Learn Computer Networking PNETLab - Multi-Area OSPF (Advanced OSPF Routing)

#Introduction

In episode 11 all routers were in area 0. In a large-scale network, that design is a problem: every change at one end forces all routers to recalculate SPF. Episode 12 brings you to multi-area OSPF — a design that splits the network into an area hierarchy so the LSDB stays small and SPF calculations stay light.

We discuss the reasons for an area hierarchy, the role of each router type, the LSA types that are OSPF's "language", and then ABR configuration and route summarization in PNETLab.

#The Need for a Multi-Area OSPF Hierarchy

#Shrinking the LSDB and CPU Load

All routers in one area store the same LSDB. The bigger the area, the bigger the LSDB, the longer SPF calculations take, and the more LSAs get flooded when a change occurs. Dividing the network into several areas means:

• Routers only store their own area's LSDB, not the whole network.
• A change in one area does not trigger SPF in another area.
• LSA flooding is limited per area, saving bandwidth.

#Router Hierarchy in Multi-Area OSPF

#Backbone Router, ABR, and ASBR

There are three special router roles in multi-area OSPF:

• Backbone Router: a router whose entire interfaces are in area 0.
• Area Border Router (ABR): a router that connects a non-0 area to area 0, for example having interfaces in area 0 and area 1.
• Autonomous System Boundary Router (ASBR): a router connected to a network outside OSPF, for example to EIGRP or an ISP.

All non-0 areas must connect to area 0, which is why an ABR always intersects the backbone area.

#Understanding Link-State Advertisement (LSA) Types

#LSA Type 1 through Type 5

An LSA is a message that forms the LSDB. The five most important types:

• Type 1 (Router LSA): information about a router and its links, valid only within an area.
• Type 2 (Network LSA): information about a multi-access network, created by the DR.
• Type 3 (Summary LSA): a summary of networks between areas, created by the ABR.
• Type 4 (ASBR Summary LSA): information about the ASBR's location, created by the ABR.
• Type 5 (AS External LSA): external routes from the ASBR to networks outside OSPF.

Understand these five types because they determine how routes are propagated and summarized between areas.

#Multi-Area OSPF Configuration

#Applying It in PNETLab

Build four routers: R1 and R2 in area 0, R3 and R4 in area 1. Router R2, which connects the two areas, is the ABR. Configure R2:

configure terminal
router ospf 1
 router-id 2.2.2.2
 network 10.0.0.0 0.0.0.3 area 0
 network 10.0.1.0 0.0.0.3 area 1
exit

The command network 10.0.0.0 0.0.0.3 area 0 advertises one link into area 0 and network 10.0.1.0 0.0.0.3 area 1 places another link into area

1. The router with interfaces in two areas is the one acting as the ABR.

#Inter-Area Summarization on the ABR

#area range to Save Routing Table Space

Without summarization, an ABR advertises every subnet of area 1 one by one into area 0. With route summarization, the ABR summarizes many subnets into a single route. Assume R3 and R4 advertise the networks 172.16.0.0/24 through 172.16.3.0/24. Summarize on R2:

configure terminal
router ospf 1
 area 1 range 172.16.0.0 255.255.0.0
exit

area 1 range 172.16.0.0 255.255.0.0 makes R2 advertise only a single summary route 172.16.0.0/16 into area 0 instead of four /24 routes. The routing tables of all routers in area 0 become slimmer, and the flapping of one subnet does not disturb the summary route.

Verify that summarization is working:

R2# show ip ospf database summary
R2# show ip route ospf

show ip ospf database summary displays the Type 3 LSAs carrying inter-area summary routes, and show ip route ospf proves the summarized routes enter the routing table.

#Comparing Area Designs in PNETLab

In PNETLab you can test two scenarios: single-area with all routers in area 0, and multi-area split into areas 0 and 1. Observe the number of entries in show ip route and the load when the topology changes. The multi-area scenario will look far more economical on large networks — the reason large companies apply this design in the real world.

#Closing

Key takeaways:

• Multi-area shrinks the LSDB and limits SPF calculations per area.
• ABR connects non-0 areas to area 0; ASBR connects to outside OSPF.
• LSA Types 1 and 2 are area-local, Types 3 and 4 are created by the ABR, Type 5 by the ASBR.
• network <network> <wildcard> area <n> places a link into a specific area.
• area <n> range <network> <mask> summarizes routes on the ABR.
• Multi-area design is mandatory once a network outgrows a single large area.

In the next episode, episode 13, we leave the IGP world for the Internet protocol: BGP fundamentals — the path-vector protocol that governs routing between Autonomous Systems across the globe, the difference between eBGP and iBGP, basic eBGP configuration, BGP attributes and the path selection order, and verification with show ip bgp summary and show ip bgp.