OSPF Configuration Example
Figure 2-17 demonstrates a two-router topology. Figure 2-17 displays three OSPF areas with Area 2 partitioned from the backbone, necessitating a virtual link.
Figure 2-17 Typical Cisco IOS OSPF topology
Virtual Link Required
Figure 2-17 Typical Cisco IOS OSPF topology
Virtual Link Required
R1's Loopbacks in Area 0 Loopback0 131.108.2.1/24 Loopback1 131.108.3.1/24 Loopback2 131.108.4.1/24 Loopback3 131.108.5.1/24 Loopback4 131.108.6.1/24 Loopback5 131.108.7.1/24
R2's Loopbacks in Area 1 Loopback0 131.108.9.1/24 Loopback1 131.108.10.1/24 Loopback2 131.108.11.1/24 Loopback3 131.108.12.1/24 Loopback4 131.108.13.1/24 Loopback5 131.108.14.1/24 Loopback6 131.108.15.1/24
Example 2-18 displays the full working configuration of R1.
Example 2-18 R1's OSPF Configuration
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hostname R1 enable password cisco interface Loopback0 ip address 131.108.2.1 255.255.255.0 |
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ip address 131.108.3.1 255.255.255.0 |
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interface Loopback2 ip address 131.108.4.1 255.255.255.0 |
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ip ospf network point-to-point |
Example 2-18 R1's OSPF Configuration (Continued)
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interface Loopback3 |
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ip address 131.108.3.1 |
255.255.255.0 |
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ip ospf network point- |
to-point |
interface Loopback4 |
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ip address 131.108.6.1 |
255.255.255.0 |
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ip ospf network point- |
to-point |
interface Loopback5 |
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ip address 131.108.7.1 |
255.255.255.0 |
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ip ospf network point- |
to-point |
interface Ethernet0/0 |
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ip address 131.108.1.1 ! |
255.255.255.0 |
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interface Serial0/0 |
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bandwidth 256 |
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ip address 131.108.255 |
.1 255.255.255 |
252 |
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encapsulation frame-relay |
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ip ospf network point- |
to-point |
interface Serial0/1 |
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bandwidth 256 |
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ip address 131.108.255 |
.5 255.255.255 |
252 |
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encapsulation frame-relay |
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ip ospf network point- |
to-point |
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router ospf 1 |
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router-id 131.108.7.1 |
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area 1 virtual-link 131 |
.108.15.1 |
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network 131.108.0.0 0. |
0.7.255 area 0 |
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network 131.108.255.0 |
0.0.0.3 area 0 |
i |
0.0.0.3 area 0 |
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end |
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By default, loopback interfaces are stub hosts in OSPF and are advertised as 32-bit hosts. The IOS command ip ospf network point-to-point advertises the loopback networks as /24 networks (in this case, you use /24 subnet mask). The Frame Relay connection is configured as point-to-point to ensure that no manual OSPF neighbor configuration is required to form OSPF neighbors. The virtual link is configured across the transit area, 1, to R2 router ID of 131.108.14.1.
Example 2-19 displays R2's full working configuration.
Example 2-19 R2's OSPF Configurations hostname R2
enable password cisco interface Loopback0 ip address 131.108.9.1 255.255.255.0 ip ospf network point-to-point
interface Loopback1
ip address 131.108.10.1 255.255.255.0 ip ospf network point-to-point
interface Loopback2 ip address 131.108.11.1 255.255.255.0 ip ospf network point-to-point
interface Loopback3 ip address 131.108.12.1 255.255.255.0 ip ospf network point-to-point
interface Loopback4 ip address 131.108.13.1 255.255.255.0 ip ospf network point-to-point
interface Loopback5 ip address 131.108.14.1 255.255.255.0 ip ospf network point-to-point
interface Loopback6 ip address 131.108.15.1 255.255.255.0 ip ospf network point-to-point
interface Ethernet0/0 ip address 131.108.8.1 255.255.255.0 half-duplex
interface Serial0/0 ip address 131.108.255.2 255.255.255.252 encapsulation frame-relay ip ospf network point-to-point interface Serial0/1 ip address 131.108.255.6 255.255.255.252 encapsulation frame-relay ip ospf network point-to-point
router ospf 1 router-id 131.108.15.1 area 1 virtual-link 131.108.7.1 network 131.108.8.0 0.0.0.255 area 2 network 131.108.9.0 0.0.0.255 area 1
Example 2-19 R2's OSPF Configurations (Continued)
network 131 network 131 network 131 network 131 network 131 network 131 network 131 network 131 end
.10.0 0 .11.0 0 .12.0 0 .13.0 0 .14.0 0 .15.0 0 .255.0 .255.4
) .255 area 1 ) .255 area 1 ) .255 area 1 ) .255 area 1 ) .255 area 1 ) .255 area 1 .0.3 area 0 .0.3 area 0
Example 2-20 displays the IP OSPF routing table on R1. Example 2-20 show ip route ospf on R1
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R1#show |
ip route ospf |
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131 |
.108. |
0.0/16 is variably |
subnetted, |
17 subnets, 2 masks |
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O |
131.1 |
08.15.0/24 |
[110/391 |
via |
131. |
08.255 |
6, |
00 |
00 |
41, |
Serial0/1 |
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[110/391 |
via |
131. |
08.255 |
2, |
00 |
00 |
41, |
Serial0/0 |
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O |
131.1 |
08.14.0/24 |
[110/391 |
via |
131. |
08.255 |
6, |
00 |
00 |
41, |
Serial0/1 |
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[110/391 |
via |
131. |
08.255 |
2, |
00 |
00 |
41, |
Serial0/0 |
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O |
131.1 |
08.13.0/24 |
[110/391 |
via |
131. |
08.255 |
6, |
00 |
00 |
41, |
Serial0/1 |
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[110/391 |
via |
131. |
08.255 |
2, |
00 |
00 |
41, |
Serial0/0 |
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O |
131.1 |
08.12.0/24 |
[110/391 |
via |
131. |
08.255 |
6, |
00 |
00 |
41, |
Serial0/1 |
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[110/391 |
via |
131. |
08.255 |
2, |
00 |
00 |
41, |
Serial0/0 |
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O |
131.1 |
08.11.0/24 |
[110/391 |
via |
131. |
08.255 |
6, |
00 |
00 |
41, |
Serial0/1 |
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[110/391 |
via |
131. |
08.255 |
2, |
00 |
00 |
41, |
Serial0/0 |
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O |
131.1 |
08.10.0/24 |
[110/391 |
via |
131. |
08.255 |
6, |
00 |
00 |
41, |
Serial0/1 |
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[110/391 |
via |
131. |
08.255 |
2, |
00 |
00 |
41, |
Serial0/0 |
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O |
131.1 |
08.9.0/24 |
[110/391 |
via |
131. |
08.255 |
6, |
00 |
00 |
41, |
Serial0/1 |
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[110/391 |
via |
131. |
08.255 |
2, |
00 |
00 |
42, |
Serial0/0 |
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O IA |
131.1 |
08.8.0/24 |
[110/400 |
via |
131. |
08.255 |
6, |
00 |
00 |
42, |
Serial0/1 |
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[110/400 |
via |
131. |
08.255 |
2, |
00 |
00 |
42, |
Serial0/0 |
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R1's routing table has the remote OSPF networks labeled as O IA because the network 131.108.8.0/24 is part of an area not directly attached to R1. Also, R1 is automatically load balancing across the two paths because the cost metric is the same (391). The administrative distance is 110 (the default).
NOTE The election of the designated router in networks such as Frame Relay is important, and you must ensure the hub or core network router is the elected DR so that the hub router disseminates information to all spoke routers. To ensure the hub is the DR, you can disable the DR election process on edge routers with the IOS command, ip ospf priority 0.
Continue reading here: Border Gateway Protocol
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