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ROUTING

EXCELLENT

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GUESTBOOK

15. Explain the differences between the possible types of areas, routers, and LSAs

OSPF multi-area components

1. Types of routers
   1. Internal router
      • All interfaces are in one area
   2. Backbone router
      • One of its interface is in area 0
   3. ABR (Area Border Router)
      • Used to connect various OSPF areas
      • Always in more than one area
   4. ASBR (Autonomous System Boundary Router)
      • Used only at the edge for protocol translation
      • Redistributing routing information
2. Types of LSAs
   1. LSA type 1 (O)
      • Router link entry
      • Sent in LSU's by each router for each area
      • "O" routing table code
   2. LSA type 2 (O)
      • Network link entry
      • Describes the routers in the area
      • Sent in LSU's by DR's
   3. LSA type 3 (IA)
      • Summary link entry
      • Sent in LSU's by ABR's
      • Describes the routers to networks within the local area to area 0
   4. LSA type 4 (IA)
      • Summary link entry
      • Sent in LSU's by ABR's.
      • Describes reachability to ASBR's.
   5. LSA type 5
      • Sent in LSU's by ASBR's.
      • Describes external routes to AS.
      • Has 2 sub-types:
        1. LSA type 5 External-type 1 (E1)
           • Adds external cost to the internal cost of each link in the path
        2. LSA type 5 External-type 2 (E2) (default)
           • Use external cost only, disregard all other costs of all internal links in the path
   6. LSA type 6 - Multicast OSPF LSA
   7. LSA type 7 - NSSA
   8. LSA type 8 - External LSA for BGP
   9. LSA type 9,10,11 - Experimental
3. Types of areas
   1. Standard area
   2. Backbone / transit area
      • The central entity to which all other areas connect
      • Always entitled as area 0
   3. Stub area
      • Adjacent to ABR
      • Only get static 0.0.0.0 routes and type 3,4 updates
   4. Totally stub area
      • Adjacent to ABR
      • Only get static 0.0.0.0 routes