First, a recap about LSAs. Type 1 - Represents a routerType 2 - Represents a DR for a multi-access linkType 3 - Represents a summary for IA routesType 4 - Represents an ASBR, usually accompanied by Type 5Type 5 - External LSA for routes originating outside the OSPF domainType 7 - Allows for advertisement of … Continue reading 7.11 Other Stubby Area Types →

default-information originate This command is used to inject a default route into the OSPF process as a Type 5 (external) LSA. Command arguments: always - By default, OSPF will not inject the default route unless there is already one resident in the local routing table. By using the always command argument, the router will inject … Continue reading 7.10 Default-information Originate Command →

The cost of the default route injected into a stub area is 1 by default. This can be modified with the command area <area id> default-cost <cost>. Consider the graphic. Two routers are configured for redundancy between Area 0 and Area 1. For more predictable traffic patterns, the administrator configures ABR2 with a cost of … Continue reading 7.9 Cost of the Default Route in a Stub Area →

Configure a stub area on the ABR: ... note that each router within the stub area needs to have this command set, since the hello packets contain a stub area flag that must match between neighbors. The adjacency will remain down until all OSPF neighbors have this configured, as shown below. The stub command will … Continue reading 7.8 OSPF Stub Areas →

... and here they are. Method 1 - default-information originate The default-information originate command is configured on an ASBR to inject a default route into the OSPF process, which is flooded as a Type 5 LSA with the route pointed back to the ASBR where it was configured. Method 2 - Stub Area A stub … Continue reading 7.7 Two Ways of Directing Traffic to the Internet →

Summarization on ASBRs affects Type 5 LSAs. Here are some characteristics of summarization on an ASBR: The summary will be entered into the ASBRs routing table with a route to null; the null interface ensures that traffic is not forwarded to any subnets that are not defined within the summary range.Only the summary range will … Continue reading 7.6 Summarization on ASBRs →

To summarize at the ABR...

Summarization on ABRs affects Type 3 LSAs. Here are some characteristics of summarization on an ABR: The summary will be entered into the ABRs routing table with a route to null if there is at least one subnet that falls within the specified range; if all subnets that are a part of the range are … Continue reading 7.5 Summarization on ABRs →

There are two focal objectives for summarization in ANY routing protocol: Reduce the size of the routing tableReduce the number of routing update messages Therefore, the purpose of route summarization is to maintain connectivity with fewer routing entries, thereby decreasing overhead. **** Route summarization is the key to scalability in OSPF. In OSPF, there are … Continue reading 7.2 OSPF Route Summarization →