Chapter 9: Advanced OSPF
Instructor Materials
CCNP Enterprise: Core Networking
,Chapter 9 Content
This chapter covers the following content:
• Areas - This section describes the benefits and functions of areas within an Open Shortest Path First
(OSPF) routing domain.
• Link-State Announcements - This section explains how OSPF stores, communicates, and builds a
topology from the link-state announcements (LSAs).
• Discontiguous Networks - This section demonstrates a discontiguous network and explains why
such a network cannot distribute routes to all areas properly.
• OSPF Path Selection - This section explains how OSPF makes path selection choices for routes
learned within the OSPF routing domain.
• Summarization of Routes - This section explains how network summarization works with OSPF.
• Route Filtering - This section explains how OSPF routes can be filtered on a router.
The (OSPF) protocol scales well with proper network planning. IP addressing schemes, area segmentation,
address summarization, and hardware capabilities for each area should considered when designing a network.
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2
,Areas
• An OSPF area is a logical grouping of routers or, more specifically, a logical grouping
of router interfaces.
• Area membership is set at the interface level, and the area ID is included in the OSPF
hello packet.
• An interface can belong to only one area.
• All routers within the same OSPF area maintain an identical copy of the link-state
database (LSDB).
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3
, Areas
OSPF Areas
An OSPF area grows in size as the number of network links and routers increase in the area.
While using a single area simplifies the topology, there are trade-offs:
• Full shortest path first (SPF) tree calculation runs when a link flaps within the area.
• The LSDB increases in size and becomes unmanageable.
• The LSDB for the area grows, consuming more memory, and lengthening the SPF
computation process.
• No summarization of route information occurs.
Proper design addresses each of these issues by segmenting the routers into multiple OSPF
areas, thereby keeping the LSDB to a manageable size. Sizing and design of OSPF networks
should account for the hardware constraints of the smallest router in that area.
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4
Instructor Materials
CCNP Enterprise: Core Networking
,Chapter 9 Content
This chapter covers the following content:
• Areas - This section describes the benefits and functions of areas within an Open Shortest Path First
(OSPF) routing domain.
• Link-State Announcements - This section explains how OSPF stores, communicates, and builds a
topology from the link-state announcements (LSAs).
• Discontiguous Networks - This section demonstrates a discontiguous network and explains why
such a network cannot distribute routes to all areas properly.
• OSPF Path Selection - This section explains how OSPF makes path selection choices for routes
learned within the OSPF routing domain.
• Summarization of Routes - This section explains how network summarization works with OSPF.
• Route Filtering - This section explains how OSPF routes can be filtered on a router.
The (OSPF) protocol scales well with proper network planning. IP addressing schemes, area segmentation,
address summarization, and hardware capabilities for each area should considered when designing a network.
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2
,Areas
• An OSPF area is a logical grouping of routers or, more specifically, a logical grouping
of router interfaces.
• Area membership is set at the interface level, and the area ID is included in the OSPF
hello packet.
• An interface can belong to only one area.
• All routers within the same OSPF area maintain an identical copy of the link-state
database (LSDB).
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3
, Areas
OSPF Areas
An OSPF area grows in size as the number of network links and routers increase in the area.
While using a single area simplifies the topology, there are trade-offs:
• Full shortest path first (SPF) tree calculation runs when a link flaps within the area.
• The LSDB increases in size and becomes unmanageable.
• The LSDB for the area grows, consuming more memory, and lengthening the SPF
computation process.
• No summarization of route information occurs.
Proper design addresses each of these issues by segmenting the routers into multiple OSPF
areas, thereby keeping the LSDB to a manageable size. Sizing and design of OSPF networks
should account for the hardware constraints of the smallest router in that area.
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4
