OSPF Discontiguous Backbone Area
How does OSPF deal with a discontiguous backbone area?
TL;DR: ABRs are responsible for generating Summary LSAs and in the process information is lost. One OSPF rule to know and skip reading this entire post is - ABRs do not take into SPF calculation consideration Summary LSAs received over a non-backbone area. In contrast, if an ABR receives Summary LSA from another ABR over the backbone area, it regenerates a new Summary LSA and lists itself as the advertising router with the additional cost metric.
Consider the following topology:
R1 and R2 are Area Border Routers (ABRs) with interfaces in both area 0 and area 1. Both the physical and loopback interfaces of R3 are in area 0. R4 has a single interface in area 0 and a loopback interface that is redistributed into OSPF.
Area 0 in this topology is not contiguous. All link-state updates sent from R3 will have to cross over the non-backbone area 1 between R1 and R2 in order to reach R4. Similarly, all link-state updates sent by R4 will need to cross over the non-backbone area 1 between R1 and R2 in order to reach R3.
All required OSPF adjacencies have been formed using basic settings in OSPF process 1. The loopback of R4 has been redistributed into OSPF.
R1#sh ip ospf nei
Neighbor ID Pri State Dead Time Address Interface
192.168.13.3 1 FULL/BDR 00:00:32 192.168.13.3 GigabitEthernet0/1
192.168.12.2 1 FULL/BDR 00:00:33 192.168.12.2 GigabitEthernet0/0R2#sh ip ospf nei
Neighbor ID Pri State Dead Time Address Interface
192.168.24.4 1 FULL/BDR 00:00:32 192.168.24.4 GigabitEthernet0/1
192.168.12.1 1 FULL/DR 00:00:34 192.168.12.1 GigabitEthernet0/0R1 and R2 are ABRs.
R1#sh ip ospf | i router
It is an area border router
Router is not originating router-LSAs with maximum metric
Number of areas in this router is 2. 2 normal 0 stub 0 nssaR2#sh ip ospf | i router
It is an area border router
Router is not originating router-LSAs with maximum metric
Number of areas in this router is 2. 2 normal 0 stub 0 nssaR4 is an ASBR.
R4#sh ip ospf | i router
It is an autonomous system boundary router
Router is not originating router-LSAs with maximum metric
Number of areas in this router is 1. 1 normal 0 stub 0 nssaHow does OSPF behave as this configuration stands?
Let's check R3's OSPF LSDB. Notice that the link between R2 and R4 (192.168.24.0/24) is not learned by R3. However, R3 learns the loopback of R4 4.4.4.4/32 as an external LSA.
R3#sh ip ospf database
OSPF Router with ID (192.168.13.3) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
192.168.12.1 192.168.12.1 1056 0x80000004 0x00341F 1
192.168.13.3 192.168.13.3 1044 0x80000005 0x004CE1 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.13.1 192.168.12.1 1056 0x80000001 0x00F269
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.12.0 192.168.12.1 1284 0x80000001 0x00F659
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
4.4.4.4 192.168.24.4 959 0x80000001 0x0062A8 0Looking into the details of the external LSA shows that it is advertised by R4 (OSPF router ID 192.168.24.4). It also shows that the forward address is set to 0.0.0.0 indicating that R3 needs to perform recursive lookup to the advertising router R4. But the 192.168.24.0/24 network is also unknown to R3's OSPF process as observed in the LSDB output above. This poses a problem and makes the 4.4.4.4/32 LSA unusable.
R3#sh ip ospf database external 4.4.4.4
OSPF Router with ID (192.168.13.3) (Process ID 1)
Type-5 AS External Link States
LS age: 1628
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 4.4.4.4 (External Network Number )
Advertising Router: 192.168.24.4
LS Seq Number: 80000001
Checksum: 0x62A8
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0Checking in R3's routing table shows that a 4.4.4.4/32 route is not installed in the RIB.
R3#sh ip route | b Gateway
Gateway of last resort is not set
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
O IA 192.168.12.0/24 [110/2] via 192.168.13.1, 00:20:00, GigabitEthernet0/0
192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.13.0/24 is directly connected, GigabitEthernet0/0
L 192.168.13.3/32 is directly connected, GigabitEthernet0/0R3's OSPF RIB also doesn't show anything about R4's Lo0 4.4.4.4/32.
R3#sh ip ospf rib
OSPF Router with ID (192.168.13.3) (Process ID 1)
Base Topology (MTID 0)
OSPF local RIB
Codes: * - Best, > - Installed in global RIB
* 3.3.3.3/32, Intra, cost 1, area 0, Connected
via 3.3.3.3, Loopback0
*> 192.168.12.0/24, Inter, cost 2, area 0
via 192.168.13.1, GigabitEthernet0/0
* 192.168.13.0/24, Intra, cost 1, area 0, Connected
via 192.168.13.3, GigabitEthernet0/0An important rule in OSPF is that ABRs do not take into SPF calculation consideration Summary LSAs received over a non-backbone area.
Therefore, both R4's Lo0 4.4.4.4/32, which was redistributed into OSPF, and the route for the link between R2 and R4 (192.168.24.0/24) are not available for R3 to use. In fact, an LSA for the latter (192.168.24.0/24) does not even appear in R3's view.
Before we get to configuration modifications, let's check how R4 sees the topology from its perspective. R4's LSDB shows that it is not receiving any LSA about R3's Lo0 3.3.3.3/32. R4 also has no knowledge of the link between R1 and R3 (192.168.13.0/24).
R4#sh ip ospf database
OSPF Router with ID (192.168.24.4) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
192.168.12.2 192.168.12.2 384 0x80000005 0x002D0B 1
192.168.24.4 192.168.24.4 371 0x80000005 0x0053C5 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.24.2 192.168.12.2 384 0x80000002 0x000F32
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.12.0 192.168.12.2 628 0x80000002 0x00EE5F
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
4.4.4.4 192.168.24.4 371 0x80000002 0x0060A9 0Finally, let's look at R1 and R2's views of the topology. These are our two Area Border Routers and we expect them to see a lot more.
R1's LSDB:
R1#sh ip ospf database
OSPF Router with ID (192.168.12.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
192.168.12.1 192.168.12.1 600 0x80000005 0x003220 1
192.168.13.3 192.168.13.3 632 0x80000006 0x004AE2 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.13.1 192.168.12.1 600 0x80000002 0x00F06A
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.12.0 192.168.12.1 849 0x80000002 0x00F45A
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
192.168.12.1 192.168.12.1 849 0x80000004 0x001E37 1
192.168.12.2 192.168.12.2 767 0x80000004 0x001C36 1
Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
192.168.12.1 192.168.12.1 849 0x80000002 0x00E07D
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
3.3.3.3 192.168.12.1 600 0x80000002 0x00AA0D
192.168.13.0 192.168.12.1 849 0x80000002 0x00E964
192.168.24.0 192.168.12.2 767 0x80000002 0x006AD7
Summary ASB Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
192.168.24.4 192.168.12.2 523 0x80000002 0x003409
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
4.4.4.4 192.168.24.4 512 0x80000002 0x0060A9 0Our ABR R1's routing table shows that it does not install the 192.168.24.0/24 and 4.4.4.4/32 routes since it learned about them over the non-backbone area 1.
R1#sh ip route | b Gateway
Gateway of last resort is not set
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/2] via 192.168.13.3, 00:45:40, GigabitEthernet0/1
192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.12.0/24 is directly connected, GigabitEthernet0/0
L 192.168.12.1/32 is directly connected, GigabitEthernet0/0
192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.13.0/24 is directly connected, GigabitEthernet0/1
L 192.168.13.1/32 is directly connected, GigabitEthernet0/1R2's LSDB:
R2#sh ip ospf database
OSPF Router with ID (192.168.12.2) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
192.168.12.2 192.168.12.2 606 0x80000005 0x002D0B 1
192.168.24.4 192.168.24.4 595 0x80000005 0x0053C5 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.24.2 192.168.12.2 606 0x80000002 0x000F32
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.12.0 192.168.12.2 850 0x80000002 0x00EE5F
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
192.168.12.1 192.168.12.1 934 0x80000004 0x001E37 1
192.168.12.2 192.168.12.2 850 0x80000004 0x001C36 1
Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
192.168.12.1 192.168.12.1 934 0x80000002 0x00E07D
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
3.3.3.3 192.168.12.1 685 0x80000002 0x00AA0D
192.168.13.0 192.168.12.1 934 0x80000002 0x00E964
192.168.24.0 192.168.12.2 850 0x80000002 0x006AD7
Summary ASB Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
192.168.24.4 192.168.12.2 606 0x80000002 0x003409
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
4.4.4.4 192.168.24.4 595 0x80000002 0x0060A9 0Similarly, R2 does not install the 192.168.13.0/24 as well as the 3.3.3.3/32 routes it learned over the non-backbone area 1.
R2#sh ip route | b Gateway
Gateway of last resort is not set
4.0.0.0/32 is subnetted, 1 subnets
O E2 4.4.4.4 [110/20] via 192.168.24.4, 00:45:36, GigabitEthernet0/1
192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.12.0/24 is directly connected, GigabitEthernet0/0
L 192.168.12.2/32 is directly connected, GigabitEthernet0/0
192.168.24.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.24.0/24 is directly connected, GigabitEthernet0/1
L 192.168.24.2/32 is directly connected, GigabitEthernet0/1What are some of the options we can attempt to resolve this issue and allow the routers to use the LSAs they already learn.
- Option 1: Add a link between R1 and R2 in Area 0 to make the backbone contiguous. This should be pretty straightforward. But in some cases there may be physical restrictions that prohibit us from doing so.
- Option 2: Build a virtual link over Area 1 to make the backbone contiguous. If R1 was a stub or non-transit area, we would not be able to implement this option.
- Option 3: Build a GRE tunnel between R1 and R2 with tunnel interfaces placed in Area 0. There's not only configuration overhead and risk of intermittency, but also data plane overhead when it comes to tunnels.
- Option 4: Use multi-area adjacency on the link between R1 and R2. OSPF Multi-area adjacency is a feature that allows OSPF to run separate adjacencies in multiple areas over the same physical link. A key requirement to use the multi-area adjacency feature is the link has to be an OSPF point-to-point network type.
We'll try Option 4 and configure multi-area adjacency.
Let's see details of the adjacency between R1 and R2 before we make the change.
R1#show ip ospf nei 192.168.12.2
Neighbor 192.168.12.2, interface address 192.168.12.2
In the area 1 via interface GigabitEthernet0/0
Neighbor priority is 1, State is FULL, 6 state changes
DR is 192.168.12.1 BDR is 192.168.12.2
Options is 0x12 in Hello (E-bit, L-bit)
Options is 0x52 in DBD (E-bit, L-bit, O-bit)
LLS Options is 0x1 (LR)
Dead timer due in 00:00:38
Neighbor is up for 00:53:57
Index 1/1/1, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0)/0x0(0) Next 0x0(0)/0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 msecIf multi-area adjacency is configured on a link that is different from an OSPF point-to-point network, a logical multi-area interface (MA0) will be created but remains in DOWN state.
R1#sh ip ospf int br
Interface PID Area IP Address/Mask Cost State Nbrs F/C
MA0 1 0 Unnumbered Gi0/0 1 DOWN 0/0
Gi0/1 1 0 192.168.13.1/24 1 DR 1/1
Gi0/0 1 1 192.168.12.1/24 1 BDR 1/1Now we configure multi-area adjacency on R1 and R2 with OSPF point-to-point type network.
R1
interface GigabitEthernet0/0
ip address 192.168.12.1 255.255.255.0
ip ospf network point-to-point
ip ospf multi-area 0
ip ospf 1 area 1
!
R2
interface GigabitEthernet0/0
ip address 192.168.12.2 255.255.255.0
ip ospf network point-to-point
ip ospf multi-area 0
ip ospf 1 area 1The OSPF interface output shows that a logical MA0 interface is created and uses the Unnumbered mechanism to borrow IP address from the Gi0/0 interface. The state of the link is P2P.
R1#show ip ospf int br
Interface PID Area IP Address/Mask Cost State Nbrs F/C
MA0 1 0 Unnumbered Gi0/0 1 P2P 1/1
Gi0/1 1 0 192.168.13.1/24 1 DR 1/1
Gi0/0 1 1 192.168.12.1/24 1 P2P 1/1Now let's check the R1-R2 OSPF adjacency details again. We see two adjacencies between R1 and R2. One adjaceny is in Area 0 over the OSPF_MA0 logical interface while another is formed in Area 1 over the Gi0/0 physical interface.
R1#show ip ospf nei
Neighbor ID Pri State Dead Time Address Interface
192.168.12.2 0 FULL/ - 00:00:39 192.168.12.2 OSPF_MA0
192.168.13.3 1 FULL/BDR 00:00:31 192.168.13.3 GigabitEthernet0/1
192.168.12.2 0 FULL/ - 00:00:36 192.168.12.2 GigabitEthernet0/0R1#show ip ospf nei 192.168.12.2
Neighbor 192.168.12.2, interface address 192.168.12.2
In the area 0 via interface OSPF_MA0
Neighbor priority is 0, State is FULL, 6 state changes
DR is 0.0.0.0 BDR is 0.0.0.0
Options is 0x12 in Hello (E-bit, L-bit)
Options is 0x52 in DBD (E-bit, L-bit, O-bit)
LLS Options is 0x1 (LR)
Dead timer due in 00:00:34
Neighbor is up for 00:02:29
Index 1/2/3, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0)/0x0(0) Next 0x0(0)/0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 msec
Neighbor 192.168.12.2, interface address 192.168.12.2
In the area 1 via interface GigabitEthernet0/0
Neighbor priority is 0, State is FULL, 12 state changes
DR is 0.0.0.0 BDR is 0.0.0.0
Options is 0x12 in Hello (E-bit, L-bit)
Options is 0x52 in DBD (E-bit, L-bit, O-bit)
LLS Options is 0x1 (LR)
Dead timer due in 00:00:31
Neighbor is up for 00:02:29
Index 1/1/1, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0)/0x0(0) Next 0x0(0)/0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 msecLet's circle back to R3's LSDB and verify how (or if) this changes its view of the topology. We can now see that R3 has a lot more in its LSDB and sees more of the topology.
R3#show ip ospf database
OSPF Router with ID (192.168.13.3) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
192.168.12.1 192.168.12.1 257 0x80000009 0x00E7DD 2
192.168.12.2 192.168.12.2 65 0x80000007 0x00FDB0 2
192.168.13.3 192.168.13.3 145 0x80000007 0x0048E3 2
192.168.24.4 192.168.24.4 18 0x80000006 0x0051C6 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.13.1 192.168.12.1 120 0x80000003 0x00EE6B
192.168.24.2 192.168.12.2 65 0x80000003 0x000D33
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.12.0 192.168.12.1 760 0x80000003 0x00F25B
192.168.12.0 192.168.12.2 308 0x80000003 0x00EC60
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
4.4.4.4 192.168.24.4 18 0x80000003 0x005EAA 0Checking the route table on R3 shows that R3 now installs the R4 loopback as an O E2, the link between R1 & R2 as an O IA, and the link between R2 & R4 as an intra-area O route.
R3#sh ip route | b Gateway
Gateway of last resort is not set
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
4.0.0.0/32 is subnetted, 1 subnets
O E2 4.4.4.4 [110/20] via 192.168.13.1, 00:05:05, GigabitEthernet0/0
O IA 192.168.12.0/24 [110/2] via 192.168.13.1, 00:13:41, GigabitEthernet0/0
192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.13.0/24 is directly connected, GigabitEthernet0/0
L 192.168.13.3/32 is directly connected, GigabitEthernet0/0
O 192.168.24.0/24 [110/3] via 192.168.13.1, 00:05:05, GigabitEthernet0/0R4 similarly has a more complete view of the topology.
R4#show ip ospf database
OSPF Router with ID (192.168.24.4) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
192.168.12.1 192.168.12.1 422 0x80000009 0x00E7DD 2
192.168.12.2 192.168.12.2 228 0x80000007 0x00FDB0 2
192.168.13.3 192.168.13.3 312 0x80000007 0x0048E3 2
192.168.24.4 192.168.24.4 179 0x80000006 0x0051C6 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.13.1 192.168.12.1 286 0x80000003 0x00EE6B
192.168.24.2 192.168.12.2 228 0x80000003 0x000D33
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.12.0 192.168.12.1 925 0x80000003 0x00F25B
192.168.12.0 192.168.12.2 471 0x80000003 0x00EC60
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
4.4.4.4 192.168.24.4 179 0x80000003 0x005EAA 0R4 also installs the appropriate routes in its RIB.
R4#sh ip route | b Gateway
Gateway of last resort is not set
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/4] via 192.168.24.2, 00:07:38, GigabitEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
O IA 192.168.12.0/24 [110/2] via 192.168.24.2, 01:10:42, GigabitEthernet0/0
O 192.168.13.0/24 [110/3] via 192.168.24.2, 00:07:38, GigabitEthernet0/0
192.168.24.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.24.0/24 is directly connected, GigabitEthernet0/0
L 192.168.24.4/32 is directly connected, GigabitEthernet0/0This demonstrates that if the backbone area in OSPF is discontiguous, we run into all sorts of routing problems. Routers will not be able to build a proper view of the topology.
The case we covered in this post only considered discontiguous backbone area. But the problem of incomplete topology views in OSPF is also experienced if non-backbone areas are built in a discontiguous manner. There are duct-tape type fixes to these problems but good design practice is critical to avoid such situations. ABR and ASBR placement considerations play a great deal into optimal OSPF routing implementations.
Further Reading:
- Configure OSPF with Multi-Area Adjacency
- OSPF Version 2 - RFC 2328
- Understanding OSPF Transit Capability
Tags: ospf
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