OSPF Route Summarization – Exclusive
Route summarization helps convert multiple routes into a single route, which reduces routing tables. 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 types 1 and 2 are generated inside each area, translated into type 3 LSAs, and sent to other areas. Suppose area 1 had 20 networks to advertise, and 20 types of 3 LSAs would be forwarded into the backbone. In summary, the ABR combines the 20 networks into one of two advertisements.
OSPF doesn’t support automatic summarization, and we cannot summarize routes on every router in an EIGRP network. OSFP can summarize routes only on ABRs and ASBRs routers. Route summarization helps minimize OSPF traffic and reduce route computation.
In Figure 1, R2 combines all 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. In this case, the R3 forwarded the summary LSA to all respected routers in area 20 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 further into the OSPF backbone, 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). However, R2 (ABR) does not propagate the update to the backbone because it configures a summary route. Specific-link LSA flooding outside the area does not occur.
Interarea and External Route-Summarization
As I already discussed earlier in this lesson, 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. Summarization can be configured as follows:
Inter-area route summarization
OSPF interarea route summarization enables an ABR to summarize neighboring networks into a single network and advertise the network to other areas. The summarization does not apply to external routes joining the OSPF via redistribution. For better route summarization, it is important to plan network addresses closely so that these addresses can be summarized into the fewest summary addresses.
External route summarization
This is the OSPF route summarization of the injected route via redistribution. It is important to ensure the continuity of the external address ranges 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.
