notes: OSPF Troubleshooting Route Summarization

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.
  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).
  
  
  
  
  router ospf 1  
  network 131.108.2.0 0.0.0.255 area 0  
  area 3 range 131.108.3.0 255.255.255.0 
  
  
  
  
  There is an easy way to check whether a router is summarizing the routes properly. 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.
  
  
  R1#show ip ospf 
  Area 3 
  Number of interfaces in this area is 0
    Area has no authentication 
    SPF algorithm executed 1 times 
    Area ranges are 
   131.108.3.0/24 Passive 
  
  
  show ip route @ R1 will still show individual routes. 
  Solution:
  configure area-range @ ABR.
  
  
  Verification: @ R2 configured as ABR with area-range command.
  
  
  R2#show ip ospf 
  Area 3 
  Number of interfaces in this area is 4 
  Area has no authentication 
  SPF algorithm executed 1 times 
  Area ranges are  
      131.108.3.0/24 Active (64)
   
• A router is not summarizing external routes.
  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).
  
  
  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.
  
  
  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 
  
  
  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.
  
  
  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
  
  
  Solution:  configure the summary-address @ ASBR router.
  
  
  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 
  
  
  verification:
  
  
   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.
  
  
  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