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41 Cards in this Set
- Front
- Back
When a large OSPF area is divided into smaller areas, this is called |
multiarea OSPF. |
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An OSPF area is a group of routers that share the same |
link-state information in their link-state databases. |
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Multiarea OSPF is useful in larger network deployments to |
reduce processing and memory overhead. |
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Multi-Area OSPF benefits: |
Reduces routing tables Reduces link-sate database overhead Reduces SPF algorithm calculation |
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Multiarea OSPF is implemented in a two-layer area hierarchy: |
Backbone (Transit) area Regular (Non-backbone) area |
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Backbone (Transit) area is an area whose primary function is the |
fast and efficient movement of IP packets. |
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Regular (Non-backbone) area connects the |
users and resources. |
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By default, a regular area does not allow |
traffic from another area. |
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All traffic moving from one area to another area must traverse the |
backbone area. |
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This traffic is referred to as |
interarea traffic. |
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The OSPF routers are categorized based on the function they perform in the |
routing domain. |
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There are four different types of OSPF routers: |
Internal router Backbone router Area Border Router (ABR) Autonomous System Boundary Router (ASBR) |
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Internal router is a router that has all of its interfaces |
in the same area. |
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Backbone router is a router in the |
backbone area. |
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Area Border Router (ABR) is a router that has interfaces attached to |
multiple areas. |
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Autonomous System Boundary Router (ASBR) is a router that has at least one interface attached to an |
external internetwork. |
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Redistribution in multiarea OSPF occurs when an ASBR connects different routing domains (e.g., EIGRP and OSPF) |
routing domains (e.g., EIGRP and OSPF). |
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LSAs are the building blocks of the |
OSPF LSDB. |
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LSAs and the LSDB describe the entire |
topology of an OSPF network or area. |
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The LSA includes a link ID field that identifies the |
object to which the link connects. |
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Each router link is defined as 1 of 5 LSA types: |
Type 1 Type 2 Type 3 Type 4 and type 5 |
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Type 1 LSAs include a list of directly connected |
network prefixes and link types. |
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The type 2 LSA contains |
The router ID and IP address of the DR. Router IDs of all other routers on the multiaccess segment. |
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The purpose of a type 2 LSA is to give other routers information about |
multiaccess networks within the same area. |
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Type 3 LSAs are used by ABRs to advertise networks from |
other areas. |
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ABRs collect type 1 LSAs in the |
LSDB. |
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After an OSPF area has converged, the ABR creates a type 3 LSA for each of its |
learned OSPF networks. |
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Type 4 and type 5 LSAs are used collectively to |
Identify an ASBR. Advertise external networks into an OSPF routing domain. |
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A type 4 summary LSA is generated by an ABR only when an ASBR |
exists within an area. |
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Type 5 external LSAs describe routes to networks outside the |
OSPF autonomous system. |
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Type 5 LSAs are originated by the ASBR and are flooded to the entire |
autonomous system. |
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It is strongly recommended that manual route summarization be configured on the |
ASBR. |
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OSPF Routing Table Entries: O |
Best paths to destinations within their area (intra-area) Type 1 and type 2 LSAs. |
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OSPF Routing Table Entries: O IA |
Best paths to the other areas within the interarea. Type 3 and type 4 LSAs. |
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OSPF Routing Table Entries: O E1 or O E2 |
best paths to the external autonomous system (type 5) destinations. |
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When converged, a router can communicate with any network within or outside the |
OSPF autonomous system. |
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Summarization helps keep routing tables |
small. |
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Summarization involves consolidating multiple routes into a |
single advertisement. |
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Summarization also helps increase the network’s stability, because it |
reduces unnecessary LSA flooding. |
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Interarea route summarization occurs on ABRs and applies to routes from |
within each area. |
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External route summarization is specific to external routes that are injected into OSPF via |
route redistribution. |