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Link-State Advertisements (LSAs) 221
LSAs are flooded only inside a router's area, and other LSAs advertise only interarea links.
Other LSAs might show routes from external links. The following sections describe the
different types of LSAs.
Type 1: Router LSA
Type 1 LSAs contain information about sending information to neighbor routers. Type 1 LSAs
are transmitted in only a single area and are called router links. Type 1 LSAs have the following
characteristics:
·
Are originated by all routers
·
Contain information about the number of links and the type of router interfaces
·
Contain the link's metric
Type 2: Network LSA
Type 2 LSAs contain lists of routers connected to a network segment. Type 2 LSAs are usually
sent by a designated router in a broadcast or nonbroadcast multiaccess network and are called
network links. Type 2 LSAs have the following characteristics:
·
Are originated by all designated routers
·
Are flooded throughout a single area
·
Contain lists of connected routers
Type 3: Summary LSA
Type 3 LSAs originate at area border routers and are sent into an area to advertise destinations
outside the area. Type 3 LSAs refer to summary links. Type 3 LSAs have the following
characteristics:
·
Are originated by the ABR
·
Contain summary route information
Type 4: Summary LSA (ASBR)
Type 4 LSAs originate as area system border routers. Type 4 LSAs advertise the ASBR router
and refer to summary links. Type 4 LSAs have the following characteristics:
·
Contain router links into the area
·
Contain summary route information
87200333.book Page 221 Wednesday, August 22, 2001 2:37 PM