How do networks communicate?

Network communication is a system that allows devices located far apart to exchange data.
Communication follows a flow of "source → network → destination". Network devices such as switches and routers are present along this communication path.

​Two fundamental keywords within the network are the "OSI reference model" and "protocol."
The OSI reference model is a framework for organizing network communication by function. Network communication is based on the OSI reference model. In this hands-on seminar, we mainly explained L2 and L3.

A protocol is a set of rules or agreements necessary for communication to function correctly.

Ethernet frame

Next, let's talk about Ethernet frames.
An Ethernet frame is a unit (a box) for transmitting data. A frame contains the following information:
• Destination information (MAC address)
- Sender information (MAC address)

In a network, communication takes place by relaying and transmitting these frames.

Operation of the virtual simulator GNS3, basic operation of OcNOS

OcNOS employs a hierarchical command mode.
Specifically, after logging in, the system is designed to transition to modes such as Executive Mode and Configure Mode, where you actually enter settings.
Furthermore, while Cisco switches and similar devices immediately apply settings after a command is entered, OcNOS requires the "commit" command to apply (activate) the entered settings, which means that even if there is a mistake in the entered command, you can revert to the previous state.

Next, let's look at commit-related commands.

commit related commands
commit: Apply the settings you have entered.
show transaction current: Displays the commands used before commit.
commit dry-run: Verifies whether the commands executed before commit can be committed without errors (they are not actually applied).
abort transaction: Deletes all commands before commit.


Confirmation command
show running-config: Check the running configuration.
show startup-config: Check the configuration file (startup-config) that is loaded at startup.

MAC address and switching

A MAC address is a unique number assigned to the NIC (Network Interface Card) of each device.
Specifically, this refers to the individual numbers assigned to the LAN cable ports on a laptop or the Wi-Fi interface.
By using MAC addresses, switches can efficiently transfer data only to the necessary recipients. This communication method is called switching.

For example, in the diagram below, if you want to communicate from PC-A to PC-C, the switch that receives the data will refer to its internal database (FDB/MAC address table) to determine that if the destination MAC address of the transmitted data is "C", it will forward it to port "eth4". This mechanism ensures that the data is delivered efficiently to the intended recipient.

VLAN (Virtual LAN)

Next, let's talk about VLANs (Virtual LANs).
VLAN is a technology that separates "networks" using a single switch.
If two devices are on different VLANs, they are considered to be on different networks, and even if their MAC addresses are known, they cannot communicate with each other.

From here, we will introduce the commands for configuring and verifying VLANs.
*Prerequisite: Create a bridge using Spanning Tree Protocol (STP)

OcNOS#configure terminal
OcNOS(config)#bridge 1 protocol ieee vlan-bridge
OcNOS(config)#no bridge 1 spanning-tree enable bridge-forward
OcNOS(config)#vlan database
OcNOS(config-vlan)#vlan 5 bridge 1 state enable
OcNOS(config-vlan)#exit
OcNOS(config)#commit

OcNOS#show vlan brief
Bridge  VLAN ID     Name         State   H/W Status     　　　　　　　 Member ports
                                             　　　　　      　　　　　　　　  　　　(u)-Untagged, (t)-Tagged
======= ======= ================ ======= ========== ==========================
1     　　  1       　default       　　　　　  ACTIVE  Success
1     　　  5       　VLAN0005       　　　 　ACTIVE  Success

OcNOS#configure terminal
OcNOS(config)#interface eth1
OcNOS(config-if)#switchport
OcNOS(config-if)#bridge-group 1

OcNOS(config-if)#switchport mode access
OcNOS(config-if)#switchport access vlan 5
OcNOS(config-if)#exit
OcNOS(config)#commit

OcNOS#show vlan brief
Bridge  VLAN ID     Name         State   H/W Status      Member ports
                                                     (u)-Untagged, (t)-Tagged
======= ======= ================ ======= ========== ==========================
1       1       default          ACTIVE  Success
1       5       VLAN0005         ACTIVE  Success    eth1(u)

localhost:~# ip address add 192.168.0.1/24 dev eth0
Localhost:~# ip link set up dev eth0

localhost:~# ip address
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 0c:ee:bc:a7:00:00 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.1/24 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::eee:bcff:fea7:0/64 scope link
       valid_lft forever preferred_lft forever

localhost:~# ping 192.168.0.2
PING 192.168.0.2 (192.168.0.2): 56 data bytes
64 bytes from 192.168.0.2: seq=0 ttl=64 time=5.048 ms
64 bytes from 192.168.0.2: seq=1 ttl=64 time=4.825 ms
64 bytes from 192.168.0.2: seq=2 ttl=64 time=4.975 ms
64 bytes from 192.168.0.2: seq=3 ttl=64 time=4.724 ms
^C
--- 192.168.0.2 ping statistics ---
4 packets transmitted, 4 packets received, 0% packet loss
round-trip min/avg/max = 4.724/4.893/5.048 ms

About IP addresses and static routing

Next, we will explain IP addresses and static routing.

An IP address is like a network address. Devices with the same network address can communicate directly with each other, but devices with different network addresses belong to different networks and cannot communicate directly.
Routing, using devices such as "routers" and "Layer 3 (L3) switches," acts as a bridge to connect these different networks.
These devices have a destination table called a routing table, which, when data arrives, looks at the destination IP address to determine which network to forward it to.

IP address configuration and verification command (show ip interface)

next, IP This article introduces the address configuration and verification command (show ip interface).

IP address for VLAN interface
(config)# interface vlan<bridge group ID>. <VLAN ID>
- Switch to Interface mode
-<bridge group ID> specifies the bridge you created.
-<VLAN ID> specifies the target VLAN.

(config-if)# ip address <IP Address>/<Mask>
-<IP Address> specifies the IP address to be assigned.
-<Mask> specifies the mask to be assigned.

Static Route
(config)# ip route <IP Address>/<Mask> <Gateway>
-<IP Address> specifies the IP address to be routed.
-<Mask> specifies the mask to be routed.
-<Gateway> specifies the IP address to be used as the gateway.

IP/GW configuration commands on a virtual PC for communication verification
# ip address add <IP Address>/<Mask> dev <IFNAME>
- <IP Address> specifies the IP address to be assigned.
- <Mask> specifies the mask to be assigned.
- <IFNAME> specifies the interface to be configured.

# ip link set up dev <IFNAME>
-<IFNAME> specifies the interface to be configured.

# ip route add default via <Gateway>
- Specify the default gateway
-<Gateway> specifies the IP address to be used as the gateway.

IPAddress settings (VLANinterface)

Configuration command: Set the IP address 192.168.0.1 to the VLAN interface.

OcNOS#configure terminal
OcNOS(config)#interface vlan1.5
OcNOS(config-if)#ip address 192.168.0.1/24
OcNOS(config-exit)#exit
OcNOS(config)#commit

OcNOS#show ip interface vlan1.5 brief
'*' - address is assigned by dhcp client
Interface            IP-Address      Admin-Status          Link-Status
vlan1.5              192.168.0.1     up                    up

Next are the commands for configuring and verifying routing tables (static routes).
In the route information displayed by the confirmation command, lines beginning with "C" are direct connect routes, while lines beginning with "S" are static routes (manually entered).

Configuration command: Set up a static route that sends packets destined for 192.168.1.0/24 to 192.168.0.2.

OcNOS#configure terminal
OcNOS(config)#ip route 192.168.1.0/24 192.168.0.2
OcNOS(config)#commit

OcNOS#show ip route
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
       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, L1 - IS-IS level-1, L2 - IS-IS level-2,
       ia - IS-IS inter area, E - EVPN,
       v - vrf leaked
       * - candidate default
IP Route Table for VRF "default"
C            127.0.0.0/8 is directly connected, lo, installed 03:07:28, last update 03:07:28 ago
C            192.168.0.0/24 is directly connected, vlan1.5, installed 00:48:43, last update 00:48:43 ago
S            192.168.1.0/24 [1/0] via 192.168.0.2, vlan1.5, installed 00:48:43, last update 00:48:43 ago
Gateway of last resort is not set

localhost:~# ip address
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 0c:ee:bc:a7:00:00 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.1/24 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::eee:bcff:fea7:0/64 scope link
       valid_lft forever preferred_lft forever

localhost:~# ip route add default via 192.168.0.2

localhost:~# ip route
default via 192.168.0.2 dev eth0
192.168.0.0/24 dev eth0 scope link  src 192.168.0.1