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

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.

Figure 9-85. OSPF Network Suffering from a Router Not Properly Summarizing External Routes

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

Figure 9-86. Problem-Resolution Flowchart

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

 R1#  router ospf 1   network 131.108.2.0 0.0.0.255 area 0   summary-address 132.108.3.0 255.255.255.0  

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 2 ²⁴ 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

 R1#  show ip ospf summary-address  OSPF Process 1, Summary-address  132.108.3.0/255.255.255.0 Metric 16777215, Type 0, Tag 0  R1# 

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

 R2#  router ospf 1   network 131.108.2.0 0.0.0.255 area 0    summary-address 132.108.3.0 255.255.255.0    !   router rip  network 132.108.0.0 

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

 R2#  show ip ospf summary-address  OSPF Process 1, Summary-address  132.108.3.0/255.255.255.0 Metric 5, Type 0, Tag 0  R2#