Chapter 3

• Switching


  Connecting devices to a network

  
• Router


  • Connects one network to another network
  • Determines the best route to the destination before forwarding traffic to the next router along the path
  • Responsible for routing traffic between networks
  • Uses a routing table to determine the most efficient path to reach the destination

  
• Router


  • Specialized computer with CPU, operating system, memory, and storage
  • Has specialized ports and network interface cards to interconnect devices to other networks

  
• Packet forwarding


  1. Determine the best path to send packets

  2. Forward packets toward their destination

  
• Longest match


  The best path in the routing table, determined by the route with the greatest number of far-left matching bits with the destination IP address

  
• Ways routers learn about remote networks

  • Static routes
  • Dynamic routing protocols

  
• Default route


  Specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address

  
• Home office device connections

  • Laptops/tablets connect wirelessly to home router
  • Network printer connects via Ethernet to home router
  • Home router connects to ISP cable modem via Ethernet

  
• Branch site device connections

  • Desktops, phones, servers connect to Layer 2 switches via Ethernet
  • Laptops, smartphones connect wirelessly to wireless access points
  • Wireless access points connect to switches via Ethernet
  • Layer 2 switches connect to edge router via Ethernet
  • Edge router connects to WAN service provider

  
• Central site device connections

  • Desktops, phones connect to Layer 2 switches via Ethernet
  • Layer 2 switches connect redundantly to Layer 3 switches via fiber-optic Ethernet
  • Layer 3 switches connect to edge router via Ethernet
  • Corporate website server connects to edge router
  • Edge router connects to WAN SP and ISP for backup

  
• Routing decisions


  1. Directly connected network - forward packet out that interface

  2. Remote network - use routing table to determine best path

  3. Unknown network - use default route

  
• Central site device connections

  1. Desktop PCs and VoIP phones connect to Layer 2 switches using Ethernet cables

  2. Layer 2 switches connect redundantly to multilayer Layer 3 switches using Ethernet fiber-optic cables

  3. Layer 3 multilayer switches connect to an Ethernet interface on the edge router using Ethernet cables

  4. The corporate website server connects to the edge router interface

  5. The edge router connects to a WAN SP and also to an ISP for backup purposes

  
• Routing Decisions


  The primary function of a router is to determine the best path to send packets

  
• Routing table search results

  1. Directly connected network - Packet is forwarded out of that interface

  2. Remote network - Packet is forwarded to another router

  3. No route determined - Packet is sent to Gateway of Last Resort

  
• Router switching function


  • Accepts a packet on one interface and forwards it out of another interface. This is not to be confused with the function of a Layer 2 switch.
  • Encapsulates the packets in the appropriate data link frame type for the outgoing interface

  
• Packet forwarding decision process


  1. The data link frame with an encapsulated IP packet arrives on the ingress interface

  2. The router examines the destination IP address in the packet header and consults its IP routing table

  3. The router finds the longest matching prefix in the routing table

  4. The router encapsulates the packet in a data link frame and forwards it out the egress interface

  5. If there is no matching route entry the packet is dropped

  
• Forwarding packet to a device on a directly connected network


  The router needs to determine the destination MAC address associated with the destination IP address of the packet

  
• Forwarding packet to a next-hop router

  The router will search for the IP address of the next-hop router in its ARP table or neighbor cache, instead of the destination IP address of the packet

  
• Dropping the packet - No match in routing table

  If there is no match between the destination IP address and a prefix in the routing table, and if there is no default route, the packet will be dropped

  
• End-to-end packet forwarding


  The primary responsibility is to encapsulate packets in the appropriate data link frame type for the outgoing interface

  
• Sending a packet from PC1 to PC2

  1. PC1 must determine if the destination IPv4 address is on the same network

  2. If it is on a different network, PC1 forwards the packet to its default gateway

  3. PC1 checks its ARP table for the IPv4 address of the default gateway and its corresponding MAC address

  
• Router R1 forwarding the packet

  1. R1 examines the destination MAC address which matches the MAC address of the receiving interface and copies the frame into its buffer

  2. R1 identifies the Ethernet Type field as 0x800 which indicates that the Ethernet frame contains an IPv4 packet

  3. R1 de-encapsulates the Ethernet frame

  4. R1 searches the routing table for a corresponding route

  5. R1 encapsulates the IPv4 packet in a new Ethernet frame with the destination MAC address of the IPv4 address of the next-hop router

  
• Router R2 forwarding the packet

  1. R2 examines the destination MAC address and copies the frame into its buffer

  2. R2 determines that the frame contains an IPv4 packet

  3. R2 de-encapsulates the Ethernet frame

  4. R2 searches the routing table to find a corresponding route for the destination IPv4 address

  5. R2 encapsulates the IPv4 packet into a new data link frame used by the exit interface and sends it out

  
• Router R3 forwarding the packet

  1. R3 copies the data link PPP frame into its buffer

  2. R3 de-encapsulates the data link PPP frame

  3. R3 searches the routing table for the destination IPv4 address of the packet

  4. R3 resolves the destination IPv4 address of the packet with a destination MAC address

  
• Packet forwarding mechanisms


  • Process switching
  • Fast switching
  • Cisco Express Forwarding (CEF)

  
• Process switching


  An older packet forwarding mechanism where the CPU matches the destination address with an entry in its routing table, and then determines the exit interface and forwards the packet for every packet in a stream

  
• Fast switching


  Uses a fast-switching cache to store next-hop information. If a match is found in the cache, the next-hop information is re-used without CPU intervention
• Cisco Express Forwarding (CEF)

  The most recent, fastest and default Cisco IOS packet-forwarding mechanism
• Fast Switching


  An older packet forwarding mechanism which was the successor to process switching. It uses a fast-switching cache to store next-hop information.

  
• Fast Switching


  1. Packet arrives on interface

  2. Forwarded to control plane

  3. CPU searches for match in fast-switching cache

  4. If not found, process-switched and forwarded to exit interface

  5. Flow information stored in fast-switching cache

  
• Cisco Express Forwarding (CEF)

  The most recent, fastest and default Cisco IOS packet-forwarding mechanism. It builds a Forwarding Information Base (FIB) and an adjacency table.
• The FIB and adjacency tables contain all the information that a router would have to consider when forwarding a packet when the network has converged.

  
• Layer 2 switches support LANs and have multiple FastEthernet or Gigabit Ethernet ports.
• Routers support LANs and WANs and have many types of interfaces including Gigabit Ethernet and High-Speed WAN Interface Card (HWIC) slots to support WAN connections.

  
• Configuring an IPv4 router interface

  1. Set IP address

  2. Set subnet mask

  3. Activate interface with no shutdown

  
• Configuring an IPv6 router interface

  1. Set IPv6 address and prefix length

  2. Activate interface with no shutdown

  3. Can generate link-local address without global unicast address

  
• IPv6 interfaces will typically have more than one IPv6 address, including a link-local and global unicast address.

  
• Configuring an IPv4 loopback interface


  Configure a logical interface internal to the router for testing and management purposes

  
• Common verification commands


  • show ip interface brief
  • show running-config interface interface-type number
  • show interfaces
  • show ip interface
  • show ip route
  • ping
• Output filters can be used in combination with any show command.