Lab 9-15 – Configuring OSPF Default Route Propagation

In this lab you will learn how to configure Open Shortest Path First (OSPF) to advertise a default route through out the entire OSPF autonomous system.

Real World Application & Core Knowledge

Configuring OSPF to advertise a default route through out the OSPF domain is very similar configuring the default route propagation in the Routing Information Protocol (RIP) found in Lab 7-7 – Configuring Rip Default Information Originate except there are two differences.


In RIP you use the default-information originate command in RIP router configuration mode and this will advertise a default route through out the RIP domain. However if you issue this command in OSPF router configuration mode this will advertise a default route from the router as a type 5 LSA in a normal area (Non-Backbone/Non Stub Area). This command will not advertise a default route unless a static default route exist on the router you’ve issued this command on.

In order to advertise a default route through out area 0 such as a route to an ISP you can use the default-information originate always in OSPF router configuration mode which will advertises a type 3 0.0.0.0/0 LSA to the entire network or you can redistribute a static default route into the OSPF routing process which will be discussed in Section 10.

In this lab you will configure R1 to advertise a default route to all routers in the OSPF autonomous system.

Familiarize yourself with the following new command(s);

area # network netmask – This command is executed in OSPF router configuration mode to advertise the default route as a type 5 LSA to 0.0.0.0/0.

default-information originate always – This command is executed in OSPF router configuration mode to advertise the default route as a type 3 summary LSA to 0.0.0.0/0.

The following logical topology shown below is used in labs found through out Section 9 – Configuring OSPF;

Lab Prerequisites

  • If you are using GNS3 than load the Stub Area Networking GNS3 topology than start devices; R1, R2, R3, R4, R5 and SW1.
  • Establish a console session with devices R1, R2, R3, R4, R5 and SW1 than load the initial configurations provided below by copying the config from the textbox and pasting it into the respected routers console.

Lab Objectives

  • Configure R1 to always originate a default route through out the entire OSPF autonomous system.
  • Verify that the default route is being propagated correctly by viewing the routing table on R4.

Lab Instruction

Objective 1. – Configure R1 to always originate a default route through out the entire OSPF autonomous system.

R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#default-information originate always
R1(config-router)#end
R1#


Objective 2. – Verify that the default route is being propagated correctly by viewing the routing table on R4.

R4#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 10.90.245.1 to network 0.0.0.0

     10.0.0.0/8 is variably subnetted, 15 subnets, 5 masks
O IA    10.90.50.1/32 [110/64767] via 10.90.245.5, 00:54:52, Serial0/0
C       10.90.40.0/24 is directly connected, Loopback0
O IA    10.90.23.0/30 [110/129532] via 10.90.245.2, 00:54:52, Serial0/0
C       10.44.6.0/24 is directly connected, Loopback6
C       10.44.7.0/24 is directly connected, Loopback7
O IA    10.90.30.1/32 [110/129533] via 10.90.245.2, 00:26:12, Serial0/0
C       10.44.4.0/24 is directly connected, Loopback4
O       10.44.4.0/22 is a summary, 00:54:52, Null0
C       10.44.5.0/24 is directly connected, Loopback5
C       10.90.45.2/32 is directly connected, Serial0/1
C       10.90.145.0/24 is directly connected, FastEthernet0/0
C       10.90.45.0/30 is directly connected, Serial0/1
O IA    10.90.20.1/32 [110/64767] via 10.90.245.2, 00:54:53, Serial0/0
O IA    10.90.10.1/32 [110/64767] via 10.90.245.1, 00:54:53, Serial0/0
C       10.90.245.0/29 is directly connected, Serial0/0
O*E2 0.0.0.0/0 [110/1] via 10.90.245.1, 00:00:17, Serial0/0
R4#

2 comments so far

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  1. Babak.v said: October 31, 2010 3:09 PM

    in lab 9-9 print out
    Gateway of last resort is 10.1.23.1 to network 0.0.0.0
    lab 9-15
    Gateway of last resort is 10.1.245.1 to network 0.0.0.0
    so they have a same reason –>default route by LSA3, the only different is O*E2 with O*IA

  2. Matthew George said: November 1, 2010 9:45 AM

    You are Correct Babak. OSPF Inter Area (O*IA) routes are prefered over OSPF External routes (O*E1 & O*E2). However both ways of propogating the default route serve acomplish the same goal but there are benifits of using Inter-Area vs External default routes and vice versa.

    -Free CCNA Workbook Author

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