CCNA

CCNA Study Guide
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These tutorials are prepared with single slogan; “provide best online CCNA training absolutely free”. This comprehensive collection of CCNA Study material is exactly; what you need to prepare for following exams CCNA Routing and Switching; Cisco Certified Entry Networking Technician (CCENT), Interconnecting Cisco Networking Devices – part 1 (ICND-1); Interconnecting Cisco Networking Devices – part 2 (ICND-2). CCNA certificate is a goal in your career journey. So to get this goal we arranged this exclusive CCNA training program in such a way that you get advantage from these CCNA tutorials in exam and after getting certificate in your job life.

Legacy Inter-VLAN Routing

As we learned that each VLAN is usually working on its subnet. The network switches mainly work at layer 2 of the OSI model, so they do not examine the logical addresses. Therefore for traffic between VLAN inter-VLAN routing is required. The Legacy Inter-VLAN routing is the first solution for traffic between different VLAN. The legacy Inter-VLAN routing relied on routers with multiple physical interfaces. All interface had to be connected to a separate network and configured with a separate subnet.

The legacy inter-VLAN routing connects different physical router interfaces to different physical ports on the switch. The switch ports connected to the router must be placed in access mode. Each physical interface of the router is assigned to a different VLAN and the router interface then accepts the traffic from the VLAN related with the switch interface it is connected to. Then the router sends the traffic to other VLANs connected to the other interfaces. The figure below illustrates the legacy inter-VLAN routing process.

Legacy Inter-VLAN Routing 3

We can see that host 1 is VLAN 100 and host 2 is in VLAN 200. So if host 1 wants to send data to host 2, the steps would be following.

Host – 1 on VLAN 100 is communicating with Host – 2 on VLAN 200 through the Router.
The router has separate interfaces configured for both VLANs.
Both hosts are located in different VLANs, therefore both have different broadcast domains and cannot send traffic directly without a default gateway.
Host – 1 will check its ARP cache for MAC address of the default gateway, if the MAC address is found in the ARP cache of the host – 1, and then host – 1 will send the data packet to the router. But if the ARP cache has no MAC address of the default gateway, then the host -1 will generate the ARP request for the default gateway MAC address.
After getting the MAC address of the gateway; the host -1 will send the packet to its default gateway (the router interface fa0/0) When the router receives the frame. It compares the destination IP address by referring to its routing table to know to which interface it should forward the data towards the destination host.
The router then forwards an ARP request out the interface connected to the destination VLAN; when the switch receives the message, it would flood it to its ports and in this case, host – 2 would reply with its MAC address.
Router – 1 would then use this information to send it to host – 2 as a unicast frame.

December 28, 2018

Configure Legacy Inter-VLAN Routing

Multiple Physical interfaces required on the router to configure Legacy inter-VLAN. The router can make a routing with each of its physical interfaces connected to a unique VLAN. Configure each physical interface with the unique IP address for the subnets related to the particular VLAN. After configuring the IP address on physical interfaces, each device connected to the LAN can communicate to the router using this physical interface. The router would be the gateway for each device on the VLAN. All the VLAN can communicate with each other without configuring any routing protocol on the router. The legacy Inter-VLAN routing required configuration on the switch as well as on the router. The figure below illustrates the Legacy inter-VLAN routing:

Legacy Inter-VLAN Routing – Switch Configuration

Legacy Inter-VLAN Routing 4

To configure legacy inter-VLAN routing starts by configuring the switch. As shown in the figure, the Router is connected to switch ports Fa0/2 and Fa0/7, which have been configured for VLANs 100 and 200, accordingly.

Use the following command to create VLAN and assign ports to that VLAN. Remember that issue the command in global configuration mode. The port must be in access mode.

In this example, the interfaces Fa0/1 to Fa05 have assigned to VLAN-100 and the interfaces Fa0/6 to Interfaces Fa 0/10 have assigned to VLAN 200. Using the name command have assigned the name to VLANs and finally using the wr (write) command save the work into the startup configuration file. We have used the command “do wr” because we are in global configuration mode. The command is originally not using in global configuration mode. So, if we are in “User Privileged Mode” then we would use the command “wr or write”. We can also use the copy “running-config startup-config” instead of the “wr” command. Watch the following video for Legacy Inter-VLAN routing configuration:

Legacy Inter-VLAN Routing – Router Configuration

In the Legacy Inter-VLAN routing, there are no static or dynamic protocols needed for the routing. We just required to configured the IP addresses of the router according to the subnet of the connected VLANs. We can configure the IP address of the interface using “IP address <ip address subnet mask> command in global configuration mode. Remember that the switch must be in the status of “no shutdown”.

What is Inter-VLAN Routing?

We know that VLANs segment network switch into different portions and assign a different subnet to each VLAN. Switches mainly work at layer 2 of the OSI model, such as the Catalyst 2960 Series. The 2960 series switches support over 4,000 VLANs. But, these switches have very limited IPv4 and IPv6 functionality and they do not look at the logical addresses or layer 3 packets. We also know that VLAN is a broadcast domain, so one broadcast domain cannot communicate with other broadcast domains. Therefore, computers on separate VLANs are unable to communicate without the intervention of a routing device.

In simple words, VLANs logically segment the switch into different subnet or broadcasts and without layer 3 device and some configuration communication between different hosts not possible. So, any device that supports Layer 3 routing, such as a router or a multilayer switch, can be used to do the necessary routing functionality.

The process of forwarding network traffic from one VLAN to another VLAN using routing is known as inter-VLAN routing. The hosts in the VLANs forwards the traffic to the Layer 2 switches, and then the layer 2 switch sends the traffic to layer 3 device then layer 3; devices decides the destination for the traffic according to the to information in the packet. There are three types of inter-VLAN routing we can use to send traffic between different VLANs.

Legacy Inter-VLAN Routing
Router-on-a-Stick Inter-VLAN Routing
Multilayer Switch Inter-VLAN Routing

Dynamic Routing Algorithms

Dynamic routing is a networking technique that provides optimal data routing. The network administrators and engineers configure a dynamic routing protocol on the network interfaces. The protocol running on the router learns about others routers automatically and also dynamically exchange routing information with each other. Dynamic routing protocols perform several activities, including network discovery and maintaining routing tables. Unlike static routing, dynamic routing protocol automatically selects the best route to put into the routing table as well as the network changes update automatically into the routing table accordingly. Cisco ISR routers can support a variety of dynamic IPv4 and IPv6 routing protocols including:

EIGRP and EIGRP for IPv6– Enhanced Interior Gateway Routing Protocol
OSPF– Open Shortest Path First for IPv4 and OSPFv3 for IPv6
IS-IS– Intermediate System-to-Intermediate System
RIP and RIPng(RIP for next Generation for IPv6)– Routing Information Protocol

All the dynamic routing protocols use routing algorithms. There are two types of routing algorithms:

Distance Vector Routing algorithms
Link state routing algorithms

Distance Vector Routing algorithms

A distance-vector routing protocol informs its neighbors about topology changes periodically. It is a simple protocol used in packet-switched networks that use distance to decide the best packet forwarding path. It is also known as the Bellman-Ford algorithm, where all routers maintain a Distance Vector table containing the distance between the router itself and all other possible destination and the way to the destination. A hop is a trip that a packet takes from one router to another as it traverses a network on the way to its destination. In simple words, the distance vectors protocols count the hop between the source to the destination.

Each Router configured distance vector algorithm transmits its distance as well as the vector to all neighbors. Other routers using distance vector protocol receives and saves the most recent information from each of its neighbors. The Distance Vector calculates distance using minimizing the cost to each destination. The Routing Information Protocol(RIP) uses Distance Vector Technique. Using the distance vector, each router advertises its routing table to its adjacent neighbors. Each advertisement has the following information:

Distance – The hop count for the router

Vector – The direction where the route is located

The receiving router does not generate acknowledgements, so it reduces the overhead of routing protocol traffic. The router selects the best path with the lowest cost to the possible destination for the packet. Routers add the selected route to its routing tables and propagate it to the neighbor using hop to hop until all router spread the information to the entire network.

Links State Routing Algorithms

The Link-State keeps complete record and roadmap of the router running link-state routing protocol in the network. Each router running link-state protocol share information about the router to its directly connected interfaces and the state of all interfaces configuring with the link-state protocol. Link-state routing constantly attempts to keep full networks topology by updating itself incrementally when a change happens in the network.

The router sends routing information to all the routers in the network as multicast messages. After starting up, the router sends its first link-state information to its neighbors. So, this reduces the network load by only sending updates to its link information. The Open Shortest Path First (OSPF) is the most important routing protocol type of Link-State routing protocol. The important terms of using link-state are following.

Link-state advertisements (LSAs) –It is an update on their link status, so router send LSA when a link has changed from the current state. It is a small packet of routing information flooded out to all routers in their area or zone.

Topological database – A topological database is a set of information gathered from the exchange of several LSAs between routers, they describe the network topology in great detail. All routers in the network store the received LSA packets in the link-state database (LSDB).

SPF algorithm – The shortest path first (SPF) algorithm also known as the Dijkstra’s algorithm, Performed the calculation of the database and builds the SPF tree. All routers in an area run this algorithm in parallel, storing the results in their topological databases.

Routing tables – A list of the known destination and interfaces.

Static Route Configuration

We can configure either static or dynamic routes after the configuration of directly connected interfaces. The static routes are manually configured and it provides a clear path between two networking devices. It must be manually reconfigured if there are changes in the network topology. This is the main disadvantage of static routes. It is more secure and more efficient than dynamic routes. It uses less bandwidth than dynamic routing protocols because of no CPU cycles required to calculate and communicate routes. There are two main types of static routes.

• A route between two specific network
• Static Default Route

Static Route between two specific network

We can configure a static route to reach a specific remote network. The command syntax for static IPv4 routes is following.
Router(config)# ip route network address network mask {next-hop-ip | exit-interface}

The configuration command must be issued in global configuration mode. We can identify the static routes with the code ‘S’ in the routing table. The figure below shows the configuration of static IPv4 route on Router2 to the Serial 0/3/0 interface.

Static Route Configuration 6

The static route on Router2 is configured to reach to network 172.16.17.0/24 on Router 3. It is configured using the exit interface towards Router3. We can also configure the router using the IP address of the next hop. In this example, the next hop is the serial 0/3/0 interface of Router 0.

Both, the route with the next-hop address and exit interface are acceptable. There is no difference between both, only they are looking different in the routing table. We can also configure the static IPv6 route between two specific networks. The command should be issued in global configuration mode. The command syntax for static IPv6 route is following.
Router(config)#ipv6 route ipv6-prefix/prefix-length{ipv6-address|interface-type interface-number}

Static Default Route

We can also examine another route “S” with an asterisk pointing to gigabitEthernet 0/1. Asterisk illustrates that it is the default route. It is also known as the gateway of last resort because it is not set for any specific network. If the packet destination is unknown for the router, the router search routing table for the default route. A default route role is similar to a default gateway on a host. It specifies the path for the packet when the router has no information about the destination of the packet. To configure an IPv4 default route, use the following command in global configuration mode.

Router(config)# ip route 0.0.0.0 0.0.0.0 {exit-interface | next-hop-ip}

Notice that the next-hop address for the default route is the exit interface of the router towards Router 0. We can also configure the default static route with the next-hop IP address similar to static route configuration. In the same way, we can configure the default static IPv6 route, using the following command in global configuration mode.

Router(config)#ipv6 route ::/0 {ipv6-address |interface-type interface-number}

Download the topology for practice

Routing Table Explained

A routing table has information usually viewed in table format; decide where to send data packets. All IP-enabled devices including routers use routing tables to direct a packet to the destination.

The router gets route information from the routing table and selects the best path for the destination. Each packet has information about its source and destination. The router examines the packet and matches it to the routing table entry providing the best match for its destination and sends the packet to the next hop on its route across the network.

We can configure routes manually or dynamically. The static routes do not change unless a network administrator manually changes them but the dynamic routes automatically update and change by routing protocols. The routing protocols exchange information about the network topology and network changes and update the routing table. Dynamic routing protocols also allow devices to listen to the network and react to occurrences like device failures and network congestion. The routing table is data file store route information about directly connected and remote networks.

Directly connected routes– When we configure and activate the interface, the router adds directly connected route against the interface.
Remote routes– This is the route to remote networks to other routers. We can configure these routes statically or dynamically.

Routing Table Sources

We can check the routing information on a Cisco router using the show ip route command. The router also provides additional route information, including the source of the route with this command. Following are the different sources of the routing entries.

Local Route interfaces– The router adds the route when we configure and activate the router interface. This entry is available in all IOS for IPv6 and for IPv4 the option is available only in IOS 15 or newer versions.
Directly connected interfaces– The directly connected routes added to the routing table when we activate and configured the interface.
Static routes– The static route is added to the routing table when a route is manually configured and the exit interface is active.
Dynamic routing protocol– The routing protocols that dynamically learn the network networks information and add the information to the routing table, such as RIP, EIGRP, and OSPF.

We can find the routing entry sources with a code. The code tells us the source of the route information. The figure below illustrates the codes of the route sources including the entries in a single route:

Routing Table Explained 10

Some common codes are:

C–This code for is for the directly connected network.
L– This code is for Local Router/Switch Interface route.
S– We can find a static route with this code.
D– This is the identification code for dynamically learned network from another router using EIGRP.
O– This code Identify a dynamically learned network from another router using the OSPF.
R– This code Identify a dynamically learned network from another router using the RIP.
S*– This is the default route.

Remote Network Routing Entries

To understand the content of an IPv4 and IPv6 routing table is most important. We have marked the route to destination network 172.16.17.0 in the above figure. The marked entry for 172.16.17.0 identifies the following information:

Route source– This entry identifies how the router adds this route. In this example, the entry is “D” which means that the router learns this route from dynamic routing protocol EIGRP.
Destination network– This is the entry for remote network Identification. In this example, the remote network is 172.16.17.0.
Administrative distance– This is the trustworthiness of the route source. Lower values indicate more trustworthiness route to the destination network.
Metric– The metric the cost to each available route so that the router select the most cost-effective path. The Lower values indicate preferred routes to the destination.
Next-hop– This is the IPv4 address of the next connected router to send the packet.
Route timestamp– This entry shows the timing since the route added.
Outgoing interface–This entry identifies the exit interface of the router to send a packet toward the destination.

Directly Connected Interfaces

A newly Installed router, without any configured and active interface, has an empty routing table, as shown in the figure below.

Routing Table Explained 11

Before the interface state is up/up and adds it to the routing table, the interface must be assigned a valid IPv4 or IPv6 address. The interface must be in no shutdown state. It should also be in a position to receive the carrier signals from another device e.g. router, switch, host etc. When the interface is up, the network of that interface is added automatically to the routing table as a directly connected network. For example, when we configured the interfaces of the Router5 with IPv4 addresses and issue the no shutdown command and it receives the carrier signals from the router and hosts. It updates the routing table from an empty routing table as shown in the figure below.

Directly Connected routes (C) and Local Routes (L) Entries

The properly configured connected interface creates two routing table entries. The figure below displays the IPv4 routing table entries on Router5 for the directly connected network 172.16.19.0. The directly connected router interfaces routing entries contain the following information:

Routing Table Explained 12

Route source– This entry identifies the route source. Directly connected interfaces have two route source codes. “C” and “L”. The “C” is for directly connected network and “L” is for IPv4 address assigned to the router interface.
Destination network– The address of the remote network.
Outgoing interface–This is the router outgoing interface for the destination network.

Another route it is showing is the Local (L) route. The difference between Local and Directly connected routes is that directly connected route is a route to network that is directly attached to the interface and Local is the route that belongs to the router/switch itself in the above example you can see that in the directly connected route the destination is 172.16.19.0/24 address but the destination in Local route is 172.16.19.1 which is configured on the same Router (Router5)

Administrative Distance

Administrative distance is the router feature that selects the best path when there are more than one different routes to the same destination with different routing protocols and static routes. If the routing table has more than one route source for the same destination network. For example, if both Enhance Interior Routing Gateway Protocol (EIGRP); and Routing Information Protocol (RIP) are configured on a router for the same destination network.

So, both routing protocol may decide a different path to the destination based on that routing protocol’s metrics because RIP selects a path based on hop count, and EIGRP selects a path based on its composite metric. So, the administrator distance is the value which tells the router which path to use first.

Administrative Distance (AD) counts the reliability and trustworthiness of the route. It is a numeric value which can range from 0 to 255. A smaller AD value is more reliable and trustworthiness, therefore the best AD value is 0 and the worst is 255. The static route AD is 1, whereas the AD of EIGRP is 110; So the static route is more reliable and trustworthiness.

When there are both static and EIGRP routes to the same destination; the router chooses the static route because it is the route with lowest AD value. if there EIGRP and OSPF are configured to the same destination, the router will choose EIGRP; because the AD of EIGRP is 90 and the AD of OSPF is 110; So the router will choose the route with the lowest value which is EIGRP.

Default Administrative Distance Values

The figure below illustrates the default values of the router administrative distance. We can change and modify the administrative distance of a routing protocol through the distance command in the routing sub configuration mode. However, a modification in the AD value can show the way to routing loops and black holes. So, use caution if you change the administrative distance.

Administrative Distance

Network Load Balancing

Routers preferably select routes with lower administrative distance when multiple paths available to the destination. When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using all paths equally called equal-cost load balancing. So, for balance the load multiple routes needed through a single routing protocol. The protocol should have equal administrative distance to a destination when forwarding packets.

The router calculates multiple routes to a destination with metrics, and the shortest path is installed in the routing table. If the destination has more than one path with the same administrative distance and a metric value; all least-metric and same routes are installed in the routing table. We can configure EIGRP to install multiple paths even if the metric is not equal. This is the only protocol which supports unequal cost load balancing. The Cisco router uses two types for balancing the network load, per-destination basis, and per-packet basis. Balancing the load also depending on the switching mechanism configured on the router interface.

Per-destination load balancing

In this type, the router distributes the packets based on the destination address and distribute all packets for a specific destination over the first path, and uses the second path for the second destination on that same network. For most Cisco routers, per-destination load-balancing is the default technique.

Per-Packet load balancing

In this type, the router sends one packet for the same destination over the first path, and the second packet for the same destination over the second path, and so on. Per-Packet guarantees equal load across all links. The forwarding process determines the outgoing interface for each packet from the routing table and selects the least used interface. This also ensures equal use of all links. The video below demonstrates the process of per-packet type.

Routing Decisions

The path choice for the packet is the basic function of the router. The router ever selects the best path for packet sending. For selecting the best path router searches its routing table information for a network address that matches the packet destination IP address. The router selects the best route with the following three results.

Directly connected network

The directly connected routes are always the best path to any subnet. If the destination IP address of the packet belongs to directly connected device that is on the same network to one of the interfaces of the router and connected to this interface, and the packet directly forwarded to the destination device. The directly connected network is the host address on the same network as the interface of the router.

The administrative distance of the directly connected network is 0. When an IP address is configured to an interface of the router, the router will automatically create a directly connected route in its routing table. The figure below illustrates the directly connected routes and remote routes. If we see the Router1, where host1 and Fa0/0 of Router2 are a directly connected network for Router1 and remaining all network is the remote network for Router1.

Remote network

If the packet’s destination IP address is not on the same network with an interface of the router; so, this is a remote network. If the destination address is on the remote network then the router forward packet to another router connected to the current router. The packet for remote networks reached to the destination through the router to router. For complete process consult my earlier article Packet sending over a routed network.

No route determined

If the destination IP address of the IP packet not belongs to the directly connected or remote network; then the router determines for the Gateway of Last Resort. A Gateway of Last Resort is a route using the router when no other known route exists to send the IP packet. The Known routes are present in the routing table. Therefore, any route not known by the routing table forwarded to the default route. If there is a default route, the router forwards the IP packet to the default route (Gateway of Last Resort). If the router does not have a default route, then the router discards the packet. The default route is 0.0.0.0.

Best Path

The router determines the best path for sending a packet by assessment of multiple paths to the same destination network and selecting the best and shortest path. In the case of multiple paths to the same destination network, each path uses a different exit interface on the router. The router selects the best path using a protocol based value or metric to calculate the distance to the destination network. The lowest metric value is the best Path to the destination network. Dynamic routing protocols use their own rules and metrics to build and update routing tables.

The algorithm of the protocol generates a metric, for each path through the network. Metrics can be based on a single characteristic or several characteristics of a path. Some routing protocols select a route on multiple metrics, combining them into a single metric. Some examples of routing protocol are Routing Information Protocol (RIP), Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP). The RIP is using hop count and OSPF is using Cisco’s cast based metric on available bandwidth. The Enhanced Interior Gateway Routing Protocol (EIGRP) is an example of multiple metrics type which uses bandwidth, delay, load, and reliability. The video demonstrates the difference between hop count and bandwidth metric.

Packet Sending Through Routed Network

For packet sending over the routed network, the IP address of destination network required. If the destination address belongs to the same network, and then the source does not use the default gateway. The host applies AND operation for determining the network address. I have already discussed the AND operation in my earlier article ANDing and Determining Network Address.

If the destination belongs to the same network, then the source device uses its ARP cache for the MAC address of the destination. If the MAC address not listed in the ARP cache then the source generates an ARP request to acquire the MAC address to complete the packet sending to the destination.

When a packet completed, router send it to the destination. If the destination network address belongs to a different network, then source consults its ARP cache for gateway MAC address, if the MAC address listed in the ARP cache the source forwards the packet to its default gateway. If the address is not listed in ARP cache; then the source first generates ARP request to get the MAC address of the gateway and then send the packet to the gateway for further processing. See the video for the complete operation of a packet sending.

In the video, you can see the Laptop 0 packet sending to the webserver. The Laptop 0 first determines that the destination IPv4 address is not on the same network. So, Laptop 0 check for default gateway IP address and MAC address. The MAC address is not listed in ARP cache of Laptop 0; So Laptop 0 generate ARP request for acquiring the MAC address of the default gateway. When Laptop 0 received MAC address then the Laptop 0 encapsulates the layer3 Packet into layer2 frames and sends it to Layer1. The Layer 1 send Layer 2 frame in shape of Ethernet frame (0 and 1).

The IPv6 also uses a similar procedure for IPv6 packets. But in place of the ARP process, IPv6 address resolution uses ICMPv6 Neighbor Solicitation and Neighbor Advertisement messages. IPv6-to-MAC address mapping is kept in a table similar to the ARP cache, called the neighbor cache.

Packet Forwarding and Routing

When Router1 receives the Ethernet frames from Laptop 0. The Router1 examines the destination MAC address, which matches the MAC address of the Router receiving interface Gig0/1. So, Router1 copies the frame into its buffer. The router identifies the frame type field as 0x800, which indicates that the frame has an IPv4 packet in the data part. The router de-encapsulates the Ethernet frame to check the destination IPv4 address.

When examining that the destination address does not match any of the directly connected interfaces of the Router1. The Router1 then searches the routing table for matching the destination address network. When the address matched to any of network address in the routing table, the router selects an outgoing interface for the packet.

When the router selects an outgoing interface, For example, in the video; the destination address is matching to route 192.168.10.0/24. The exit interface for 192.168.10.0/24 is Gig0/0, so, the router encapsulates the packet in Ethernet layer 2 frames. The frame included source MAC address and destination MAC address. The router1 exit interface Gig0/0 is a source and router-2 interface Fa0/0 is the destination for layer-2 frames this time.

When Router2 receives the Ethernet frame from Router1. It examines the MAC address and de-encapsulates the layer 2 frames to check the layer3 information. Then search the proper route in the routing table for address 192.168.10.2. This time the exit interface is serial port s0/3/0. The router encapsulates the layer2 frame this time for a serial port which protocol is PPP. The PPP frames not use MAC addresses. Each router is doing the same procedure with the packet sending until the original destination mentioned in layer 3 information, received the packet.

Reach the Destination

When the packet arrives at router4 directly connected network 192.168.10.0/24. The Router4 copies the data link Ethernet frame into its buffer and de-encapsulates the data link Ethernet frame. The Router3 searches the routing table for the destination IPv4 address of the packet. The routing table has a route to a directly connected network on Router4. So, the Router4 sent the packet directly to the destination.

The exit interface is directly connected to the Ethernet network; So, the router must resolve the destination IPv4 address of the packet with a destination MAC address. The Router4 searches for the destination IPv4 address of the packet in its ARP cache. If the entry is not in the ARP cache; the Router4 sends an ARP request out of its Fast Ethernet 0/0 interface. The Web Server sends back an ARP reply with its MAC address. Router4 then updates its ARP cache with an entry for 192.168.10.2 and the MAC address that is returned in the ARP reply.

The IPv4 packet is encapsulated into a new Ethernet data link frame and sent out to the Fast Ethernet 0/0 interface. When Web Server receives the frame, it examines the destination MAC address in the frame; which matches the MAC address of the receiving interface; its Ethernet network interface card (NIC). Then the Web Server copies the rest of the frame into its buffer and identifies the Ethernet Type field as 0x800; which means that the Ethernet frame has an IPv4 packet in the data portion. It de-encapsulates the Ethernet frame and passes the IPv4 packet to the IPv4 process of its operating system and send a reply according to the packet.

Router Switching Function

The router is a device which received a packet from the source on any interface and forwards to their destination on another interface. This is done by router switching function. The router switching function encapsulates packets in the data link frame type for the outgoing data link. The router routing function selects the best path for packet destination and the router switching function encapsulate the packet into the data link frame of the outgoing interface. The router switching function performs the following function with receiving a packet from one network and destined for another network.

1. Router received Layer 2 encapsulated frame and then de-encapsulates the Layer 2 frame header and trailer.

2. After De-encapsulation the router read Layer 3 information and the destination IP address of the IP packet to select the best path in the routing table.

3. When selecting a path for the packet destination, router encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface.

Figure 1 to Figure 6 illustrates the packet switching over the routed network. As shown in figure 1, the laptop generates an ICMP message for the server in the topology. The packet is containing layer 3 information of source and destination layer 3 addresses. The source layer 3 address is the address of the laptop and the destination layer 3 address is the IP address of the server. As a packet travels from the source to the destination, the Layer 3 IP addresses do not change because the Layer 3 PDU does not change.

But, the Layer 2 data link addresses change at every hop as the packet is de-encapsulated and re-encapsulated in a new Layer 2 frame by each router. In figure 1 the Layer 3 packet encapsulated for the wireless access point and forwarded to Layer 2 (wireless card of the laptop) and then to Layer 1 for transmitting on port 1. The wireless port is a virtual port, not the physical port.

Encapsulation into a different type of Layer 2 frame than the one type is common for receiving packets. For example, a home router received a frame from the wireless port and then sends it to router 3 over an Ethernet interface. So, the encapsulation for wireless and Ethernet is different. Also, encapsulation for Fast Ethernet, Giga Ethernet, and serial interfaces are different.

Figure 2 illustrates that the home router receives the layer 2 packets in the shape of bits from layer 1 and then de-encapsulate; the packet to read layer 3 information. When reading the source and destination the router select the proper outgoing interface and again encapsulate the packet, and send it to Layer 2 and then to Layer 1.

Router Switching Function 18

Notice in the source and destination MAC addresses and IP addresses; at the home router and then on all the router. At each router, the source and destination MAC addresses are changing; but the IP address at each router is not changing.

Router Switching Function 19

Also, notice figure 3 and Figure 4 that the ports between Router3; and Router2 have no MAC addresses in the frame. Because this is a serial link and MAC addresses are only required on multi-access networks, such as Ethernet. A serial link is a point-to-point connection and uses a different Layer 2 frame that not required the MAC address.

Router Switching Function 20

For example, when Ethernet frames received on Router3 from the Fa0/0 interface; destined for Server0, it is de-encapsulated and then re-encapsulated for the serial interface. When Router2 receives the frame, it is de-encapsulated again; and then re-encapsulated into an Ethernet frame with a destination MAC address.

Router Switching Function 21

Router Switching Function 22

The table below better summarizes the process of sending a packet from Laptop 1 to server 0. You can see the packet source IP and MAC address and Destination IP and MAC address. Notice that the source and destination IP address is not changing until the packet is reached to the final destination. But the Source and destination MAC addresses are changing on each hop. At stage 11^th where server response to Laptop 1 the source and destination; addresses accordingly changed because now server 0 is sending a reply message to Laptop 1. So, this time the source is server 0.

Serial	Hop	Source IP Address	Destination IP Address	Source MAC Address	Destination MAC Address
1	Laptop1	192.168.0.100	10.200.100.2	-	-
2	Wireless Router	172.16.16.2	10.200.100.2	0001.C9B3.A501	0060.5C05.3401
3	Router0	172.16.16.2	10.200.100.2	0060.5C05.3401	00E0.F93C.A302
4	Router1	172.16.16.2	10.200.100.2	-	-
5	Router2	172.16.16.2	10.200.100.2	000A.4152.0701	0001.6374.4A7A
6	Server0	10.200.100.2	172.16.16.2	0001.6374.4A7A	000A.4152.0701
7	Router2	10.200.100.2	172.16.16.2	-	-
8	Router1	10.200.100.2	172.16.16.2	00E0.F93C.A302	0060.5C05.3402
9	Router0	10.200.100.2	172.16.16.2	0060.5C05.3401	0001.C9B3.A501
10	Wireless Router	10.200.100.2	192.168.0.100	-	-