Route summarization helps to convert multiple routes into a single route which make routing tables small. After converting a large routing table into a small routing table it is propagated into the backbone area. We have already discussed the type of LSAs in the previous article.
The LSA type1 and type2 are generated inside each area, and then translated into type 3 LSAs, and sent to other areas. Suppose area 1 had 20 networks to advertise, and then 20 types 3 LSAs would be forwarded into the backbone. With summarization, the ABR combine the 20 networks into one of two advertisements.
OSPF doesn’t support automatic summarization and also we cannot summarize routes on every router in an EIGRP network. OSFP can summarize routes only on ABRs and ASBRs routers. Route summarization helps minimizes OSPF traffic and reduces route computation.
In Figure 1, R2 combines all of the network advertisements into one summary LSA. Instead of forwarding individual LSAs for each route in area 10, R2 forwards a summary LSA to area 0. The R3 forward the summary LSA all respected routers in area 20, in this case to R4.
Route summarization increases the network’s stability because it reduces gratuitous LSA flooding. It also reduces the extra overhead on the bandwidth, CPU, memory resources and routing table process. With route summarization, every specific-link LSA is not propagated into the OSPF backbone and further than, which causing unnecessary network traffic and router overhead.
Figure 2 illustrates that if the network link on R1 fails. R1 sends an LSA to R2 (ABR). But, R2 (ABR) does not propagate the update to the backbone, because it has a summary route configured. Specific-link LSA flooding outside the area does not occur.
Interarea and External Route Summarization
As I already discussed earlier in this lesson that summarization in OSPF can only be configured on ABRs or ASBRs. The ABR and ASBR routers advertise only a summary route. ABR routers summarize type 3 LSAs and ASBR routers summarize type 5 LSAs. By default the type 3 and type 5 LSAs do not contain summarized routes; because, by default, summary LSAs are not summarized. Route summarization can be configured as follows:
Inter-area route summarization
OSPF interarea route summarization enables an ABR to summarize neighbouring networks into a single network and advertise the network to other areas. The summarization does not apply to external routes join the OSPF via redistribution. For better route summarization it is important to plan network addresses closely so that these addresses can be summarized into least number of summary addresses.
External route summarization
This is the OSPF route summarization of injected route via redistribution. It is important to make sure the continuity of the external address ranges that are being summarized. The ASBRs can summarize the external routes. We can configure the external route summarization on ASBRs using the summary-address address mask router configuration mode command.