Troubleshooting Route Summarization in OSPF

Troubleshooting Route Summarization in OSPF

This section discusses a feature in OSPF called route summarization. The idea is that if there are contiguous ranges of addresses, instead of advertising every network, you can form a group of contiguous networks and summarize those networks in one, two, or fewer blocks and advertise those blocks. This feature helps reduce the size of the routing table. Reducing the routing table size decreases the convergence time and increases OSPF performance. Thus, summarization needs to be configured manually on the router.

OSPF can use two types of summarization:

  • Interarea summarization that can be done on the ABR
  • External summarization that can be done on the ASBR
  • Two common problems related to summarization in OSPF are as follows:
  • A router is not summarizing interarea routes.
  • A router is not summarizing external routes.

Problem: Router Is Not Summarizing Interarea Routes—Cause: area range Command Is Not Configured on ABR

You must ensure that the area range command is configured on the correct router. Area range summarization can be done only on the ABR. In summarization, instead of originating separate LSAs for each network, the ABR originates summary LSAs to cover those ranges of addresses.

Sometimes, the network mask is configured wrong and summarization doesn’t work because of the misconfiguration. When configuring the area range command, make sure that the summarization mask is in the form of a prefix mask rather than a wildcard mask (that is, 255.255.255.0 instead of 0.0.0.255).

Figure 9-83 shows a network suffering from this problem. In this example, the area range com-mand is configured on R1. This command should be configured only on the router that belongs to the area for which routes are being summarized. In addition, the router must be an ABR.

Troubleshooting Route Summarization in OSPFfig9.83

Figure 9-84 shows the flowchart to follow to solve this problem.

Troubleshooting Route Summarization in OSPFfig9.84

Debugs and Verification
Example 9-232 shows that R1 has the area range command for summarization of area 3 routes. The command highlighted in Example 9-232 defines the range to be summarized. This means that any address in the range 131.108.3.0–255 will be summarized into a single route—131.108.3.0/24. Also, the mask should be in the format of 255.255.255.0 instead of 0.0.0.255. The format 0.0.0.255 is used for the access list range, but in OSPF, the format should be 255.255.255.0.

The syntax of this area range command is correct, but the problem is that R1 is not an ABR. R1 indeed is included in area 0, but it is not connected to area 3 and thus cannot summarize area 3 routes.

Example 9-232 R1 Summarizes Area 3 Routes

There is an easy way to check whether a router is summarizing the routes properly. Example 9-233 shows the output of show ip ospf, which shows that the area 3 range is passive. Passive means that no addresses within this area fall within this range. In fact, R1 does have the routes in this range; however, because R1 is not connected to area 3, the range appears as passive. Also note that the number of interfaces in this area is 0, meaning that this router is not connected to area 3. As a result, summarization will not work properly.

Example 9-233 Area 3 Interfaces Are Defined as Passive

Because summarization is not happening, R1 is receiving four routes in its routing table instead of one summarized route. Example 9-234 shows that R1 is receiving four routes in the routing table known through OSPF interarea.

Example 9-234 R1’s Routing Table Shows Four Routes Instead of One Summarized Route

Solution
To solve this problem, the command must be configured on R2, which is connected to area 3 and also is an ABR. Example 9-235 shows that the area range command is configured on the ABR, which is R2.

Example 9-235 Configuring the area range Command on R2 (ABR)

Again, after configuring the range, check the range status to see whether it is active or passive. If it appears active, the router properly is summarizing the routes. Example 9-236 shows the output of show ip ospf, which indicates that area 3 has area ranges defined and active.

Example 9-236 Determining the Area Range Status in Area 3

After verifying that R2 indeed is summarizing and that the range is active, check R1’s routing table to see whether R1 still is seeing four routes. Example 9-237 shows that after summarization, R1 starts receiving one route instead of four routes.

Example 9-237 R1 Receives a Single Summarized Route

Problem: Router Is Not Summarizing External Routes—Cause: summary-address Command Is Not Configured on ASBR

An OSPF ASBR originates the external LSA whenever any external, static, or connected protocols are redistributed into OSPF. These LSAs are generated at the ASBR. So, when summarization is configured, it always should be configured on the ASBR that is originating these external LSA; otherwise, summarization will not work properly. Again, the summary mask syntax is the same as the area range—that is, 255.255.255.0 instead of 0.0.0.255 (pre-fix mask rather than wildcard mask).

Figure 9-85 shows a network setup in which a router is not summarizing external routes properly. R2 is an ASBR that is redistributing RIP routes into OSPF.

Troubleshooting Route Summarization in OSPFfig9.85

Figure 9-86 shows the flowchart to follow to solve this problem.

Troubleshooting Route Summarization in OSPFfig9.86

Debugs and Verification
Example 9-238 shows the summary-address configuration on R1. Note that R1 is not an ASBR. Also note that the range is using the format 255.255.255.0 instead of 0.0.0.255, as explained in the previous problem. In addition, in the previous example, the area range command was used to summarize the area routes, but that command cannot be used here because these are external routes. To summarize the external routes, summary-address must be used.

Example 9-238 R1 Is Configured for Address Summarization Even Though It Is Not an ASBR

Example 9-239 shows the output of show ip ospf summary-address, which indicates that the metric on this summary route is 16777215—this is infinity because the external LSA metric is 24 bits long and 224 is equal to 16,777,215. A metric of infinity means that no valid addresses belong to this range.

Example 9-239 Summary Route Has a Metric of Infinity

Solution
To fix this problem, configure the summarization on R2, which is the ASBR. Example 9-240 shows that summarization is now configured on the OSPF ASBR, which is R2.

Example 9-240 Configuring Address Summarization on the Correct Router, the ASBR

Be sure to remove the summary-address command from R1. After configuring the summary-address command on R2, the output of show ip ospf summary-address should be checked again for the right metric. Example 9-241 gives the output of show ip ospf summary-address, which shows that the range is valid with a correct metric. The metric for the summarized route is the largest metric of all the addresses in that summary range. This is as of RFC 2178. In RFC 1583, the metric for the summary route used to be the lowest metric of all the addresses in summary range.

Example 9-241 Verifying That the Summarized Address Range Is Now Valid

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