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route summarization exercises

Interarea Route Summarization

Interarea Route Summarization – Exclusive Explanation

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Interarea route summarization in the OSPF must be manually configured on ABRs because OSPF does not support auto-summarization. We can perform internal routes summarization only on ABRs. When route summarization is configured on ABRs, a single type 3 LSA (summary LSA) describing the summary route are forwarded to the backbone area.

Multiple routes inside the area are summarized by the single LSA. A summarized route is generated if at least one subnet within the area comes down in the summary-address range. The summarized route metric is the same as the lowest cost of all subnets in the summary-address range. The ABR can only sum up routes that are within the areas connected to the ABR.

Figure 1 illustrates a multiarea OSPF topology. Figure 2 illustrates the routing tables of R3 before the route summarization is configured on R2.

Interarea Route Summarization
Interarea Route Summarization

Calculating the Summary Route

Summarizing different adjacent subnetworks into a single address and mask can calculate in three steps. The steps are the following:-

  1. List all networks in binary format. For example, the two networks of area 10, 192.168.0.0/24 and 192.168.1.0/24 are listed in binary format in figure 3 (1).
  2. Count the matching bits from left to right. The matching bits determine the mask for the summary route. As masked, the first 23 far left bits match. This results in the prefix/23 or subnet mask 255.255.254.0. Figure 3(2) illustrates the counting of matching bits.
  3. Copy the matching bits from left to right and then add zero bits to remaining positions to determine the summarized network address as shown in figure 3(3). In this example, the matching bits with zeros at the result is a network address of 192.168.0.0/23. This summary-address summarizes two networks: 192.168.0.024, and 192.168.1.0/24. In the example the two networks matched out of three networks.
Interarea Route Summarization

Configuring Interarea Route Summarization

We can manually configure the route summarization on ABRs. The R2 is ABR for area 10 and area 0. In the example, R1 is configured to summarize the internal area 10 routes.

To configure route summarization manually on ABR, use the area <area-id> range <address> <mask> command on router configuration mode. This command instructs the ABR to summarize routes for a specific area before injecting them into a different area, via the backbone as type 3 summary LSAs. In our example the configuration on R2 is:

  • R2(config)#router ospf 10
  • R2(config-router)# area 10 range 192.168.0.0 255.255.254.0
  • R2(config-router)# do wr

In OSPFv3, the command is the same except for the IPv6 network address. The command syntax for OSPFv3 is area area-id range prefix/prefix-length.

Figure 4 displays the updated R3 routing table. You can examine that there is only one interarea route entry now available in the routing table for both routes. Though this example only reduced the routing table by one entry, summarization could be implemented, to sum up a lot of networks to reduce the size of routing tables extremely.

Interarea Route Summarization
OSPF Route Summarization

OSPF Route Summarization – Exclusive

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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.

OSPF Route Summarization

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.

OSPF Route Summarization

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.