OSPF Unnumbered Interfaces
This post explains how to use Unnumbered Interfaces to simplify OSPF network setup and maintenance in Infix.
Introduction
When using OSPF over Ethernet point to point links, the work to make IP plans for the PtP connections can be tedious and limits flexibility as configurations must be coordinated and matched with each neighbor. The figure below is used as illustration.
Figure 1: OSPF network with point-to-point links.
The setup disregards any user networks attached to the routers, and focus solely on establishing connectivity between routers. (Adding host subnets is described further below).
With OSPF, router connectivity would be easy to configure if the links can be treated as unnumbered point-to-point OSPF interfaces. With this approach, each router can be configured individually with minimum coordination with its neighbours. Ideally, R1 should have IP address 10.1.1.1 assigned to its loopback, advertise 10.1.1.1 via OSPF, and then connect one port (say eth0) towards R2 and another (eth1) towards R3.
In contrast, the custom way is to make an IP plan for each connection, e.g., 10.0.1.0/30 between R1 and R2, assign address 10.0.1.1/30 to eth0 on R1 and 10.0.1.2/30 to eth0 on R2. And do the same for subnet 10.0.1.4/30 between R1 and R3, etc. This work we wish to avoid.
With OSPF unnumbered interfaces we can skip coordinating and configuring IP addresses on Ethernet LANs used as point-to-point links between routers.
Unnumbered interfaces in a nutshell
- Make an IP plan for your routers: In this example R1 is allocated the address 10.1.1.1, R2 address 10.1.1.2, etc.
- Configure the allocated address as /32 to loopback and to all Ethernet interfaces intended as unnumbered interfaces On R1, we would configure 10.1.1.1/32 to lo, eth0 and eth1.
- In OSPF configuration there are two settings (done per OSPF area)
- enable OSPF on intended interfaces
- set Ethernet interfaces as OSPF point-to-point networks
- Plug the network together
The routers should detect each other, build routing tables for connectivity.
Simple setup, two nodes
It’s always a good idea to start with the smallest setup to get a feel for the system. So we start out with a simple network, just two nodes:
Figure 2: OSPF network with point-to-point link between two routers.
The first step, the IP plan, is already shown in Figure 2.
Setting hostname (R1, R2, etc.)
By default, Infix units are assigned the name infix-12-34-56 where the last part (12-34-56) matches the last 3 octets of the unit’s base MAC address. In the examples below, we change the hostname to R1, R2, etc. To change it, login to the unit (console or SSH), and issue cli command to enter Infix CLI. Then change the hostname as shown below.
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admin@infix-04-00-00:~$
admin@infix-04-00-00:~$ cli
admin@infix-04-00-00:/> configure
admin@infix-04-00-00:/config/> set system hostname R1
admin@infix-04-00-00:/config/>
Apply changes by leaving the configuration context with leave, then store changes to the startup configuration.
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admin@infix-04-00-00:/config/> leave
admin@R1:/> copy running-config startup-config
admin@R1:/>
Remember to copy the running configuration to the startup configuration to make the changes permanent.
copy running-config startup-configis not shown in remaining examples.
Configuring IP address (and enabling IP forwarding)
Below we show how to set the address on R1 using the Infix CLI. As the unit is intended to work as IPv4 router, ensure IP forwarding is enabled on the Ethernet interface(s).
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admin@R1:/>
admin@R1:/> configure
admin@R1:/config/> set interface lo ipv4 address 10.1.1.1 prefix-length 32
admin@R1:/config/> set interface eth0 ipv4 address 10.1.1.1 prefix-length 32
admin@R1:/config/> set interface eth0 ipv4 forwarding
To view the changes done so far, use the diff command.
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admin@R1:/config/> diff
interfaces {
interface eth0 {
+ ipv4 {
+ forwarding true;
+ address 10.1.1.1 {
+ prefix-length 32;
+ }
+ }
}
interface lo {
ipv4 {
+ address 10.1.1.1 {
+ prefix-length 32;
+ }
}
}
}
Then issue leave to activate the changes:
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admin@R1:/config/> leave
admin@R1:/>
Status of IP address assignment can be viewed using show interfaces command.
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admin@R1:/> show interfaces
INTERFACE PROTOCOL STATE DATA
eth0 ethernet UP 0c:ec:d1:04:00:00
ipv4 10.1.1.1/32 (static)
ipv6 fe80::eec:d1ff:fe04:0/64 (link-layer)
eth1 ethernet DOWN 0c:ec:d1:04:00:01
eth2 ethernet DOWN 0c:ec:d1:04:00:02
eth3 ethernet DOWN 0c:ec:d1:04:00:03
eth4 ethernet DOWN 0c:ec:d1:04:00:04
eth5 ethernet DOWN 0c:ec:d1:04:00:05
eth6 ethernet DOWN 0c:ec:d1:04:00:06
eth7 ethernet DOWN 0c:ec:d1:04:00:07
eth8 ethernet DOWN 0c:ec:d1:04:00:08
eth9 ethernet DOWN 0c:ec:d1:04:00:09
lo ethernet UP 00:00:00:00:00:00
ipv4 127.0.0.1/8 (static)
ipv4 10.1.1.1/32 (static)
ipv6 ::1/128 (other)
admin@R1:/>
OSPF configuration
OSPF configuration takes place under the routing, control-plane-protocol and ospfv2 subcontext. In line with the IETF OSPF YANG model, an OSPF instance name has to be given. The name has local significance only; here the instance is called default. Enabling OSPF per interface and configuring interface-type are done within OSPF area context (here the backbone area 0.0.0.0 is used).
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admin@R1:/config/> edit routing control-plane-protocol ospfv2 name default
admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> set ospf area 0.0.0.0 interface lo enabled
admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> set ospf area 0.0.0.0 interface eth0 enabled
admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> set ospf area 0.0.0.0 interface eth0 interface-type point-to-point
admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/>
Changes can be shown with the diff command.
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admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> diff
+routing {
+ control-plane-protocols {
+ control-plane-protocol ospfv2 name default {
+ ospf {
+ areas {
+ area 0.0.0.0 {
+ interfaces {
+ interface eth0 {
+ interface-type point-to-point;
+ enabled true;
+ }
+ interface lo {
+ enabled true;
+ }
+ }
+ }
+ }
+ }
+ }
+ }
+}
Then issue leave to activate the changes:
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admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> leave
admin@R1:/>
When the above configuration is done on R1, and correspondingly on R2 with address 10.1.1.2 (Figure 2), routing information will be exchanged using OSPF.
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admin@R1:/> show ospf routes
============ OSPF network routing table ============
N 10.1.1.1/32 [0] area: 0.0.0.0
directly attached to lo
N 10.1.1.2/32 [10] area: 0.0.0.0
via 10.1.1.2, eth0
============ OSPF router routing table =============
============ OSPF external routing table ===========
admin@R1:/>
Larger setup
Expanding the simple setup, depicted in Figure 2, to the larger in Figure 1, we should include the remaining interfaces on R1 and R2, and the do the corresponding configuration for R3-R5.
On R1, we need include eth1 for IP settings and OSPF.
Doing IP setting via Infix CLI is shown below (eth1 should get same config as eth0 got before).
First setting IPv4 address and enable forwarding
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admin@R1:/> configure
admin@R1:/config/> set interface eth1 ipv4 address 10.1.1.1 prefix-length 32
admin@R1:/config/> set interface eth1 ipv4 forwarding
Then OSPF configuration
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admin@R1:/config/> edit routing control-plane-protocol ospfv2 name default
admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> set ospf area 0.0.0.0 interface eth1 enabled
admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> set ospf area 0.0.0.0 interface eth1 interface-type point-to-point
OSPF configuration can be inspected
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admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> show
ospf {
areas {
area 0.0.0.0 {
interfaces {
interface eth0 {
interface-type point-to-point;
enabled true;
}
interface eth1 {
interface-type point-to-point;
enabled true;
}
interface lo {
enabled true;
}
}
}
}
}
admin@R1:/config/routing/control-plane-protocol/ospfv2/name/default/> leave
admin@R1:/>
Corresponding IP and OSPF configuration are then applied to all routers in Figure 1. OSPF is able to establish routes to all routers. Here is the result at R1.
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admin@R1:/> show ospf routes
============ OSPF network routing table ============
N 10.1.1.1/32 [0] area: 0.0.0.0
directly attached to lo
N 10.1.1.2/32 [10] area: 0.0.0.0
via 10.1.1.2, eth0
N 10.1.1.3/32 [10] area: 0.0.0.0
via 10.1.1.3, eth1
N 10.1.1.4/32 [20] area: 0.0.0.0
via 10.1.1.2, eth0
N 10.1.1.5/32 [20] area: 0.0.0.0
via 10.1.1.3, eth1
N 10.1.1.6/32 [30] area: 0.0.0.0
via 10.1.1.2, eth0
via 10.1.1.3, eth1
============ OSPF router routing table =============
============ OSPF external routing table ===========
admin@R1:/>
Adding host subnets
In addition to the point-to-point links, the router likely has regular LAN subnets where hosts can be connected. In Figure 3, host subnets have been added to R1 and R6.
Figure 3: LAN broadcast networks added at R1 and R6.
On R1, eth2 is configured with IP address 10.0.1.1/24, and enabled for OSPF within area 0.0.0.0. The eth2 interface is kept as (regular) broadcast interface (not point-to-point), and as no neighbor OSPF router is expected on this LAN, eth2 can be configured as a passive OSPF interface.
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admin@R1:/> configure
admin@R1:/config/> set interface eth2 ipv4 address 10.0.1.1 prefix-length 24
admin@R1:/config/> set interface eth2 ipv4 forwarding
admin@R1:/config/> set routing control-plane-protocol ospfv2 name default ospf area 0.0.0.0 interface eth2 enabled
admin@R1:/config/> set routing control-plane-protocol ospfv2 name default ospf area 0.0.0.0 interface eth2 passive
admin@R1:/config/> leave
admin@R1:/>
If corresponding setup is done on R6, OSPF would now establish the following routing table
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admin@R1:/> show ospf routes
============ OSPF network routing table ============
N 10.0.1.0/24 [10] area: 0.0.0.0
directly attached to eth2
N 10.0.6.0/24 [40] area: 0.0.0.0
via 10.1.1.2, eth0
via 10.1.1.3, eth1
N 10.1.1.1/32 [0] area: 0.0.0.0
directly attached to lo
N 10.1.1.2/32 [10] area: 0.0.0.0
via 10.1.1.2, eth0
N 10.1.1.3/32 [10] area: 0.0.0.0
via 10.1.1.3, eth1
N 10.1.1.4/32 [20] area: 0.0.0.0
via 10.1.1.2, eth0
N 10.1.1.5/32 [20] area: 0.0.0.0
via 10.1.1.3, eth1
N 10.1.1.6/32 [30] area: 0.0.0.0
via 10.1.1.2, eth0
via 10.1.1.3, eth1
============ OSPF router routing table =============
============ OSPF external routing table ===========
admin@R1:/>
Verify by letting PC1 ping PC2, where attached to R1 and R4 respectivly, see Figure 3.
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PC1> ping 10.0.6.26
84 bytes from 10.0.6.26 icmp_seq=1 ttl=60 time=2.536 ms
84 bytes from 10.0.6.26 icmp_seq=2 ttl=60 time=2.386 ms
84 bytes from 10.0.6.26 icmp_seq=3 ttl=60 time=2.629 ms
^C
PC1>
Done!