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Straight-through cable

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1. Straight-through cable (39-40): connects the wire at pin 1 on one end of the cable to pin 1 at the other end of the cable; the wire at pin 2 needs to connect to pin 2 on the other end of the cable; pin 3 on one end connects to pin 3 on the other; and so on. (To create a straight-through cable, both ends of the cable use the same TIA pinout standard on each end of the cable.) Crossover cable: a cable that swaps the wire pairs inside the cable

2. Collision domain (43): the set of devices whose frames could collide; switches increase the size and number of collision domains

3. IPv4 Header Fields (98)
Version: Version of the IP protocol. Most networks use version 4 today. TTL: Time to live. A value used to prevent routing loops.
Header Checksum: A value used to store an FCS value, whose purpose is to determine if any bit errors occurred in the IP header. Source IP address: The 32-bit IP address of the sender of the packet. Destination IP address: The 32-bit IP address of the intended recipient of the packet.

4. Passive-interface (342): stops the sending of RIP updates on the interface

5. Classless and Classful Routing (454): For a routing protocol to support VLSM, the routing protocol must advertise not only the subnet number but also the subnet mask when advertising routes. Additionally, a routing protocol must include subnet masks in its routing updates to support manual route summarization.

6. Overlapping VLSM Subnets (455): The subnets chosen to be used in any IP internetwork design must not overlap their address ranges. With a single subnet mask in a network, the overlaps are somewhat obvious; however, with VLSM, the overlapping subnets might not be as obvious. When multiple subnets overlap, a router’s routing table entries overlap. As a result, routing becomes unpredictable, and some hosts can be reached from only particular parts of the internetwork. In short, a design that uses overlapping subnets is considered to be an incorrect design, and should not be used.

7. Manual Route Summarization (461): Route summarization reduces the size of routing tables while maintaining routes to all the destinations in the network. As a result, routing performance can be improved and memory can be saved inside each router. Summarization also improves convergence time, because the router that summarizes the route no longer has to announce any changes to the status of the individual subnets. The term manual refers to the fact that manual route summarization only occurs when an engineer configures one or more commands. Summary routes, which replace multiple routes, must be configured by a network engineer.

8. ICMP Unreachable Codes (479)
Network unreachable: There is no match in a routing table for the packet’s destination. Host unreachable: The packet can be routed to a router connected to the destination subnet, but the host is not responding. Can’t fragment: The packet has the Don’t Fragment bit set, and a router must fragment to forward the packet. Protocol unreachable: The packet is delivered to the destination host, but the transport layer protocol is not running on that host. Port unreachable: The packet is delivered to the destination host, but the destination port has not been opened by an application.

9. The ICMP Time Exceeded Message (481): The ICMP Time Exceeded message notifies a host when a packet it sent has been discarded because it was “out of time.” Packets are not actually timed, but to prevent them from being forwarded forever when there is a routing loop, each IP header uses a Time to Live (TTL) field. Routers decrement the TTL by 1 every time they forward a packet; if a router decrements the TTL to 0, it throws away the packet.

10. Open Shortest Path First (OSPF) (506): the most popular link-state IP routing protocol

11. Dijkstra Shortest Path First (SPF) algorithm (507): The SPF algorithm can be compared to how humans think when taking a trip using a road map. Anyone can buy the same road map, so anyone can know all the information about the roads. However, when you look at the map, you first find your starting and ending locations, and then you analyze the map to find the possible routes. If several routes look similar in length, you might decide to take a longer route if the roads are highways rather than country roads.

12. Neighbor States (513): OSPF defines a large set of potential actions that two neighbors use to communicate with each other. To keep track of the process, OSPF routers set each neighbor to one of many neighbor states. An OSPF neighbor state is the router’s perception of how much work has been completed in the normal processes done by two neighboring routers.

13. Database Exchange (517): When two neighbors complete [database exchange] process, they are considered to have fully completed the database exchange process. So OSPF uses the Full neighbor state to mean that the database exchange process has been completed.

14. OSPF Areas and Design Terminology (520-21)
Area Border Router (ABR): An OSPF router with interfaces connected to the backbone area and to at least one other area Autonomous System Border Router (ASBR): An OSPF router that connects to routers that do not use OSPF for the purpose of exchanging external routes into and out of the OSPF domain Backbone Router: A router in one area, the backbone area

Internal Router: A router in a single, nonbackbone area
Area: A set of routers and links that share the same detailed LSDB information, but not with routers in other areas, for better efficiency Backbone area: A special OSPF area to which all other areas must connect: Area 0 External route: A route learned from outside the OSPF domain and then advertised into the OSPF domain Intra-area route: A route to a subnet inside the same area as the router Inter-area route: A route to a subnet in an area of which the router is not a part Autonomous system: In OSPF, a reference to a set of routers that use OSPF

15. OSPF Metrics (Cost) (530): OSPF calculates the metric for each possible route by adding up the outgoing interfaces’ OSPF costs.

16. Neighbor Requirements for EIGRP and OSPF (550)
Interfaces must be in an up/up state
Interfaces must be in the same subnet
Must pass neighbor authentication (if configured)
Must use the same ASN/process-ID on the router configuration command Yes
Hello and hold/dead timers must match
IP MTU must match
Router IDs must be unique
K-values must match
Must be in the same area
1Having duplicate EIGRP RIDs does not prevent routers from becoming neighbors, but it can cause problems when external EIGRP routes are added to the routing table.

17. IPv6 Neighbor Discovery and Stateless Autoconfiguration and Router
Advertisements (579) The stateless autoconfiguration process uses one of many features of the IPv6 Neighbor Discovery Protocol (NDP) to discover the prefix on a LAN. NDP performs many functions for IPv6, all related to something that occurs between two hosts in the same subnet. Stateless autoconfiguration uses two NDP messages, namely router solicitation (RS) and router advertisement (RA) messages, to discover the IPv6 prefix used on a LAN. The host sends the RS message as an IPv6 multicast message, asking all routers to respond to the questions “What IPv6 prefix(s) is used on this subnet?” and “What is the IPv6 address(s) of any default routers on this subnet?” IPv6 does not use broadcasts. In fact, there is no such thing as a subnet broadcast address, a network-wide broadcast address, or an equivalent of the all-hosts broadcast IPv4 address. Instead, IPv6 uses multicast addresses.

18. Summary of the steps a host takes when first booting (584): a. Host calculates its IPv6 link local address (begins with FE80::/10 b. Host sends and NDP RS message with its link local address as the source address and the all-routers FF02::2 multicast destination address, to ask routers to supply a list of default routers and the prefix/length used on the LAN c. The router(s) replies with an RA message, sourced from the router’s link local address, sent to the all-IPv6-hosts-on-the-link multicast address (FF02::1), supplying the default router and prefix information d. If the type of dynamic address assignment is stateless autoconfiguration, the following occur: 1. The host builds the unicast IP address it can use to send packets through the router by using the prefix learned in the RA message and calculating an EUI-64 interface ID based on the NIC MAC address 2. The host uses DHCP messages to ask a stateless DHCP server for the DNS server IP addresses and domain name e. If the type of dynamic address assignment is stateful DHCP, the host uses DHCP messages to ask a stateful DHCP server for a lease of an IP address/prefix length, as well as default router addresses, the DNS server IP addresses, and domain name

19. Three categories of IPv6 addresses (581):
Unicast: IP addresses assigned to a single interface for the purpose of allowing that one host to send and receive data Multicast: IP addresses that
represent a dynamic group of hosts for the purpose of sending packets to all current members of the group. Some multicast addresses are used for special purposes, like with NDP messages, while most support end-user applications Anycast: A design choice by which servers that support the same function can use the same unicast IP address, with packets sent by clients being forwarded to the nearest server, allowing load balancing across different servers

20. IPv6 Address Types (585)
Type of Address
Easily Seen Hex Prefix(es)
Global unicast
Unicast packets sent through the public Internet
2 or 3
Unique local
Unicast packets inside one organization
Link local
Packets sent in the local subnet
Multicast (link local scope)
Multicasts that stay on the local subnet

21. Updates to Routing Protocols for IPv6 (585)
Routing Protocol
Full Name
RIP Next Generation
OSPF version 3
Multiprotocol BGP-4
EIGRP for IPv6
EIGRP for IPv6

22. IPv4/IPv6 Dual Stacks (589): The term dual stacks means that the host or router uses both IPv4 and IPv6 at the same time.

23. Microsegmentation (601): A switch’s effect of segmenting and Ethernet LAN into one collision domain per interfaces is sometimes called microsegmentation.

24. Switching Logic (602): The IEEE defines three general categories of Ethernet MAC addresses: a. Unicast addresses: MAC addresses that identify a single LAN interface card b. Broadcast addresses: A frame sent with a destination address of the broadcast address (FFFF.FFFF.FFFF.FFFF) implies that all devices on the LAN should receive and process the frame c. Multicast addresses: Multicast MAC addresses are used to allow a dynamic subset of devices on a LAN to communicate

25. The Forward Versus Filter Decision (603): To decide whether to forward a frame, a switch uses a dynamically built table that lists MAC addresses and outgoing interfaces. Switches compare the frame’s destination MAC address to this table to decide whether the switch should forward a frame or simply ignore it.

26. How Switches Learn MAC Addresses (604): Switches build the address table by listening to incoming frames and examining the source MAC address in the frame. If a frame enters the switch and the source MAC address is not in the MAC address table, the switch creates an entry in the table. The MAC address is placed in the table, along with the interface from which the frame arrived.

27. Flooding Frames (605): When there is not matching entry in the table, switches forward the frame out all interfaces (except the incoming interface). Switches forward these unknown unicast frames (frames whose destination MAC addresses are not yet in the bridging table) out all other interfaces, with the hope that the unknown device will be on some other Ethernet segment and will reply, allowing the switch to build a correct entry in the address table.

28. Avoiding Loops Using Spanning Tree Protocol (606): The third primary feature of LAN switches is loop prevention, as implemented by Spanning Tree Protocol (STP). Without STP, frames would loop for an indefinite period of time in Ethernet networks with physically redundant links. To prevent looping frames, STP blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments (collision domains). The result of STP is good: frames do not loop infinitely, which makes the LAN usable. However, although the network can use some redundant links in case of a failure, the LAN does not load-balance the traffic. To avoid Layer 2 loops, all switches need to use STP. STP causes each interface on a switch to settle into either a blocking state or a forwarding state. Blocking means that the interface cannot forward or receive data frames. Forwarding means that the interface can send and receive data frames. If a correct subset of the interfaces is blocked, a single currently active logical path exists between each pair of LANs.

29. Internal Processing on Cisco Switches (607-09)
Switching Method
The switch full receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame. Cut-through
The switch forwards the frame as soon as it can. This reduces latency but does not allow the switch to discard frames that fail the FCS check. Fragment-free
The switch forwards the frame after receiving the first 64 bytes of the frame, thereby avoiding forwarding frames that were errored due to a collision.

30. The following list summarizes the terms that describe the roles of campus switches (617): Access: Provides a connection point (access) for end-user devices. Does not forward frames between two other access switches under normal circumstances. Distribution: Provides an aggregation point for access switches, forwarding frames between switches, but not connecting directly to end-user devices. Core: Aggregates distribution switches in very large campus LANs, providing very high forwarding rates.

31. Configuring the Switch IP Address (625): An IOS-based switch configures its IP address and mask on a special virtual interface called the VLAN 1 interface.

32. Port Security (629): If the network engineer knows what devices should be cabled and connected to particular interfaces on a switch, the engineer can use port security to restrict that interface so that only the expected devices can use it.

33. Actions When Port Security Violation Occurs (632):
Option on the switchport port-security violation command
Discards offending traffic
Sends log and SNMP messages
Disables the interface, discarding all traffic

34. The following summarizes the most common reasons for separating hosts into different VLANs (644): To create more flexible designs that group users by department, or by groups that work together, instead of by physical location To segment devices into smaller LANs (broadcast domains) to reduce overhead caused to each host in the VLAN To reduce the workload for the Spanning Tree Protocol (STP) by limiting a VLAN to a single access switch To enforce better security by keeping hosts that work with sensitive data on a separate VLAN To separate traffic sent by an IP phone from traffic sent by PCs connected to the phones

35. Trunking with ISL and 802.1Q (644): When using VLANs in networks that have multiple interconnected switches, the switches need to use VLAN trunking on the segments between the switches. VLAN trunking causes the switches to use a process called VLAN tagging, by which the sending switch adds another header to the frame before sending it over the trunk. This extra VLAN header includes a VLAN identifier (VLAN ID) field so that the sending switch can list the VLAN ID and the receiving switch can then know in what VLAN each frame belongs.

36. ISL (646): ISL fully encapsulates each original Ethernet frame in an ISL header and trailer. The original Ethernet frame inside the ISL header and trailer remains unchanged.

37. IEEE 802.1Q (646): 802.1Q uses a different style of header than does ISL to tag frames with a VLAN number. In fact, 802.1Q does not actually encapsulate the original frame in another Ethernet header and trailer. Instead, 802.1Q inserts an extra 4-byte VLAN header into the original frame’s Ethernet header. As a result, unlike ISL, the frame still has the same original source and destination MAC addresses. Also, because the original header has been expanded, 802.1Q encapsulation forces a recalculation or the original frame check sequence (FCS) field in the Ethernet trailer, because the FCS is based on the contents of the entire frame. 802.1Q recognizes the concept of native VLAN.

38. ISL and 802.1Q Compared (648)
Defined by
Inserts another 4-byte header instead of completely encapsulating the original frame No
Supports normal-range (1-1005) and extended-range (1006-4094) VLANs Yes
Allows multiple spanning trees
Uses a native VLAN

39. Three Requirements for VTP to Work Between Two Switches (651): The link between the switches must be operating as a VLAN trunk (ISL or 802.1Q) The two switches’ case-sensitive VTP domain name must match If configured on at least one of the switches, the two switches’ case-sensitive VTP password must match

40. Trunking Administrative Mode Options with the switchport mode Command (661) Command Option
Prevents the use of trunking, making the port always act as an access (non-trunk) port trunk
Always uses trunking
dynamic desirable
Initiates negotiation messages and responds to negotiation messages to dynamically choose whether to start using trunking, and defines the trunking encapsulation dynamic auto
Passively waits to receive trunk negotiation messages, at which point the switch will respond and negotiate whether to use trunking, and if so, the type of trunking

41. VTP Modes: Servers, Clients, and Transparent (669-75)

42. PPP Concepts (689): Built-in authentication tools: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP)

43. PPP Multilink (692): When multiple PPP links exist between the same two routers—referred to as parallel links—the routers must then determine how to use those links. With HDLC links, and with PPP links using the simplest configuration, the routers must use Layer 3 load balancing. This means that the routers have multiple routes for the same destination subnets…Multilink PPP load-balances the traffic equally over the links while allowing the Layer 3 logic in each router to treat the parallel links as a single link. When encapsulating a packet, PPP fragments the packet into smaller frames, sending one fragment over each link.

44. Troubleshooting Serial Links (699-700): The serial link verification and troubleshooting process should begin with a simple three-step process: From one router, ping the other router’s serial IP address If the ping fails, examine the interface status on both routers, and investigate problems related to the likely problem areas listed in the following table:

Line Status
Protocol Status
Likely Reason
Down (stable) on both ends
Down (stable) on one end, flapping between up and down on the other Mismatched encapsulation commands
Down on one end, up on the other
Keepalive is disabled on the end in an up state
Down (stable) on both ends
PAP/CHAP authentication failure

If the ping works, also verify that any routing protocols are exchanging routes over the link

45. Frame Relay Overview (713): Unlike with LANs, you cannot send a data link layer broadcast over Frame Relay. Therefore, Frame Relay networks are called nonbroadcast multi-access (NBMA) networks. Also, because Frame Relay is multi-access, it requires the use of an address that identifies to which remote router each frame is addressed…A leased line is installed between the router and a nearby Frame Relay switch; this link is called the access link. To ensure that the link is working, the device outside the Frame Relay network, called the data terminal equipment (DTE), exchanges regular messages with the Frame Relay switch. These keepalive messages, along with other messages, are defined by the Frame Relay Local Management Interface (LMI) protocol. The routers are considered DTE, and the Frame Relay switches are data communications equipment (DCE).

46. Frame Relay Terms and Concepts (714-15)
Virtual circuit (VC)
A logical concept that represents the path that frames travel between DTEs.
VCs are particularly useful when you compare Frame Relay to leased physical circuits. Permanent virtual circuit (PVC)
A predefined VC. A PVC can be equated to a leased line in concept. Switched virtual circuit (SVC)
A VC that is set up dynamically when needed. An SVC can be equated to a dial connection in concept. Data terminal equipment (DTE)
DTEs are connected to a Frame Relay service from a telecommunications company. They typically reside at sites used by the company buying the Frame Relay service. Data communications equipment (DCE)

Frame Relay switches are DCE devices. DCEs are also known as data circuit-terminating equipment. DCEs are typically in the service provider’s network. Access link
The leased line between the DTE and DCE.
Access rate (AR)
The speed at which the access link is clocked. This choice affects the connection’s price. Committed Information Rate (CIR)
The speed at which bits can be send over a VC, according to the business contract between the customer and provider. Data-link connection identifier (DLCI)
A Frame Relay address used in Frame Relay headers to identify the VC. Nonbroadcast multi-access (NBMA)
A network in which broadcasts are not supported, but more than two devices can be connected Local Management Interface (LMI)
The protocol used between a DCE and DTE to manage the connection. Signaling messages for SVCs, PVC status messages, and keepalives are all LMI messages

47. Frame Relay LMI Types (719):
IOS LIM-Type Parameter
T1.617 Annex D
Q.933 Annex A

48. A Fully Meshed Network with One IP Subnet (737): A network with one IP subnet in which all devices are connected all other devices

49. Inverse ARP (742): Inverse ARP dynamically creates a mapping between the Layer 3 address (for example, the IP address) and the Layer 2 address (the DLCI). The end result of Inverse ARP is the same as IP ARP on a LAN: The router builds mapping between a neighboring Layer 3 address and the corresponding Layer 2 address. However, the process used by Inverse ARP differs for ARP on a LAN. After the VC is up, each router announces its network layer address by sending an Inverse ARP message over that VC.

50. A Partially Meshed Network with One IP Subnet Per VC (745): A network with one IP subnet in which all devices connect to neighboring devices.

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