Problem: OSPF Neighbor (ABR) Not Advertising the Summary Route

When OSPF is configured with more than one area, one area has to be a backbone area. The router that sits at the border of the backbone and any other area is the ABR. The ABR generates the summary LSA for one area and sends it to another area. When the ABR fails to generate the summary LSA, the areas become isolated from each other.

The most common possible causes of this problem are as follows :

• An area is configured as a totally stubby area.

• An ABR is not connected to area 0.

• A discontiguous area 0 exists.

OSPF Neighbor (ABR) Not Advertising the Summary Route ‚ Cause: Area Is Configured as Totally Stubby Area

When an area is configured as a stubby area, no external LSA can be leaked into that area. Similarly, an area can be configured as a totally stubby area, which means that no external or summary LSAs can be leaked into this area.

Figure 9-50 shows an OSPF network setup used to produce this problem. R1 is an ABR, and area 2 is defined as a totally stubby area.

Figure 9-50. Network Setup Used to Produce This Problem

Figure 9-51 shows the flowchart to follow to solve this problem.

Figure 9-51. Problem-Resolution Flowchart

Debugs and Verification

Example 9-133 shows the configuration of R1 that shows area 2 configured as a totally stubby area.

Example 9-133 R1 Configuration Shows Area Types

 R1#  router ospf 1   network 131.108.1.0 0.0.0.255 area 2   network 131.108.2.0 0.0.0.255 area 3   network 131.108.3.0 0.0.0.255 area 0   area 2 stub no-summary  

Example 9-134 shows the output of show ip ospf database summary for 131.108.2.0, which indicates that the summary LSA is generated only in area 0 and not in area 2.

Example 9-134 show ip ospf database summary Command Output Shows Summary LSA Information

 R1#  show ospf database summary 131.108.2.0  OSPF Router with ID (131.108.3.1) (Process ID 1)  Summary Net Link States (Area 0)  LS age: 58   Options: (No TOS-capability, DC)   LS Type: Summary Links(Network)   Link State ID: 131.108.2.0 (summary Network Number)   Advertising Router: 131.108.3.1   LS Seq Number: 8000000E   Checksum: 0x4042   Length: 28   Network Mask: /24        TOS: 0  Metric: 1 

Solution

In this example, area 2 is configured as a totally stubby area, so no summary LSA is originated for this area by the ABR.

If there is only a single exit point, there is no need to receive specific summary LSAs in an area; the default summary LSA of 0.0.0.0 is sufficient. This is also true in the case of a totally stubby NSSA area.

Because this is normal behavior, no solution is given for this. However, if multiple ABRs exist for this area and receiving a specific summary LSA is necessary to avoid suboptimal routing, just remove the no-summary keyword from the area 2 configuration on ABR. This will make area 2 a stub and all the summary LSAs will be leaked into this area.

OSPF Neighbor (ABR) Not Advertising the Summary Route ‚ Cause: ABR Is Not Connected to Area 0

When a router is connected to more than one area, one of those areas must be area 0. The ABR will not generate summary LSAs if it is not connected to area 0. This is standard OSPF behavior.

Figure 9-52 shows an OSPF network experiencing this problem. R1 is between area 2 and area 3, but it is not connected to area 0.

Figure 9-52. OSPF Network in Which the ABR Is Not Connected to Area 0

Figure 9-53 shows the flowchart to follow to solve this problem.

Figure 9-53. Problem-Resolution Flowchart

Debugs and Verification

Example 9-135 shows that R1 is not originating a summary.

Example 9-135 R1 Is Not Originating a Summary Route

 R1#  show ip ospf database summary  OSPF Router with ID (131.108.3.1) (Process ID 1) R1# 

Example 9-136 shows the configuration on R1, which shows that R1 is not connected to area 0.

Example 9-136 R1's Configuration Indicates That It Is Not Connected to Area 0

 router R1#  router ospf 1   network 131.108.1.0 0.0.0.255 area 2  network 131.108.3.0 0.0.0.255 area 3 

There is another way to see whether an area is connected to the backbone. Example 9-137 shows the output of show ip ospf. If this router were connected to area 0, the output would say "It is an area border router."

Example 9-137 show ip ospf Command Output Indicates Whether the Router Is an ABR

 R1#  show ip ospf  Routing Process "ospf 1" with ID 131.108.3.1  Supports only single TOS(TOS0) routes  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs 

Solution

To solve this problem, create a backbone area. After a backbone area is created on R1, it will start generating summary LSAs.

If no area 0 exists in the network, one of the areas must be changed to area 0 for OSPF to work properly.

Example 9-138 shows the new configuration, which shows that R1 now is connected to area 0 by removing 131.108.3.0 from area 3 and putting it into area 0.

Example 9-138 Configuring R1 to Connect to Area 0

 R1#  router ospf 1   network 131.108.1.0 0.0.0.255 area 2   network 131.108.3.0 0.0.0.255 area 0  

If a backbone area already exists, create a virtual link between the nearest ABR and R1, as shown in Example 9-139.

Example 9-139 Creating a Virtual Link Between R1 and the Nearest ABR

 R1#  router ospf 1   network 131.108.1.0 0.0.0.255 area 2   network 131.108.3.0 0.0.0.255 area 3   area 2 virtual-link 141.108.1.1  

Example 9-140 shows the output of show ip ospf, which now shows R1 as an ABR.

Example 9-140 Verifying That R1 Is an ABR

 R1#  show ip ospf  Routing Process "ospf 1" with ID 131.108.3.1  Supports only single TOS(TOS0) routes  It is an area border router  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs 

Example 9-141 shows that R1 starts generating the summary LSA.

Example 9-141 Verifying That R1 Is Generating a Summary LSA

 R1#  show ip ospf database summary  OSPF Router with ID (131.108.3.1) (Process ID 1)  Summary Net Link States (Area 0)  LS age: 58   Options: (No TOS-capability, DC)   LS Type: Summary Links(Network)   Link State ID: 131.108.1.0 (summary Network Number)   Advertising Router: 131.108.3.1   LS Seq Number: 8000000E   Checksum: 0x4042   Length: 28   Network Mask: /24         TOS: 0  Metric: 1  Summary Net Link States (Area 2)  LS age: 58   Options: (No TOS-capability, DC)   LS Type: Summary Links(Network)   Link State ID: 131.108.3.0 (summary Network Number)   Advertising Router: 131.108.3.1   LS Seq Number: 8000000E   Checksum: 0x4042   Length: 28   Network Mask: /24         TOS: 0  Metric: 1 

OSPF Neighbor (ABR) Not Advertising the Summary Route ‚ Cause: Discontiguous Area 0

Figure 9-54 shows an OSPF network experiencing this problem. The link between R1 and R2 is broken, which makes area 0 discontiguous.

Figure 9-54. OSPF Network with a Discontiguous Area 0

Figure 9-55 shows the flowchart to follow to solve this problem.

Figure 9-55. Problem-Resolution Flowchart

Debugs and Verification

Example 9-142 shows the routing table of ABR 2. The route 131.108.1.0/24 shows up in the routing table as an interarea route. The route 131.108.1.0 truly exists in the backbone area, but because the backbone has become discontiguous, this route is coming to ABR 2 through area 2 as an interarea route. The ABR 2 will not generate the summary LSA for this route because it's an interarea route.

Example 9-142 ABR 2 Is Receiving the 131.108.1.0/24 Route as an Interarea Route Because of a Discontiguous Backbone

 ABR2#  show ip route 131.108.1.0  Routing entry for 131.108.1.0/24   Known via "ospf 1", distance 110, metric 129,  type inter area  Redistributing via ospf 1   Last update from 131.108.0.1 on Serial0.1, 00:56:02 ago   Routing Descriptor Blocks:   * 131.108.0.1, from 131.108.3.1, 00:56:02 ago, via Serial0.1        Route metric is 129, traffic share count is 1 

Example 9-143 shows that ABR 2 is not generating a summary LSA for this route into area 0.

Example 9-143 No Summary LSA Is Generated for 131.108.1.0 into Area 0

 ABR2#  show ip ospf database summary 131.108.1.0  OSPF Router with ID (131.108.4.1) (Process ID 1) ABR2# 

Solution

This is obviously a bad design. The backbones are attached to each other through one link. If that link fails, the backbone becomes discontiguous. ABR 2 receives the interarea route from ABR 1 and doesn't create summary LSAs for those routes. This is in accordance with OSPF RFC 2328 that a summary LSA should not be injected into the backbone for interarea routes.

Injecting a summary LSA into the backbone for interarea routers isolates the backbones from each other. To fix this problem, be sure to avoid a single point of failure, as shown in this example, that could make a backbone area discontiguous. Another solution is to create a virtual link between ABR 1 and ABR 2. After creating the virtual link, the area 0 database of ABR 1 and ABR 2 will be synchronized over the virtual link. It will look exactly as if there were a physical link between ABR 1 and ABR 2 and that link were in area 0. The only difference is that both ABR 1 and ABR 2 would use area 2 to forward the OSPF packets. For more infor-mation on virtual links, refer back to Chapter 8.

Example 9-144 shows the configuration of both ABR 1 and ABR 2, which shows that a virtual link is configured to make area 0 contiguous.

Example 9-144 Configuring ABR 1 and ABR 2 with a Virtual Link

 ABR1#  router ospf 1   network 131.108.0.0 0.0.0.255 area 2   network 131.108.3.0 0.0.0.255 area 0    area 2 virtual-link 131.108.4.1   _____________________________________________________________________________________ ABR2#  router ospf 1   network 131.108.0.0 0.0.0.255 area 2   network 131.108.4.0 0.0.0.255 area 0   area 2 virtual-link 131.108.3.1  

A virtual link is configured first by defining the transit area. This is the area that is common between the two ABRs. This area is used to form a virtual adjacency between the two ABRs. In this example, it is area 2. On ABR 1, the command defined under router OSPF is area 2 virtual-link 131.108.4.1. The address 131.108.4.1 is the router ID of ABR 2. Similarly, the address 131.108.3.1 is the router ID of ABR 1. The router ID is the highest IP address on the box ‚ or, if a loopback exists, the loopback becomes the router ID. It is highly recommended that you define a loopback address so that it will be elected as a router ID. One good reason is that, if a link is elected as a router ID instead of a loopback and that link goes down, the router ID will be changed. When a router ID changes, it breaks the virtual link also. On the other hand, if a loopback is defined as a router ID, the router ID always will be the same.

Example 9-145 shows that after creating the virtual link, ABR 2 starts receiving router LSAs for area 0 that include this link 131.108.1.0/24.

Example 9-145 Verifying That ABR 2 Receives Router LSAs for Area 0, Including the 131.108.1.0/24 Link

 ABR2#  show ip ospf database router 131.108.1.1  OSPF Router with ID (131.108.3.1) (Process ID 1)                 Router Link States (Area 0)   Routing Bit Set on this LSA   LS age: 6 (DoNotAge)   Options: (No TOS-capability, DC)   LS Type: Router Links   Link State ID: 131.108.1.1   Advertising Router: 131.108.1.1   LS Seq Number: 80000002   Checksum: 0xC375   Length: 48    Number of Links: 3     Link connected to: a point-to-point Link      (Link ID) Neighboring Router ID: 131.108.3.1      (Link Data) Router Interface address: 131.108.3.2       Number of TOS metrics: 0        TOS 0 Metrics: 64     Link connected to: a Stub Network      (Link ID) Network/subnet number: 131.108.3.0      (Link Data) Network Mask: 255.255.255.0       Number of TOS metrics: 0        TOS 0 Metrics: 1  Link connected to: a Stub Network   (Link ID) Network/subnet number: 131.108.1.0   (Link Data) Network Mask: 255.255.255.0  Number of TOS metrics: 0        TOS 0 Metrics: 1 

Example 9-146 shows that R2 starts receiving intra-area routes for 131.108.1.0/24 after the virtual link is created.

Example 9-146 Verifying That R2 Receives Intra-Area Routes for 131.108.1.0/24

 ABR2#  show ip route 131.108.1.0   Routing entry for 131.108.1.0/24  Known via "ospf 1", distance 110, metric 193,type intra area   Redistributing via ospf 1   Last update from 131.108.4.1 on Serial0.1, 00:56:02 ago   Routing Descriptor Blocks:   * 131.108.4.1, from 131.108.4.1, 00:56:02 ago, via Serial0.1       Route metric is 193, traffic share count is 1