LAYER 3 Static Routing, RIPv2, OSPF, NAT

Loopback Interface

The loopback interface is virtual and serves as a backup SSH connection for all ports on the network device.

It also allows for identifying the next router in a multi-area OSPF routing protocol.

In an interconnected network, the loopback interface provides a backup route to reach the desired device.

This interface acts as a VLAN, forming a network separate from other known networks on the router.

1. Create the loopback interface

   interface loopback 0
   

   A notification indicates that the loopback interface is now UP:

   %LINK-5-CHANGED: Interface Loopback0, changed state to up
   %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up
   

2. Assign addresses

   ip address 1.1.1.1
   ip address 2.2.2.2
   

3. Configure routing

   Since this is a separate network, configure routing on both devices (static in this example):

   ip route 2.2.2.0 255.255.255.0 192.168.1.2
   ip route 1.1.1.0 255.255.255.0 192.168.1.1
   

4. Test with ping

   Router1# ping 2.2.2.2
   

Static Routing

To communicate with another network (i.e., with a different addressing scheme), establish a route between the routers:

ip route 10.10.10.0 255.255.255.0 172.16.1.1

Destination network with subnet mask | Next-hop (next node) 192.168.1.0 255.255.255.0 | 172.16.1.1

Next-hop represents the path for packet forwarding.

Similarly, configure a route for the second router in the opposite direction.

ip route 192.168.1.0 255.255.255.0 172.16.1.2

To reach the 192.168.1.0 255.255.255.0 network, go through 172.16.1.2.

The Router#show ip route command displays known routes, including all active routing protocols and shared networks.

show ip route
Codes: L - local, 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
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area,
       * - candidate default, U - per-user static route, o - ODR, P - periodic downloaded static route

C 192.168.1.0/24 is directly connected, FastEthernet0/0
S 10.10.10.0/24 [1/0] via 172.16.1.1

S for static routes.

⚡ Dynamic Routing

RIPv2

RIPv2 is an example of a dynamic distance-vector routing protocol.

It enables multiple routers to communicate and announce the networks they know.

Routers update their routing tables (every 30 seconds) based on RIP requests they receive.

When discussing cost, the same principle applies to other routing protocols.

1. Configure RIP mode

   R1(config)#router rip
   

2. Switch to version 2

   R1(config)#version 2
   

3. Specify networks

   R1(config-router)#network 10.10.10.0
   R1(config-router)#network 172.16.0.0
   

4. Set passive interfaces

   The g0/0/0 interface on router 1 does not point to a router.

   R1(config-router)#passive-interface g0/0/0
   

RIPv2 Command Summary

Copy and paste these lines (adjust your configuration) directly into your terminal:

conf t
router rip
version 2
network 10.10.10.0
network 172.16.1.0
passive-interface g0/0/0

OSPF

OSPF is an example of a link-state routing protocol.

It organizes routers into “areas” for its configuration.

Four key elements for a minimal OSPF configuration:

1. OSPF Process ID

   router ospf 1
   

2. Router ID

   router-id 1.1.1.1
   

3. Passive interfaces

   passive-interface g0/0/0
   

4. Networks and areas

   1 network per router interface, including sub-interfaces.

network 192.168.1.0 0.0.0.255 area 0
network 10.20.30.40 0.0.0.3 area 0

192.168.1.0/24 is the LAN network.
10.20.30.40/30 is the point-to-point network between the two routers.

View OSPF neighbors with the command:

show ip ospf neighbor


Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.1.0           1   2WAY/DROTHER    00:00:32    192.168.1.0         GigabitEthernet0/1
10.20.30.40           1   2WAY/DROTHER    00:00:32    10.20.30.40         GigabitEthernet0/2

• Set route cost manually (in units of 1):

auto-cost reference-bandwidth 1000

OSPF Command Summary

Copy and paste these lines (adjust your configuration) directly into your terminal:

1
conf t
2
router ospf
3
router-id 1.1.1.1
4
network 192.168.1.0 0.0.0.255 area 0
5
network 10.20.30.40 0.0.0.3 area 0

The example above shows the configuration for a router with one leg in a LAN and another in the /30 network pointing to the next router.

passive-interface g0/0/0

Multi-area OSPF

Dynamic NAT

1. Configure LAN interfaces

   int g0/0/0
   ip nat inside
   

2. Configure WAN interfaces

   int g0/0/1
   ip nat outside
   

3. Configure ACL

   access-list 1 permit 192.168.0.0 0.0.0.255
   

   0.0.0.255 is the wildcard mask.

4. Configure IP pool (WAN)

   ip nat pool MY_POOL 10.10.10.1 10.10.10.1 netmask 255.255.255.0
   

   • 10.10.10.0 corresponds to the WAN network
   • 10.10.10.1 to 10.10.10.x defines an IP address range
   • If there is only one interface: 10.10.10.1 10.10.10.1

5. Link ACL to address pool

   ip nat inside source list 1 pool MON_POOL overload
   

   OR

   1
   ip nat inside source list 1 interface g0/0/1 overload