The ip unnumbered configuration command allows you to enable IP processing on a serial interface without assigning it an explicit IP address. The IP unnumbered interface can "borrow" the IP address of another interface that is already configured on the router. This conserves network and address space. For more information on the ip unnumbered command, refer to Understanding and Configuring the ip unnumbered Command. This document illustrates two Open Shortest Path First (OSPF) routers connected by an unnumbered serial link.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware versions.
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In this section, you are presented with the information to configure the features described in this document.
This document uses the network setup shown in this diagram:
This document uses these configurations:
Current configuration: hostname r184.108.40.206 interface Loopback0 ip address 220.127.116.11 255.0.0.0 interface Ethernet2/0/0 ip address 18.104.22.168 255.0.0.0 interface Serial2/1/0 ip unnumbered Ethernet2/0/0 router ospf 1 network 22.214.171.124 0.255.255.255 area 0 end
Current configuration: hostname r126.96.36.199 interface Loopback0 ip address 188.8.131.52 255.0.0.0 interface Ethernet0/0/4 ip address 184.108.40.206 255.0.0.0 interface Serial2/1/0 ip unnumbered Ethernet0/0/4 router ospf 2 network 220.127.116.11 0.255.255.255 area 0 end
This section provides information to confirm that the configuration works properly.
show ip ospf database —Displays a list of the Link State Advertisements (LSAs) in the link state database. This list shows only the information in the LSA header.
show ip ospf database [router] [link-state-id]—Displays the content of the Router LSA (Type-1 LSA) in the database. Router LSAs are produced by every router. These fundamental LSAs list all of the routers' links, or interfaces, along with the states and outgoing costs of the links. They are flooded only within the area in which they originate.
To see how the OSPF database looks when it is given this network environment, look at the output of the show ip ospf database command.
r18.104.22.168#show ip ospf database OSPF Router with ID (22.214.171.124) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 126.96.36.199 188.8.131.52 254 0x8000001A 0xA6FA 2 184.108.40.206 220.127.116.11 253 0x80000017 0x4858 2 r18.104.22.168#show ip ospf database router 22.214.171.124 OSPF Router with ID (126.96.36.199) (Process ID 2) Router Link States (Area 0) LS age: 279 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 188.8.131.52 !--- For router links, Link State Id is always the same as the !--- Advertising Router (next line). Advertising Router: 184.108.40.206 !--- This is the router ID of the router which created !--- this LSA. LS Seq Number: 8000001A Checksum: 0xA6FA Length: 48 Number of Links: 2 Link connected to: another Router (point-to-point) !--- This line shows that this router (220.127.116.11) is a neighbor !--- with 18.104.22.168. (Link ID) Neighboring Router ID: 22.214.171.124 (Link Data) Router Interface address: 0.0.0.12 !--- In the case of unnumbered link, use the MIB II IfIndex !--- value. This value usually starts with 0. Number of TOS metrics: 0 TOS 0 Metrics: 64 !--- This is the OSPF cost of the link that connects !--- the two routers. Link connected to: a Stub Network !--- This entry represents the Ethernet segment 126.96.36.199/8. (Link ID) Network/subnet number: 188.8.131.52 (Link Data) Network Mask: 255.0.0.0 Number of TOS metrics: 0 TOS 0 Metrics: 10 !--- This is the OSPF cost of the Ethernet segment. r184.108.40.206#show ip ospf database router 220.127.116.11 OSPF Router with ID (18.104.22.168) (Process ID 2) Router Link States (Area 0) LS age: 295 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 22.214.171.124 Advertising Router: 126.96.36.199 LS Seq Number: 80000017 Checksum: 0x4858 Length: 48 Number of Links: 2 Link connected to: another Router (point-to-point) (Link ID) Neighboring Router ID: 188.8.131.52 (Link Data) Router Interface address: 0.0.0.10 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 184.108.40.206 (Link Data) Network Mask: 255.0.0.0 Number of TOS metrics: 0 TOS 0 Metrics: 10
This section calculates the shortest path tree from the perspective of Router 220.127.116.11.
Router 18.104.22.168 looks in its own LSA and sees that Router 22.214.171.124 is a neighbor. It then looks at Router 126.96.36.199's LSA to verify that 188.8.131.52 sees 184.108.40.206 as a neighbor. If both routers see each other as neighbors, they are considered reachable. The routers then install routes for any stub networks listed in their neighbor's LSA.
In this example, Router 220.127.116.11 installs a route for 18.104.22.168/8 in its routing table because Router 22.214.171.124 lists 126.96.36.199/8 as a stub network in its LSA. This is seen with the help of the show ip route ospf command.
r188.8.131.52#show ip route ospf O 184.108.40.206/8 [110/74] via 220.127.116.11, 00:06:01, Serial0/1/0 r18.104.22.168#show ip route ospf O 22.214.171.124/8 [110/74] via 126.96.36.199, 00:06:16, Serial2/1/0
There is currently no specific troubleshooting information available for this configuration.
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