
All of Backbone, Standard, Stub and NSSA areas send LSA 3. Anybody can clarify?
Thanks
Hello,
Boris you are mixing a lot of things in this question what really should not be mixed.
As OSPF is link-state protocol you can summarize routes only on ABRs, but ABRs are not doing any route summarization for you by default, you have to configure route summarization manually.
However ABRs are summarizing for you LSA Type 1 and 2 into LSA Type 3 and this LSA Type 3 is after flooded to other area (that are the “O IA” routes). This is ok, because you dont need complete visibility to neighboring area, OSPF ABR is simply saying by this “If you need to send something to subnet in another area, send it to me and I manage the rest, because I have complete visibility there.”
Regarding to stub area types, there are really only 3 stub area types – Normal, Stub and NSSA. Backbone is not a stub area type. Backbone area is always Normal area, it cannot be Stub or NSSA.
Exactly opposite, there is no “Standard” area type. Its called Normal area.
Its not like areas are flooding something into other areas.
ABR routers are generatng and flooding LSA Type 3.
ABR routers that are interconnecting Backbone (Normal) area with other area, which can be Normal, pure Stub, pure NSSA or NSSA Stub, are flooding LSA Type 3 into them (the O IA routes).
It can also be Totally Stub and NSSA Totally Stub, when the area is Totally Stub or NSSA Totally Stub then LSA Type 3 is not flooded into them. Remember when you see the word “Totally” it means NO LSA Type 3 (in other words no “O IA” routes).
Now back to your question.
Question should be rephrased, because otherwise answers would not make sense.
Q: To which two areas does OSPF send a default route?
Hello Boris
I agree with @fugazz that the question does not have the best wording, but this is a question that is found on some practice exams, so we have to take the question as it is. You may find something similar on an actual exam, so you will need to be prepared for bad wording.
Otherwise, it is indeed only the stub and totally stub areas that will have summarization flooded by default. For the totally stub area, the summarization does indeed only include the default route.
I hope this has been helpful!
Laz
Thanks you very much
Thank you very much.
in all IGP , we can do sumarization until we have one of the major network in routing table which is RIB , lets we have 1.1.1.100.0/24 in RIb then we can do summarization like 1.0.0.0/8. if we don’t have 1.x.x.x/x in RIB then we cant do summarization like 1.0.0.0/8 .
Am i right ??.
in BGP , we must have the prefix in BGP table but not mandatory to have in RIB ,then we can do summarization .
kindly correct me sir @lagapidis
Hello Narad
When you perform summarization, you don’t necessarily have to summarize on classful networks. You’ll notice in the lesson, that the command is:
R1(config-router)#area 0 range 172.16.0.0 255.255.0.0
But you can change the subnet mask being used to anything you want. However, you are correct that in order to do summarization that will appear in the routing table, you must have at least one advertised or static route within that summarized range for it to appear.
For BGP, there are two ways to perform summarization. With the first, you create a static route to Null 0 interface for the prefix, and then advertise the network via the network statement under the BGP configuration. By using this technique, you are forcing the summary route to be in the RIB. Remember that BGP will not advertise a network (summary or not) unless it is in the routing table.
The other way to do it is to configure an aggregation network range. When viable routes that match the network range enter the BGP table, an aggregate route is created. On the originating router, the aggregated prefix sets the next-hop to Null 0.
These methods are further described in the following Cisco documentation:
https://www.ciscopress.com/articles/article.asp?p=2756480&seqNum=13
Finally, if you want your BGP routing process to act in a classful manner, then you can use the BGP Auto-summary feature. This will automatically advertise routes based on their classful subnet masks. For more info, take a look at this lesson:
I hope this has been helpful!
Laz
Hello Laz,
If we don’t do OSPF summarization, then fault in 1 area will be also informed and if we do summarization then fault in 1 Area will be passed on each Areas directly connected with it.
and it will save efforts of switch not to rerun SPF algorithm again and saves CPU memory of router or switch
Is understanding on OSPF summarization correct?
Hello Nitin
If you implement summarization, you get several benefits. One benefit is that you can have a single summary route that includes all of the subnets in a particular area. This is beneficial because it keeps OSPF databases as well as routing tables small.
The other benefit is if there is indeed a fault on one of those subnets in a particular area, then the summarization will remain the same, and you will not need to rerun the SPF in the neighboring areas as you suggest.
However, this may have a downside to this as well. Take a look at this diagram:
If the 172.16.1.0/24 network fails, R2 will never learn about that, because the summarization sent to R2 includes this network. So if R2 tries to reach this network, it will send traffic to R1, but R1 will drop it because it has no path to that destination. Thus the link between R2 and R1 is carrying traffic that will eventually be dropped. If we remove summarization, then R2 will know that the 172.16.1.0/24 network has failed, and will not attempt to send any traffic there, it will simply drop it. Thus, the link between R1 and R2 will not be utilized for this dropped traffic.
So you see, you gain something but lose elsewhere. It all depends on what is most important to your network topology.
I hope this has been helpful!
Laz
Many Thanks…Laz
Yes it got clear…
`got this topology:
this is the partial conf file:
– R5 –
interface Loopback0
ip address 7.7.7.7 255.255.255.0
!
interface FastEthernet3/0
ip address 192.165.15.6 255.255.255.252
duplex auto
speed auto
serial restart-delay 0
!
router ospf 1
router-id 5.5.5.5
log-adjacency-changes
network 7.7.7.0 0.0.0.255 area 2
network 192.165.15.0 0.0.0.3 area 2
–R3 –
interface Loopback0
ip address 3.3.3.3 255.255.255.0
!
interface FastEthernet0/0
ip address 192.168.14.1 255.255.255.0
duplex half
!
interface FastEthernet1/0
ip address 192.168.13.2 255.255.255.0
duplex half
!
interface FastEthernet3/0
ip address 192.165.15.2 255.255.255.252
duplex auto
speed auto
!
interface FastEthernet3/1
ip address 192.165.15.5 255.255.255.252
duplex auto
speed auto
!
router ospf 1
router-id 3.3.3.3
log-adjacency-changes
network 3.3.3.0 0.0.0.255 area 0
network 192.165.15.0 0.0.0.3 area 2
network 192.168.13.0 0.0.0.255 area 0
network 192.168.14.0 0.0.0.255 area 0
!
ospf is not running on R5, there is a prob wth subnetting, why rtr cannot establish relationship wth its neighbor R5?
i have this network:
r4 belongs to area 1
r1 ABR area 1- area 0
r2 belongs to area 0
r4 advertise 6.6.5.0/24, 6.6.0/24 on ABR-R1
on R1 apply a access list to deny 6.6.0.0 0.0.255.255 log on output to not adverstise the route on the area 0
r2 sees the 6.6.5.0, 6.6.6.0
why the access list does not cut the 6.6.0.0/16 network?
Hello Konstantinos
On both R5 and R3 your network commands are not including the subnet that you have between the two routers.
On R5, the IP address of Fe3/0 is 192.165.15.6/30, while your network command in your OSPF configuration defines the 192.165.15.0/30 subnet to be advertised. Thus the 192.165.15.6/30 network is not participating in OSPF, thus your Fe3/0 interface is also not participating in OSPF.
Similarly, on R3, the Fe3/1 interface has an IP address of 192.165.15.5/30 while your network command defines the 192.165.15.0/30 subnet to be advertised, once again, leaving out the network attached to Fe3/1.
In other words, OSPF is not active on the interfaces connecting R3 and R5 because the network commands do not include the networks on those interfaces.
To solve the problem, you can use a wildcard mask of 0.0.0.7 in the network command on both routers like so:
network 192.165.15.0 0.0.0.3 area 2
I hope this has been helpful!
Laz
Hello Konstantinos
I understand that you have an access list that denies the 6.6.5.0/24 and 6.6.6.0/24 networks. However, how are you applying this on the ABR to filter the subnets? Can you share that with us?
Just so you have an idea, in order to filter such routes on the ABR, you must use filter-lists. More details about this can be seen in the following lesson:
I hope this has been helpful!
Laz
Hi,
I just want to clarify that the area range command only summarizes networks in the area specified in the command and not Summary LSAs that are advertised into that area by another ABR. e.g…
AREA 2----(ABR_2)----AREA 0-- (ABR_1)—AREA 1
the area 0 range x.x.x.x y.y.y.y command on ABR_2 will only summarize networks that exist in Area 0 and does not include Type 3 LSAs that were created at ABR_1 for networks in Area1.
Is that correct?
Thanks.
Hello Samir
Yes, you are correct. The “area range” command on ABR_2 will only summarize the networks in Area 0 that have been directly advertised to it by routers in that area. It will not include Type 3 LSAs that were created by ABR_1 for networks in Area 1.
I hope this has been helpful!
Laz
Hi,
Another question. I can see from labs that when I use the area range command it uses the lowest metric for the summary. And that is what it says in your notes too. However, looking through rfc2328, on page 135-6 it says:
At most a single Type 3 summary-LSA is originated for each range. When the range’s status indicates Advertise, a Type 3 summary-LSA is generated with Link State ID equal to the range’s address (if necessary, the Link State ID can also have one or more of the range’s “host” bits set; see Appendix E for details) and cost equal to the largest cost of any of the component networks.
Am I misunderstanding what it means or is that statement contradicting your lesson:
A summary route will have the cost of the subnet with the lowest cost that falls within the summary range.
and my lab findings?
Thanks.
Sam
Hello Sam
You’ve got a good eye for detail. The RFC 2328 does indeed say that the cost should be equal to the largest cost of any of the component networks. However, in practice, most OSPF implementations, including Cisco’s, use the cost of the subnet with the lowest cost that falls within the summary range when calculating the cost for an OSPF summary route. This is done to ensure that traffic is routed through the most efficient path.
So, while the RFC does state one thing, the actual implementation can vary depending on the vendor. In this case, the lesson and your lab findings are accurate for Cisco’s implementation of OSPF.
This example emphasizes the fact that vendors are not obligated to adhere absolutely to the definitions of protocols in the RFCs. However, those factors that enable compatibility between devices of different vendors are more important as far as ensuring interoperability goes. Vendors are free to tweak their own implementations, and Cisco seems to do this quite often.
I hope this has been helpful!
Laz
I have a query though, in the 4.0.0.0/8 example what if I add another subnet for example 4.4.5.0/24 is added on area 2…then do we change the summarization on the area 1 border router??
Hello Anudeep
Here’s the diagram I think you are referring to:
If you have a subnet of 4.4.5.0/24 that is connected to the router in Area 2, then you have a discontiguous set of networks. What that means is that parts of the 4.0.0.0/8 network are connected to a router in one part or area of the network, and parts of it are connected to a router in another part or area of a network. This makes summarization more difficult. In this case, you wouldn’t be able to create a summarization using the 4.0.0.0/8 summary.
To resolve such an issue, you would either have to not use summarization, or change your addressing scheme to eliminate the discontiguity of the addressing scheme.
I hope this has been helpful!
Laz
