Router Functions and Network Characteristics

Router Functions and Network Characteristics

• Networks are essential for web applications, IP telephony, video conferencing, interactive gaming, and e-commerce.
• Key network characteristics include topology, speed, cost, security, availability, scalability, and reliability.
• Physical topology involves cables, network devices, and end systems and their physical arrangement.
• Logical topology shows data paths and how network devices appear to users.
• Speed refers to the network's data rate measured in bits per second (b/s).
• Cost constitutes expenses for components, installation, and maintenance.
• Security measures how well the network and its data transmissions are protected.
• Availability indicates how likely the network is to be operational when needed.
• Scalability signifies how easily the network can handle more users and data.
• Reliability is the dependability of network components, often measured by MTBF (mean time between failures).

Routing and Router Functions

• Routers connect networks, deciding on the optimal path for traffic before forwarding it.
• A router is responsible for network traffic and uses a routing table to find the most efficient path.
• A router is a specialized computer with components such as a CPU, OS (like Cisco IOS), memory, and storage.
• Routers contain specialized ports and network interface cards to connect various networks.
• RAM provides temporary storage for the running IOS, configuration file, routing, ARP tables, and packet buffer
• ROM permanently stores bootup instructions, basic diagnostic software, and a limited IOS version for router startup.
• NVRAM permanently stores the startup configuration file.
• Flash memory provides permanent storage for the IOS and other system-related files.
• Routers forward packets between networks, using interfaces to connect to different IP networks.
• When a packet reaches a router, it directs it to the destination or sends it to another router.

Determining the Best Path

• Key router functions are identifying the best path for packets and sending the packets accordingly.
• The router consults its IP routing table to determine the path and interface for forwarding packets.
• A router can handle different data link layer frame encapsulations.
• Routers perform de-encapsulation to check the routing table and encapsulate packets for the outgoing interface.
• Key router functions are identifying the best path for packets and sending the packets accordingly.
• The best path is the "longest match" in the routing table.
• Routing tables consist of network address prefixes and prefix lengths.
• Optimal routes have the greatest number of far-left matching bits with the destination IP, which is known as the longest match.

IPv4 and IPv6 Longest March Examples

• An IPv4 packet with destination 172.16.0.10 is routed using the longest match from entries like 172.16.0.0/12, /18, and /26.
• The subnet mast determines how many number of far left matching bits are needed by a route.
• An IPv6 packet with destination 2001:db8:c000::99 selects the longest prefix match, such as /40 or /48.
• The /64 prefix can not be used with a /48 address as this requires 64 matching bits.

Building and Using Router Tables

• Directly connected networks are automatically added to the routing table when an interface is configured with an IP address and is active.
• Routers learn about remote networks through manual static route configuration or dynamic routing protocols.
• Default routes are used when the routing table lacks a specific route and has a prefix length of /0.

Packet Forwarding

• Home office devices connect using wireless from Laptops, Ethernet cable to printer, Ethernet to the ISP modem, and the cable modem then connects to the ISP network.
• Branch sites use Ethernet cables for PCs/VoIP phones, wireless for smartphones/laptops, Ethernet for WAPs, and an Ethernet interface to the edge router.
• Central sites may have L2 wth Ethernet cables for PCs and VoIP phones, redundant L3 switches using Ethernet fiber, L3 Ethernet connections to edge routers, and a corporate website server.
• A key role of the router is to determine the ideal path for packets and forward them appropriately.
• The router determines forwarding using 3 path determinations , including directly connected networks, remote networks, and no route.
• If the destination isn't on a connected or known in the routing table then it is sent to a Last Gateway Resort.
• Routers also encapsulate packets into an appropriate data link frame type for the outgoing interface.

Packet Forwarding Decision Process

• When a data link frame with an encapsulated IP packet reaches the interface, the destination IP address is examined, and the IP routing table is consulted.
• The routing table checks for destination IP address and finds the longest matching prefix.
• It is encapsulated with the exit interface, and packets are forwarded it out the destination of device or next-hop router.
• If there is no matching route, the packet is dropped

Packet Handling Scenarios

• Directly connected routes mean the packet is typically forwarded to its destination on an Ethernet LAN.
• The router needs to determine destination MAC, and can take a variety of forms for IPv4 and IPv6 packets.
• Next-hop router addresses a packet that are not on a directly connected network needs to be sent.
• The router looks up the next-hop IP in its ARP table or uses neighbor discovery.
• Packets are dropped when a route or default route match is not found.
• A packet forwarding function is to encapsulate packets in the appropriate data link frame type for the outgoing interface, such as PPP or HDLC.

Packet Routing in Practice

• PC1 sends a packet to PC2, by determining the IPv4 address is on the same network or uses ARP to find MAC.
• If it is on a different network, PC1 forwards the packet to its default gateway.
• Router R1, receives the Ethernet frame from PC1, examines destination MAC to which interface and saves to buffer.
• It is de-encapsulated before the routing table is searched for the destination IP.
• Packets will then exit out of the FastEthernet via the router, after determining the next-hop IP with a destination MAC via ARP.
• The route then determines it is going to another R2, before it searches the buffer.
• It will require that it searches the destination table with IPv4 packet be encapsulated in with destination MAC, or found with destination MAC with ARP.

Process Switching

• R2 looks at the frames that are sent to be by MAC which contains a lot of the data contained.
• Routing tables is searched and a corresponding route is assigned. It will outputted and encapsulated.

Packet Forwarding Mechanisms

• Routers supports process switching, fast switching, and Cisco Express Forwarding (CEF)..
• Process switching forwards an arriving packet to the control plane, matches the destination address to a routing table entry.
• The exit interface is determined and forwards the packet of every packet this way.
• Fast switching stores the route information in a fast-switching cache for reuse.
• The next-hop info is used without CPU intervention, and the CPU intervention cache reuses the route information.
• CEF uses a Forwarding Information Base (FIB) and an adjacency table, which are updated when network topology changes.
• The FIB and adjacency tables have all the information that a router needs to decide a routing path.

Basic Router Review and Configuration

• The chapter covers physical router setting examples
• An IPv4 configuration includes IP, subnet mask, interface, and the "no shutdown" command.
• IPv6 configuration includes the use of "ipv6 address" and "no shutdown" commands.
• The "clock rate" command is set in lab environment.
• IPv6s typically contain multiple IPv6s.

IPv4 Loopback Configuration

• IPv4 loopback is used for testing and management purposes on a router without physical ports.
• The loopback address is used by routing protocols for identification to maintain connectivity and avoid outages.
• The verification includes show ip interface brief, show running-config command, interface + code/ id and show ip interfaces.
• Replace IP with ipv6 for IPV6 version.

Filter Output Commands

• Use a pipe, or “|”, character and show command.
• Commands will be “section”, including all lines that match filtering, exclude that matches output, or begin an output

Other Router Commands

• Commands generate multiple screens of output default for 24 lines
• Spacing is used to show where to display the next file or line.
• Use terminal length to lengthen or change the display
• A history code will indicate used codes can be recalled.
• Use Ctrl + P or the up arrows command.

IP Routing Table Code Key

• The IP routing consists of code and keys
• It is common codes indicated by L, C, Sand many other codes
• L shows the inter face assigned to the router, C is a direct network, S is static, and 0 indicates dynamic learnt from another router
• It also contains a candidate for last resort, and should only appear in the routing table.

IP Routing Table Principles

• Each router is unique, as it has to make decisions on its own information of the routing table.
• There is often a missmatch as there needs to be proper communication amongst them.
• If the message does not reach the device, then information will not leave that area and may need routing to reoccur.
• Entries consist of how the routes are learned, destination and address (prefix and suffix), administration and distance.
• Routing is also used with metrics, time stamps, and for outgiong packets that reach the final destination.

Sources and Static Routes

• Directly Connected networks are assigned and added to router when an interface is active with a IP.
• This consists of C status with that has L codes. Static routes have limited use, but can maintain tables easily.
• It also relies on a single default route from to the next stream of network.