CCNA
Directly connected network,
how do routers forward packets, remote network, router packet forwarding decision,
routing decisions in production and operation management August 23, 2019 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. 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. 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. 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. 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. CCNA Introduction to Packet Sending Through Routed NetworkAsad Ijaz data packet sending, how do routers forward packets, how packet travels in network, how to understand networking, life of a packet in network, packet data sending, packet flow from source to destination, path determination, routing ip packets August 23, 2019 For packet sending over the routed network, the IP address of the destination network is 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 is 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 is completed, the router send it to the destination. If the destination network address belongs to a different network, then the source consults its ARP cache for the gateway MAC address, if the MAC address is listed in the ARP cache the source forwards the packet to its default gateway. If the address is not listed in the ARP cache; then the source first generates an 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 checks for default gateway IP address and MAC address. The MAC address is not listed in the ARP cache of Laptop 0; So Laptop 0 generate an ARP request for acquiring the MAC address of the default gateway. When Laptop 0 received the MAC address then Laptop 0 encapsulates the layer3 Packet into layer2 frames and sends it to Layer1. Layer 1 sends a Layer 2 frame in the shape of an 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 neighbour cache. Packet Forwarding and RoutingWhen Router1 receives the Ethernet frames from Laptop 0. 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. Router1 then searches the routing table for matching the destination address network. When the address is matched to any of the network addresses 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 do 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 DestinationWhen 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, 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; 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 the 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.  CCNA Router Packet Forwarding DecisionAsad Ijaz default router ip, forwarding techniques in computer networks, how do routers determine the best path, how do routers forward packets, Packet Forwarding and Routing, packet forwarding in router, packet forwarding methods, router and router table, routing decision and routing models, routing decision process June 9, 2019 When a host sends a packet to another host in the network or outside the network. It will use the host routing table where to send the packets. If the destination host belongs to the same network, the host directly sends the data to the specific host without involving the default gateway. But while the destination host not on the same network; the data is forwarded to the default gateway. When a data arrives at the default gateway, which is generally a router. So the routers consult its routing table to decide where to send this packet. The routing table of a router has information about the destination, in the form of a different route in the routing table: Directly connected routesThese routes come from the router active interfaces. When Interface has an IP address configured and both status code in up and up status. Subnet directly connected to a router’s interface added to the router’s routing table. A router will be able to route all packets destined for all hosts in subnet directly connected to its active interfaces. Remote routesThese routes come from remote networks connected to other routers. Routes to these networks can be manually or dynamically configured on the local router by the network administrator. The figure below identifies the directly connected networks and remote networks of Router-1. The network with a light blue rectangle is the directly connected networks for Router-1 and network in the light red rectangles are a remote network for Router-1. Default routeRouters also use a default route as a gateway of last resort option if there is no other route available to the desired network in the routing table of the router. The IP address 0.0.0.0 reserved for a default route. If a router has no information about the incoming packet; it routes the packet to the interface where the default route has configured. The Host Default Gateway and Routing Table We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. By clicking “Accept”, you consent to the use of ALL the cookies. How does a router forward a packet?When a router receives a packet, the router checks its routing table to determine if the destination address is for a system on one of it's attached networks or if the message must be forwarded through another router. It then sends the message to the next system in the path to the destination.
Which route would be used to forward a packet?Packet Forwarding in Router:
Routers are used on the network for forwarding a packet from the local network to the remote network. So, the process of routing involves the packet forwarding from an entry interface out to an exit interface.
Which protocol is used by routers to forward messages?Internet Control Message Protocol (ICMP) is used to send control messages to network devices and hosts. Routers and other network devices monitor the operation of the network. When an error occurs, these devices can send a message using ICMP.
How does a router forward data packets from one network to another?When a data packet comes in on one of the lines, the router reads the network address information in the packet header to determine the ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey.
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