OSPF Summarization Flashcards
Summarization of Routes
Route scalability is a large factor for the Interior Gateway Protocols used by service providers because there can be thousands of routers running in the network. Splitting up an OSPF routing domain into multiple areas reduces the size of each area’s LSDB. While the number of routers and networks remains the same within the OSPF routing domain.
LSA Reduction Through Area Segmentation
There are three Type 1 LSAs and one Type 2 LSA for the 10.123.1.0/24 network. Those four LSAs become one Type 3 LSA outside Area 1234.
Summarization Fundamentals
Another method of shrinking the LSDB involves summarizing network prefixes.
Summarization of routes also helps SPF calculations run faster. A router that has 10,000 network entries will take longer to run the SPF calculation than a router with 500 network entries. Because all routers within an area must maintain an identical copy of the LSDB, summarization occurs between areas on the ABRs.
Summarization can eliminate the SPF calculation outside the area for the summarized prefixes because the smaller prefixes are hidden.
The Impact of Summarization on SPF Topology Calculation
If the 10.1.12.0/24 link fails, all routers in Area 1 have to run SPF calculations. R4 identifies that the 10.1.13.0/24 and 10.1.34.0/24 networks change their next hop through the serial link. Both of the Type 3 LSAs for these networks need to be updated with new path metrics and advertised into Area 0. The routers in Area 0 run an SPF calculation only on those two prefixes.
Summarization Example
The networks in Area 1 being summarized at the ABR into the aggregate 10.1.0.0/18 prefix. If the 10.1.12.0/24 link fails, all the routers in Area 1 still run the SPF calculation, but routers in Area 0 are not affected because the 10.1.13.0/24 and 10.1.34.0/24 networks arent known outside Area 1.
Interarea Summarization
Interarea summarization reduces the number of Type 3 LSAs that an ABR advertises into an area when it receives Type 1 LSAs. The network summarization range is associated with a specific source area for Type 1 LSAs.
Summarization works only on Type 1 LSAs and is normally configured (or designed) so that summarization occurs as routes enter the backbone from nonbackbone areas.
R1 summarizes three prefixes with various path costs. The 172.16.3.0/24 prefix has the lowest metric, so that metric will be used for the summarized route.
Three Type 1 LSAs (172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24) being summarized into one Type 3 LSA, as the 172.16.0.0/20 network.
Interarea Summarization Configuration
You define the summarization range and associated area by using the command area area-id range network subnet-mask [advertise | not-advertise] [cost metric] under the OSPF process. The default behavior is to advertise the summary prefix, so the keyword advertise is not necessary. Appending cost metric to the command statically sets the metric on the summary route.
R2 summarizes the 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24 networks into a single summary network, 172.16.0.0/16, as those networks are advertised into Area 0. A static cost of 45 is added to the summary route to reduce CPU load if any of the three networks flap.
area 12 range 172.16.0.0 255.255.0.0 cost 45
Discard Route
The ABR performing interarea summarization installs discard routes, which are routes to the Null0 interface that match the summarized network. Discard routes prevent routing loops where portions of the summarized network range do not have a more specific route in the RIB. The administrative distance (AD) for the OSPF summary discard route for internal networks is 110, and it is 254 for external networks.
External Summarization
During OSPF redistribution, external routes advertise into the OSPF domain as Type 5 or Type 7 LSAs (NSSA). External summarization reduces the number of external LSAs in an OSPF domain. An external network summarization range is configured on the ASBR router, and network prefixes that match the network range do not generate a Type 5/Type 7 LSA for the specific prefix. Instead, a Type 5/Type 7 LSA with the external network summarization range is created, and the smaller routes in the summary range are suppressed.
The ASBR creates only one Type 5/Type 7 LSA in Area 56 when EIGRP redistributes routes into OSPF.
summary-address 172.16.0.0 255.255.240.0
