3Com 5500G, 4210 manual Route Summarization, Single route and distribute it to other areas, Routers

Configuring OSPF Route Summarization 117

■After you configure an area as a totally NSSA area, the ABR of the totally NSSA area will automatically generate a Type-3 default LSA into the totally NSSA area.

■For the ASBR of an NSSA area to generate a default Type-7 LSA, the default route with the destination address 0.0.0.0/0 must exist in the routing table and you need to execute the nssa default-route-advertisecommand.

■For the ABR of an NSSA area to generate a default Type-7 LSA, you only need to execute the nssa default-route-advertisecommand on it.

Configuring OSPF	You can configure an ABR or ASBR to summarize routes with the same prefix into
Route Summarization	a single route and distribute it to other areas.
	An AS is divided into different areas that are interconnected through ABRs.
	Through route summarization, routing information across areas and the size of
	routing tables on routers will be reduced, improving the calculation speed of
	routers.
	After calculating the intra-area routes of an area, an ABR summarizes contiguous
	networks into one route and advertises it to other areas according to the related
	configuration.
	For example, as shown in the following figure, in Area 1 are three intra-area routes
	19.1.1.0/24, 19.1.2.0/24, and 19.1.3.0/24. By configuring route summarization
	on Router A, the three routes are summarized with the route 19.1.0.0/16 that is
	advertised into Area 0.
	Figure 32	Route summarization
		19.1.0.0/16	Router A
				19.1.1.0/24
				19.1.2.0/24
	ABR	Area 0	ABR	Router B 19.1.3.0/24

Area 1

OSPF performs two types of route summarization:

1ABR route summarization

To distribute routing information to other areas, an ABR generates Type-3 LSAs on a per network segment basis. If contiguous network segments are available in the area, you can summarize them with a single network segment. In this way, the ABR in the area distributes only the summary LSA to reduce the scale of LSDBs on routers in other areas.

2ASBR route summarization

If summarization for redistributed routes is configured on an ASBR, it will summarize redistributed Type-5 LSAs that fall into the specified address range. If in an NSSA area, it also summarizes Type-7 LSAs that fall into the specified address range.