I.TsForWomen CCENT: Mod 5
Routing Table Sources
- '#show ip route' command is used to display the IPv4 routing table of a router. - Provides additional route information. - How the route was learned - How long the route has been in the table -Which interface to use to get to a destination.
default static route
- A default static route is similar to a default gateway on a host. - The default static route specifies the exit point to use when the routing table does not contain a path for the destination network. - A default static route is useful when a router has only one exit point to another router, such as when the router connects to a central router or service provider.
Dynamic Routing
- A dynamic routing protocol allows the routers to automatically learn about networks from other routers. - These networks, and the best path to each, are added to the routing table of the router, and identified as a network learned by a specific dynamic routing protocol.
ARP Table
- A list of IPs and associated macs that are on the same vlan. - The ARP table is stored in the RAM of the device. - The Arp table resolves IPv4 addresses to MAC addresses and maintains a table of mappings - Before sending the packet to the data link layer the router checks the arp table for the destination. - If the device locates the IPv4 address, its corresponding MAC address is used as the destination MAC address in the frame. -If no entry is found, the device sends an ARP request.
Routing Table
- A list of routes organized in a table format. - Each route lists the destination subnet and a mask, router interface. - It analyzes the the destination IP address forwards the packets as needed. - Routing table is stored in RAM
Local network
- A network that is that a packet originates in. - If the packet travels in to another device in the same LAN. It is staying in the local area network.
Router Switching Function
- A primary function of a router is to forward packets toward their destination. - This is accomplished by using a switching function, which is the process used by a router to accept a packet on one interface and forward it out of another interface. - The purpose of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link.
IPv4 Routing Protocols
- A router running a dynamic routing protocol does not only make a best path determination to a network, it also determines a new best path if the initial path becomes unusable (or if the topology changes). - For these reasons, dynamic routing protocols have an advantage over static routes. - Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator.
The Routing Table
- A routing table is a data file in RAM that is used to store route information about directly connected and remote networks. - The routing table contains network or next hop associations. - These associations tell a router that a particular destination can be optimally reached by sending the packet to a specific router that represents the next hop on the way to the final destination. - The next hop association can also be the outgoing or exit interface to the next destination.
ARP Cache Timer
- ARP entries expire if they are not used after a set time. - The times differ depending on the device's operating system.
Local Route Interfaces
- Added when an interface is configured and active. - This entry is only displayed in IOS 15 or newer for IPv4 routes and all IOS releases for IPv6 routes.
Dynamic routing protocol
- Added when routing protocols that dynamically learn about the network, such as EIGRP or OSPF, are implemented and networks are identified.
ARP Process
- Address Resolution Protocol. - Internet protocol used to map an IP address to a mac address. - Sends out a Broadcast message; Receives an Arp Reply, then adds the mac address to the table.
Directly Connected Routing table Entries
- An active, properly configured, directly connected interface actually creates two routing table entries. - The routing table entry for directly connected interfaces is simpler than the entries for remote networks.
Process switching
- An older packet forwarding mechanism. - An arriving packet is forwarded to the control plane where the CPU compares the destination IP to the routing table & then determines the exit interface and forwards the packet. - It is important to understand that the router does this for every packet, even if the destination is the same for a stream of packets. - This is very slow and rarely implemented in modern networks.
Hop
- Another router on the way to the packets final destination.
Directly Connected Interfaces
- Be assigned a valid IPv4 or IPv6 address - Be activated with the no shutdown command - Receive a carrier signal from another device (router, switch, host, etc.)
#no shutdown
- By default, LAN and WAN interfaces are not activated (shutdown). - To enable an interface, it must be activated using the no shutdown command. - The interface must also be connected to another device (a hub, a switch, or another router) for the physical layer to be active.
Cisco Express Forwarding (CEF)
- CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. - Like fast switching, CEF builds a Forwarding Information Base (FIB), and an adjacency table. - CF table entries are change-triggered, when something changes in the network topology. - When a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. - Cisco Express Forwarding is the fastest forwarding mechanism and the preferred choice on Cisco routers because it is cisco proprietary.
#show ipv6 route
- Can be used to verify that IPv6 networks and specific IPv6 interface addresses have been installed in the IPv6 routing table.
Administrative Distance
- Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. - The AD represents the "trustworthiness" of the route; the lower the AD, the more trustworthy the route source. - For example, a static route has an AD of 1, whereas an EIGRP-discovered route has an AD of 90. - Given two separate routes to the same destination, the router chooses the route with the lowest AD. W - Similarly, a directly connected route with an AD of 0 takes precedence over a static route with an AD of 1
#ipv6 address ipv6-address / prefix-length link-local
- Configures a static link-local address on the interface that is used instead of the link-local address that is automatically configured.
Configure an IPv6 Router Interface
- Configuring an IPv6 interface is similar to configuring an interface for IPv4. - Most IPv6 configuration and verification commands in the Cisco IOS are very similar to their IPv4 counterparts. - In many cases, the only difference uses 'ipv6' in place of 'ip' in commands.
Console Access
- Console access is really only required when initially configuring a device, or if remote access fails. Console access requires: - Console cable - RJ-45-to-DB-9 console cable - Terminal emulation software - Tera Term, PuTTY, HyperTerminal
ARP Request
- Contains a Target IPv4 address & Target MAC address. - This is encapsulated in an Ethernet frame using the Destination MAC address, Source MAC address, Source MAC address & Type as header information. - ARP requests are broadcasts, they are flooded out all ports by the switch except the receiving port.
#clock rate
- Depending on the type of interface, additional parameters may be required. - The serial cable end labeled DCE must be configured with the clock rate command.
Best Path
- Determining the best path involves the evaluation of multiple paths to the same destination network and selecting the optimum or shortest path to reach that network. - Whenever multiple paths to the same network exist, each path uses a different exit interface on the router to reach that network. - The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. - A metric is the quantitative value used to measure the distance to a given network. The best path to a network is the path with the lowest metric.
Directly-connected routes
- Directly-connected routes come from the active router interfaces. - Routers add a directly connected route when an interface is configured with an IP address and is activated. - Each of the router's interfaces is connected to a different network segment.
#show ip route
- Displays the contents of the IPv4 routing table stored in RAM.
#show ipv6 interface <int>
- Displays the interface status and all of the IPv6 addresses belonging to the interface. - shows the link local address and global unicast address, the output includes the multicast addresses assigned to the interface, beginning with prefix FF02.
Dynamic routing protocols
- Dynamic Routing Protocols use their own rules and metrics to build and update routing tables. - The routing algorithm generates a value, or a metric, for each path through the network. - Metrics can be based on either a single characteristic or several characteristics of a path.
Configure an IPv4 Loopback Interface
- Enabling and assigning a loopback address is simple: - Router(config)# interface loopback number - Router(config-if)# ip address ip-address subnet-mask - Router(config-if)# exit - Multiple loopback interfaces can be enabled on a router. - The IPv4 address for each loopback interface must be unique and unused by any other interface.
IP address
- Identifies a unique host on a local network - Routers use IPs to forward packets, unlike switches which use mac addresses. - This makes it a layer 3 device.
Route source
- Identifies how the route was learned. - Directly connected interfaces have two route source codes. - 'C' identifies a directly connected network. - 'L' identifies the IPv4 address assigned to the router's interface.
Default gateway
- Identifies the router to send a packet to when a switch determines the destination is not on the same local network subnet.
Directly connected network
- If the destination IP address of the packet belongs to a device on a network that is directly connected to one of the interfaces of the router, that packet is forwarded directly to the destination device. - This means that the destination IP address of the packet is a host address on the same network as the interface of the router.
Remote network
- If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. - Remote networks can only be reached by forwarding packets to another router.
Gateway of Last Resort
- No route determined - If the destination IP address of the packet does not belong to either a connected or remote network, the router determines if there is a Gateway of Last Resort available. - A Gateway of Last Resort is set when a default route is configured on a router. - If there is a default route, the packet is forwarded to the Gateway of Last Resort. - If the router does not have a default route, then the packet is discarded.
ARP Reply
- Only the device that originally sent the ARP request will receive the unicast ARP reply. - Once the ARP reply is received, the device will add the IPv4 address and the corresponding MAC address to its ARP table. - Packets destined for that IPv4 address can now be encapsulated in frames using its corresponding MAC address. - If no device responds to the ARP request, the packet is dropped because a frame cannot be created.
Speed
- Speed is a measure of the data rate in bits per second (b/s) of a given link in the network. - It can also be measured in packets per second.
Forwarding Information Base (FIB)
- The FIB contains pre-computed reverse lookups, next hop information for routes including the interface and Layer 2 information.
Configure Basic Router Settings
- The following basic tasks should be configured first. The device is named, management access is secured, a banner is configured and the configuration is saved: - Name the device - Distinguishes it from other routers. - Secure management access - Secures privileged EXEC, user EXEC, and Telnet access, and encrypts passwords to their highest level. - Configure a banner - Provides legal notification of unauthorized access.
#ipv6 enable
- The ipv6 enable interface command is used to automatically create an IPv6 link-local address whether or not an IPv6 global unicast address has been assigned.
IPv4 Loopback Interface
- The loopback interface is a logical interface internal to the router. - It is not assigned to a physical port and can therefore never be connected to any other device. - It is considered a software interface that is automatically placed in an UP state, as long as the router is functioning. - The loopback interface is useful in testing and managing a Cisco IOS device because it ensures that at least one interface will always be available. - Testing internal routing processes, by emulating networks behind the router.
router
- The router is responsible for the routing of traffic between networks. - Communication between networks would not be possible without a router determining the best path to the destination and forwarding traffic to the next router along that path. - When a router receives an IP packet on one interface, it determines which interface to use to forward the packet to the destination. - The interface that the router uses to forward the packet may be the final destination, or it may be another router that is used to reach the destination network.
Remote routes
- These are remote networks connected to other routers. - Routes to these networks can either be statically configured or dynamically configured using dynamic routing protocols.
Fast switching
- This is a common packet forwarding mechanism which uses a fast-switching cache to store next-hop information. - A packet arrives and is forwarded to the control plane where the CPU searches for a match in the fast-switching cache. - If it does not exist, it is process-switched and forwarded to the exit interface. - For incoming packets with the same destination IP, the next-hop information in the cache is re-used without CPU intervention.
Remote Networks
- When a host sends a packet to a device on a different IP network, then the packet is forwarded to the default gateway. - The default gateway is the destination that routes traffic from the local network to devices on remote networks. - It is often used to connect a local network to the Internet.
Load Balancing
- When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally. - This is called equal cost load balancing. The routing table contains the single destination network, but has multiple exit interfaces, one for each equal cost path. - The router forwards packets using the multiple exit interfaces listed in the routing table.
#ipv6 unicast-routing
- When the router is configured using the ipv6 unicast-routing global configuration command, the router begins sending ICMPv6 Router Advertisement messages out the interface. - This enables a PC connected to the interface to automatically configure an IPv6 address and to set a default gateway without needing the services of a DHCPv6 server.
TCP/IP
1981 TCP and IP Protocols are formalized. [RFC 793 and RFC 791]
DNS
1984 The domain name service is introduced. Translates domain names, such as cisco.com, into IP addresses
packet-forwarding mechanisms
A common analogy used to describe the three packet-forwarding mechanisms is as follows: - Process switching solves a problem by doing math long hand, even if it is the identical problem. - Fast switching solves a problem by doing math long hand one time and remembering the answer for subsequent identical problems. - CEF solves every possible problem ahead of time in a spreadsheet.
Routing Decisions
A primary function of a router is to determine the best path to use to send packets. To determine the best path, the router searches its routing table for a network address that matches the destination IP address of the packet.
Static routes
Added when a route is manually configured and the exit interface is active.
Enhanced Interior Gateway Routing Protocol (EIGRP)
Bandwidth, delay, load, reliability
BOOTP
Bootstrap Protocol Enables a diskless workstation to discover its own IP address, the IP address of a BOOTP server on the network, and a file to be loaded into memory to boot the machine.
Open Shortest Path First (OSPF)
Cisco's cost based on cumulative bandwidth from source to destination
#pv6-address/prefix-length [link-local | eui-64]
Configured with IPv6 address and subnet mask interface configuration command.
#ipv6 address ipv6-address / prefix-length eui-64
Configures a global unicast IPv6 address with an interface identifier (ID) in the low-order 64 bits of the IPv6 address using the EUI-64 process.
#ipv6 address ipv6-address / prefix-length
Creates a global unicast IPv6 address as specified.
#show ip interface brief
Displays a summary for all interfaces including the IPv4 address of the interface and current operational status.
#show interfaces
Displays interface information and packet flow count for all interfaces on the device.
#show ip interface
Displays the IPv4 related information for all interfaces on a router.
#show running-config interface interface-id
Displays the commands configured on the specified interface.
DHCP
Dynamic Host configuration protocol. Dynamically assigns IP addresses to client stations at start-up Allows the addresses to be re-used when no longer needed
Application Protocols
HTTP, DNS, DHCP, FTP.
Routing Information Protocol (RIP)
Hop count
Internet Protocols
IPv4, IPv6, ICMPv4, ICMPv6.
Route timestamp
Identifies how much time has passed since the route was learned.
Next-hop
Identifies the IPv4 address of the next router to forward the packet to.
Destination network
Identifies the address of the remote network.
Outgoing interface
Identifies the exit interface to use to forward a packet toward the final destination.
Administrative distance
Identifies the trustworthiness of the route source. Lower values indicate preferred route source.
Metric
Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.
#ip address ip-address subnet-mask
If using IPv4, configured with an address and a subnet mask - Use the ip address ip-address subnet-mask interface configuration command.
ICMP
Internet Control Message Protocol. Provides feedback from a destination host to a source host about errors in packet delivery.
IMAP
Internet Message Access Protocol. Enables clients to access email stored on a mail server Maintains email on the server
common codes
L - Identifies the address assigned to a router's interface. This allows the router to efficiently determine when it receives a packet for the interface instead of being forwarded. C - Identifies a directly connected network. S - Identifies a static route created to reach a specific network. D - Identifies a dynamically learned network from another router using EIGRP. O - Identifies a dynamically learned network from another router using the OSPF routing protocol.
NAT
Network Address Translation. Translates IP addresses from a private network into globally unique public IP addresses.
PPP
Point to Point Protocol. Provides a means of encapsulating packets for transmission over a serial link.
POP
Post Office Protocol 3. Enables clients to retrieve email from a mail server. Downloads email from the mail server to the desktop
Random Access Memory (RAM)
Provides temporary storage for various applications and processes including the running IOS, the running configuration file, various tables (i.e., IP routing table, Ethernet ARP table) and buffers for packet processing. RAM is referred to as volatile because it loses its contents when power is turned off.
SMTP
Simple Mail Transfer Protocol. Enables clients to send email to a mail server Enables servers to send email to other servers.
Transport Protocols
TCP, UDP.
TFTP
TFTP Trivial File Transfer Protocol. A simple, connectionless file transfer protocol A best-effort, unacknowledged file delivery protocol Utilizes less overhead than FTP
EGP
The Exterior Gateway Protocol (EGP) is developed to allow routers to exchange network information [RFC 827]
FTP
The File Transfer Protocol is documented [RFC 765] Sets rules that enable a user on one host to access and transfer files to and from another host over a network A reliable, connection-oriented, and acknowledged file delivery protocol.
Verify IPv6 Interface Settings
The commands to verify the IPv6 interface configuration are similar to the commands used for IPv4.
Forward to the Next Hop
The following processes take place when R1 receives the Ethernet frame from PC1: 1. R1 examines the destination MAC address, which matches the MAC address of the receiving interface, FastEthernet 0/0. R1, therefore, copies the frame into its buffer. 2. R1 identifies the Ethernet Type field as 0x800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame. 3. R1 de-encapsulates the Ethernet frame. 4. Because the destination IPv4 address of the packet does not match any of the directly connected networks of R1, R1 consults its routing table to route this packet. R1 searches the routing table for a network address that would include the destination IPv4 address of the packet as a host address within that network. In this example, the routing table has a route for the 192.168.4.0/24 network. The destination IPv4 address of the packet is 192.168.4.10, which is a host IPv4 address on that network.
Packet Routing
The following processes take place when a router receives a frame on its interface: 1. Router examines the destination MAC address, which matches the MAC address of the receiving interface. The router copies the frame into its buffer. 2. Router identifies the Ethernet Type field as 0x800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame. 3. Router de-encapsulates the Ethernet frame. 4. Because the destination IPv4 address of the packet does not match any of the interface addresses of the router, the router consults its routing table to make a decision. The router searches the routing table for the destination IPv4 address of the packet. The routing table has a route to the destination IP network and an exit interface. Because the exit interface is not an Ethernet network, the router does not have to resolve the next-hop IP address with a destination MAC address. 5. The packet is now encapsulated into a new data link frame and sent out the exit interface.
Reach the Destination
The following processes take place when the frame arrives at the next router: 1. Router copies the data link PPP frame into its buffer. 2. Roouter de-encapsulates the data link PPP frame. 3. Router searches the routing table for the destination IP address of the packet. The routing table has a route to a directly connected network. This means that the packet can be sent directly to the destination device and does not need to be sent to another router.
#show running-config interface
The output displays the current commands configured on the specified interface.
router process
The primary functions of a router are to: - Determine the best path to send packets - Forward packets toward their destination - The routing table is used to determine the best path to forward a packet. - The router examines the destination address of the packet and uses the routing table to determine for the best path to that network. - The routing table also includes the interface to be used to forward packets for each known network. - When a match is found, the router encapsulates the packet into the data link frame of the outgoing or exit interface, and the packet is forwarded toward its destination. - Routers use static routes and dynamic routing protocols to learn about remote networks and build their routing tables.
Verify Interface Settings
There are several show commands that can be used to verify the operation and configuration of an interface. The following three commands are especially useful to quickly identify an interface status: - show ip interface brief - show ip route - show running-config interface interface-id
UDP
User Datagram Protocol. Enables a process running on one host to send packets to a process running on another host Does not confirm successful datagram transmission.
router performs the following three major steps
What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps: Step 1. De-encapsulates the Layer 2 frame header and trailer to expose the Layer 3 packet. Step 2. Examines the destination IP address of the IP packet to find the best path in the routing table. Step 3. If the router finds a path to the destination, it encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface.
#show ipv6 interface brief
displays a summary for each of the interfaces.
encapsulation
encapsulation process of inserting a formatted message inside another formatted message
de-encapsulation
process of removing a formatted message from another formatted message
