Solved: OSPF route summarization

zheepern0826 · ‎11-20-2016

Hi experts,

I'm studying for CCNP Route and have some question regarding OSPF route summarization. I have attached my topology below.

In R1, i have several 172.160.0.0 subnets and i have summarize it on the ABR(R2)

In R5, I have several 200.0.0.0 subnets and i have sumarize it.

Below is my configuration.

R1#sh run | s ospf
router ospf 10
router-id 192.168.1.1
log-adjacency-changes
network 10.0.0.0 0.0.0.15 area 0
network 172.16.0.0 0.0.0.255 area 0
network 172.16.10.0 0.0.0.255 area 0
network 172.16.20.0 0.0.0.255 area 0
network 172.16.30.0 0.0.0.255 area 0
network 172.16.40.0 0.0.0.255 area 0
network 172.16.50.0 0.0.0.255 area 0
network 172.16.60.0 0.0.0.255 area 0
network 172.16.70.0 0.0.0.255 area 0

R2#sh run | s ospf
router ospf 10
router-id 192.168.1.2
log-adjacency-changes
area 0 range 172.16.0.0 255.255.0.0
area 10 virtual-link 192.168.1.5
network 10.0.0.0 0.0.0.15 area 0
network 10.0.0.16 0.0.0.15 area 10
network 10.0.0.32 0.0.0.15 area 10

R5#sh run | s ospf
router ospf 20
router-id 192.168.1.5
log-adjacency-changes
area 10 virtual-link 192.168.1.2
area 20 range 200.0.0.0 255.0.0.0
network 10.0.0.48 0.0.0.15 area 10
network 10.0.0.64 0.0.0.15 area 10
network 200.10.10.0 0.0.0.255 area 20
network 200.20.10.0 0.0.0.255 area 20
network 200.30.10.0 0.0.0.255 area 20
network 200.40.10.0 0.0.0.255 area 20
network 200.50.10.0 0.0.0.255 area 20
network 200.60.10.0 0.0.0.255 area 20
network 200.70.10.0 0.0.0.255 area 20
network 200.80.10.0 0.0.0.255 area 20
network 200.90.10.0 0.0.0.255 area 20

Below is the result of show ip route from R2, R3 & R5

R2#sh ip route
Gateway of last resort is not set

172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks
O 172.16.60.1/32 [110/2] via 10.0.0.3, 00:04:08, Vlan11
O 172.16.50.1/32 [110/2] via 10.0.0.3, 00:04:08, Vlan11
O 172.16.40.1/32 [110/2] via 10.0.0.3, 00:04:08, Vlan11
O 172.16.30.1/32 [110/2] via 10.0.0.3, 00:04:08, Vlan11
O 172.16.20.1/32 [110/2] via 10.0.0.3, 00:04:08, Vlan11
O 172.16.10.1/32 [110/2] via 10.0.0.3, 00:04:08, Vlan11
O 172.16.0.0/16 is a summary, 00:04:08, Null0
O 172.16.0.1/32 [110/2] via 10.0.0.3, 00:04:09, Vlan11
O 172.16.70.1/32 [110/2] via 10.0.0.3, 00:04:09, Vlan11
10.0.0.0/28 is subnetted, 5 subnets
C 10.0.0.0 is directly connected, Vlan11
C 10.0.0.16 is directly connected, Vlan13
C 10.0.0.32 is directly connected, Vlan15
O 10.0.0.48 [110/2] via 10.0.0.18, 00:04:20, Vlan13
O 10.0.0.64 [110/2] via 10.0.0.35, 00:04:20, Vlan15
O IA 200.0.0.0/8 [110/3] via 10.0.0.35, 00:04:09, Vlan15
[110/3] via 10.0.0.18, 00:04:09, Vlan13

R3#sh ip route
Gateway of last resort is not set

172.16.0.0/32 is subnetted, 8 subnets
O IA 172.16.60.1 [110/3] via 10.0.0.17, 00:04:48, Vlan13
O IA 172.16.50.1 [110/3] via 10.0.0.17, 00:04:48, Vlan13
O IA 172.16.40.1 [110/3] via 10.0.0.17, 00:04:48, Vlan13
O IA 172.16.30.1 [110/3] via 10.0.0.17, 00:04:48, Vlan13
O IA 172.16.20.1 [110/3] via 10.0.0.17, 00:04:48, Vlan13
O IA 172.16.10.1 [110/3] via 10.0.0.17, 00:04:48, Vlan13
O IA 172.16.0.1 [110/3] via 10.0.0.17, 00:04:48, Vlan13
O IA 172.16.70.1 [110/3] via 10.0.0.17, 00:04:49, Vlan13
10.0.0.0/28 is subnetted, 5 subnets
O IA 10.0.0.0 [110/2] via 10.0.0.17, 00:04:57, Vlan13
C 10.0.0.16 is directly connected, Vlan13
O 10.0.0.32 [110/2] via 10.0.0.17, 00:04:57, Vlan13
C 10.0.0.48 is directly connected, Vlan17
O 10.0.0.64 [110/2] via 10.0.0.51, 00:04:58, Vlan17
O IA 200.0.0.0/8 [110/2] via 10.0.0.51, 00:04:58, Vlan17

R5#sh ip route
Gateway of last resort is not set

172.16.0.0/32 is subnetted, 8 subnets
O 172.16.60.1 [110/4] via 10.0.0.66, 00:05:09, Vlan19
[110/4] via 10.0.0.50, 00:05:09, Vlan17
O 172.16.50.1 [110/4] via 10.0.0.66, 00:05:09, Vlan19
[110/4] via 10.0.0.50, 00:05:09, Vlan17
O 172.16.40.1 [110/4] via 10.0.0.66, 00:05:09, Vlan19
[110/4] via 10.0.0.50, 00:05:09, Vlan17
O 172.16.30.1 [110/4] via 10.0.0.66, 00:05:09, Vlan19
[110/4] via 10.0.0.50, 00:05:09, Vlan17
O 172.16.20.1 [110/4] via 10.0.0.66, 00:05:10, Vlan19
[110/4] via 10.0.0.50, 00:05:10, Vlan17
O 172.16.10.1 [110/4] via 10.0.0.66, 00:05:10, Vlan19
[110/4] via 10.0.0.50, 00:05:10, Vlan17
O 172.16.0.1 [110/4] via 10.0.0.66, 00:05:11, Vlan19
[110/4] via 10.0.0.50, 00:05:11, Vlan17
O 172.16.70.1 [110/4] via 10.0.0.66, 00:05:11, Vlan19
[110/4] via 10.0.0.50, 00:05:11, Vlan17
10.0.0.0/28 is subnetted, 5 subnets
O 10.0.0.0 [110/3] via 10.0.0.66, 00:05:11, Vlan19
[110/3] via 10.0.0.50, 00:05:11, Vlan17
O 10.0.0.16 [110/2] via 10.0.0.50, 00:05:21, Vlan17
O 10.0.0.32 [110/2] via 10.0.0.66, 00:05:21, Vlan19
C 10.0.0.48 is directly connected, Vlan17
C 10.0.0.64 is directly connected, Vlan19
C 200.40.10.0/24 is directly connected, Loopback3
C 200.50.10.0/24 is directly connected, Loopback4
C 200.60.10.0/24 is directly connected, Loopback5
C 200.10.10.0/24 is directly connected, Loopback0
C 200.20.10.0/24 is directly connected, Loopback1
C 200.30.10.0/24 is directly connected, Loopback2
C 200.70.10.0/24 is directly connected, Loopback6
C 200.80.10.0/24 is directly connected, Loopback7
C 200.90.10.0/24 is directly connected, Loopback8
O 200.0.0.0/8 is a summary, 00:05:32, Null0

Based on the command show ip route, R2 seems not summarize the 172.16.0.0 subnets as I still can see very single 172.16.x.x subnet.

Can any experts advise any steps i have missed and roughly explain why the 172.16.0.0 subnets is not summarize correctly.

Thank you.

Rolf Fischer · ‎11-21-2016

My confusion is since R5 can summarized 200.x.x.x, why R2 cannot summarized 172.16.x.x ?

Well, both R5 and R2 are ABRs and they both do summarization - but like I mentioned in my first post summarization takes place between areas. You cannot summarize inside an area, this is a rule by link-state design.

R2#sh run | s ospf
router ospf 10
area 0 range 172.16.0.0 255.255.0.0

This sequence tells R2 to generate a 172.16/16 summary for area 0's 172.16.x.x subnets and advertise this summary (only) to the other attached areas (10 and 20) instead of advertising summaries for all the subnets (default ABR behavior). This doesn't change the LSDB of area 0, in area 0 you still see (only) the subnets. This is why OSPF has this area hierarchy: A single area could not be extended indefinitely, you need such a mechanism to keep the LSDBs at a certain size for scalability reasons.

I know that's not easy to understand, and I often don't know where to start explaining; so just let us know when you still have doubts.

View solution in original post

Rolf Fischer · ‎11-20-2016

Hi,

R2 is an ABR, interface Fa0/0 is attached to Area 0, so it has a common LSDB with R1 for Area 0. It installs the networks advertised by R1 as intra-area Routes (O) and generates a summary in Area 10 based on this routes. The interal routers in Area 10 (R3, R4) also have a common LSDB with R2, but in Area 10; so they only have knowledge about the summary and install an inter-area route (O IA).

It is important to understand that the LSDBs need to be synchronized between all routers attached to a given area, so all the routers have the same information to calculate their routes. Between areas we have distance-vector mechanisms and they allow us to summarize or filter routes from other areas. It is a disadvantage of link-state protocols over distance-vector protocols that you don't have the flexibility to filter or summarize wherever you want.

HTH
Rolf

zheepern0826 · ‎11-21-2016

Hi Rolf,

Thank you for your prompt reply. However, i still have some confusion.
From the R1, I can see that 200.x.x.x subnet has been propagated and been summarized but from R5, the 172.16.x.x subnet is not summarized.
My confusion is since R5 can summarized 200.x.x.x, why R2 cannot summarized 172.16.x.x ?

R1#sh ip route
Gateway of last resort is not set

172.16.0.0/24 is subnetted, 8 subnets
C 172.16.60.0 is directly connected, Loopback6
C 172.16.50.0 is directly connected, Loopback5
C 172.16.40.0 is directly connected, Loopback4
C 172.16.30.0 is directly connected, Loopback3
C 172.16.20.0 is directly connected, Loopback2
C 172.16.10.0 is directly connected, Loopback1
C 172.16.0.0 is directly connected, Loopback0
C 172.16.70.0 is directly connected, Loopback7
10.0.0.0/28 is subnetted, 5 subnets
C 10.0.0.0 is directly connected, Vlan11
O IA 10.0.0.16 [110/2] via 10.0.0.4, 00:00:23, Vlan11
O IA 10.0.0.32 [110/2] via 10.0.0.4, 00:00:23, Vlan11
O IA 10.0.0.48 [110/3] via 10.0.0.4, 00:00:24, Vlan11
O IA 10.0.0.64 [110/3] via 10.0.0.4, 00:00:24, Vlan11
O IA 200.0.0.0/8 [110/4] via 10.0.0.4, 00:00:24, Vlan11
R1#

R5#sh ip route
Gateway of last resort is not set

172.16.0.0/32 is subnetted, 8 subnets
O 172.16.60.1 [110/4] via 10.0.0.66, 00:00:23, Vlan19
[110/4] via 10.0.0.50, 00:00:23, Vlan17
O 172.16.50.1 [110/4] via 10.0.0.66, 00:00:23, Vlan19
[110/4] via 10.0.0.50, 00:00:23, Vlan17
O 172.16.40.1 [110/4] via 10.0.0.66, 00:00:23, Vlan19
[110/4] via 10.0.0.50, 00:00:23, Vlan17
O 172.16.30.1 [110/4] via 10.0.0.66, 00:00:23, Vlan19
[110/4] via 10.0.0.50, 00:00:23, Vlan17
O 172.16.20.1 [110/4] via 10.0.0.66, 00:00:24, Vlan19
[110/4] via 10.0.0.50, 00:00:24, Vlan17
O 172.16.10.1 [110/4] via 10.0.0.66, 00:00:24, Vlan19
[110/4] via 10.0.0.50, 00:00:24, Vlan17
O 172.16.0.1 [110/4] via 10.0.0.66, 00:00:24, Vlan19
[110/4] via 10.0.0.50, 00:00:24, Vlan17
O 172.16.70.1 [110/4] via 10.0.0.66, 00:00:24, Vlan19
[110/4] via 10.0.0.50, 00:00:24, Vlan17
10.0.0.0/28 is subnetted, 5 subnets
O 10.0.0.0 [110/3] via 10.0.0.66, 00:00:29, Vlan19
[110/3] via 10.0.0.50, 00:00:29, Vlan17
O 10.0.0.16 [110/2] via 10.0.0.50, 00:00:29, Vlan17
O 10.0.0.32 [110/2] via 10.0.0.66, 00:00:29, Vlan19
C 10.0.0.48 is directly connected, Vlan17
C 10.0.0.64 is directly connected, Vlan19
C 200.40.10.0/24 is directly connected, Loopback3
C 200.50.10.0/24 is directly connected, Loopback4
C 200.60.10.0/24 is directly connected, Loopback5
C 200.10.10.0/24 is directly connected, Loopback0
C 200.20.10.0/24 is directly connected, Loopback1
C 200.30.10.0/24 is directly connected, Loopback2
C 200.70.10.0/24 is directly connected, Loopback6
C 200.80.10.0/24 is directly connected, Loopback7
C 200.90.10.0/24 is directly connected, Loopback8
O 200.0.0.0/8 is a summary, 00:00:41, Null0
R5#

Appreciate your reply on this. Thank you .

Rolf Fischer · ‎11-21-2016

My confusion is since R5 can summarized 200.x.x.x, why R2 cannot summarized 172.16.x.x ?

Well, both R5 and R2 are ABRs and they both do summarization - but like I mentioned in my first post summarization takes place between areas. You cannot summarize inside an area, this is a rule by link-state design.

R2#sh run | s ospf
router ospf 10
area 0 range 172.16.0.0 255.255.0.0

This sequence tells R2 to generate a 172.16/16 summary for area 0's 172.16.x.x subnets and advertise this summary (only) to the other attached areas (10 and 20) instead of advertising summaries for all the subnets (default ABR behavior). This doesn't change the LSDB of area 0, in area 0 you still see (only) the subnets. This is why OSPF has this area hierarchy: A single area could not be extended indefinitely, you need such a mechanism to keep the LSDBs at a certain size for scalability reasons.

I know that's not easy to understand, and I often don't know where to start explaining; so just let us know when you still have doubts.

zheepern0826 · ‎11-27-2016

Hi Rolf,

Thanks. Based on your explanation, I have done more test on this topic and now i have much more understanding on how the OSPF works.

Thanks again :)

Rolf Fischer · ‎11-27-2016

Thanks for your feedback, I'm really glad to hear that.

And I'm also glad to see that you're studying how nework technologie actually works instead of just memorize braindumps like so many others do in order to pass a certification exam. That's not an easy way but it is the right way to go and I'd like to encourage you to keep it up!

surahuja · ‎11-28-2016

Hi ,

I see that you are not able to see the summary on R3 . Is that your doubt ?

Rolf's answer on the topic is excellent but you should still see a summary in R2's routing table.

R2#

172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks
O 172.16.0.0/16 is a summary, 00:53:59, Null0
O 172.16.0.1/32 [110/2] via 12.12.12.1, 00:53:59, GigabitEthernet0/1
O 172.16.10.1/32 [110/2] via 12.12.12.1, 00:53:59, GigabitEthernet0/1
O 172.16.20.1/32 [110/2] via 12.12.12.1, 00:53:59, GigabitEthernet0/1
O 172.16.30.1/32 [110/2] via 12.12.12.1, 00:53:59, GigabitEthernet0/1
O 172.16.40.1/32 [110/2] via 12.12.12.1, 00:53:59, GigabitEthernet0/1
O 172.16.50.1/32 [110/2] via 12.12.12.1, 00:53:59, GigabitEthernet0/1
O 172.16.60.1/32 [110/2] via 12.12.12.1, 00:53:59, GigabitEthernet0/1
O 172.16.70.1/32 [110/2] via 12.12.12.1, 00:53:59, GigabitEthernet0/1

I can't see this summary route anywhere in your output. Is this your confusion as to why the summary is not generated ?

Rolf Fischer · ‎11-28-2016

Very good point; I didn't mention this because with every additional detail the answer becomes more complicated.

Stricktly speaking, in terms of OSPF (Standards) this is not a summary route. It is a local Discard Route, generated on ABRs/ASBRs by default in Cisco IOS except for really old versions (follow the link for more information).

surahuja · ‎11-28-2016

Hi Rolf,

Point noted. The route is indeed the internal discard route which would be seen as a summary on R3 and R4. I somehow seemed to have missed that in the output !

Surya