Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 226 The OSPF Hierarchy (continued) Now consider the above example. Router G has been added, which belongs to Area 0. However, Router G also has a connection to the Internet, which is outside this Autonomous System. This makes Router G an Autonomous System Border Router (ASBR). A router can become an ASBR in one of two ways: • By connecting to a separate Autonomous System, such as the Internet • By redistributing another routing protocol into the OSPF process. ASBRs provide access to external networks. OSPF defines two “types” of external routes: • Type 2 (E2) – Includes only the external cost to the destination network. External cost is the metric being advertised from outside the OSPF domain. This is the default type assigned to external routes. • Type 1 (E1) – Includes both the external cost, and the internal cost to reach the ASBR, to determine the total metric to reach the destination network. Type 1 routes are always preferred over Type 2 routes to the same destination. Thus, the four separate OSPF router types are as follows: • Internal Routers – all router interfaces belong to only one Area. • Area Border Routers (ABRs) – contains interfaces in at least two separate areas • Backbone Routers – contain at least one interface in Area 0 • Autonomous System Border Routers (ASBRs) – contain a connection to a separate Autonomous System * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 227 LSAs and the OSPF Topology Database OSPF, as a link-state routing protocol, does not rely on routing-by-rumor as RIP and IGRP do. Instead, OSPF routers keep track of the status of links within their respective areas. A link is simply a router interface. From these lists of links and their respective statuses, the topology database is created. OSPF routers forward link-state advertisements (LSAs) to ensure the topology database is consistent on each router within an area. Several LSA types exist: • Router LSA (Type 1) – Contains a list of all links local to the router, and the status and “cost” of those links. Type 1 LSAs are generated by all routers in OSPF, and are flooded to all other routers within the local area. • Network LSA (Type 2) – Generated by all Designated Routers in OSPF, and contains a list of all routers attached to the Designated Router. • Network Summary LSA (Type 3) – Generated by all ABRs in OSPF, and contains a list of all destination networks within an area. Type 3 LSAs are sent between areas to allow inter-area communication to occur. • ASBR Summary LSA (Type 4) – Generated by ABRs in OSPF, and contains a route to any ASBRs in the OSPF system. Type 4 LSAs are sent from an ABR into its local area, so that Internal routers know how to exit the Autonomous System. • External LSA (Type 5) – Generated by ASBRs in OSPF, and contain routes to destination networks outside the local Autonomous System. Type 5 LSAs can also take the form of a default route to all networks outside the local AS. Type 5 LSAs are flooded to all areas in the OSPF system. Multicast OSPF (MOSPF) utilizes a Type 6 LSA, but that goes beyond the scope of this guide. Later in this section, Type 7 NSSA External LSAs will be described in detail. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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