21. It is important to know the difference between a collision domain and a broadcast domain. When you use Hubs, all the nodes connected to the hub will be in the same collision domain. However, when you use switches and implement VLANs, each VLAN will be in a separate broadcast domain. The packet forwarding between VLANs is achieved through the use of routing.
22. Network layer is responsible for end to end delivery of packets. IP and IPX are examples of two routing protocols that work at the Network layer of the OSI reference model. Network layer addresses remain constant as a packet travels from source to destination network, whereas physical (such as MAC address) addresses change with each hop that a packet takes. For this reason, a Network layer address is a logical address (not hardcoded like MAC address).
23. Originating source port numbers are dynamically assigned by source
host, usually greater than 1023. The following are the recommended port
· Numbers 0 - 255 are used for public applications
· Numbers 255 - 1023 are assigned to companies so that they can use these port numbers in their applications.
· Numbers above 1023 are used by upper layers to set up sessions with other hosts and by TCP to use as source and destination addresses.
24. PAP uses 2-way handshaking. Passwords are sent in clear text across
the link. Therefore, PAP is to be used only when it not possible to use
CHAP uses 3-way handshaking. CHAP uses Challenge/ Response method which provides protection against the password capture while authenticating the user. One should use CHAP whenever it is possible.
25. PPP and Cisco HDLC support multiple upper layer protocols. SLIP and LAPB does not support multiple upper layer protocols. PPP supports Password Authentication Protocol (PAP) and, Challenge Handshake Authentication Protocol (CHAP).
26. service password-encryption command:
Directs the router to encrypt passwords that are saved in the configuration file. Service password encryption is applied to all passwords, including username passwords, authentication key passwords, the privileged command password, console and virtual terminal line access passwords. This command is primarily useful for keeping unauthorized individuals from viewing your password in your configuration file.
27. The command "ipx routing" selects IPX as a routing protocol and starts the routing process. Some other important commands are:
1. ipx maximum-paths 2: This command allows load sharing over parallel
metric paths to the destination. The parallel paths that can be used is
limited to 2.
2. Interface ethernet 0.1: Indicates the first sub interface on interface e0.
28. The following are true about HDLC encapsulation over serial links:
a. HDLC encapsulation used on Cisco routers is proprietary (uses a Type
field), and may not be compatible with routers manufactured by other vendors
b. It is recommended to use other encapsulation schemes such as PPP when connecting between a Cisco router and a non-Cisco router instead of HDLC encapsulation.
29. The following precautions may be taken to harden network infrastructure:
a. Use physical barriers such as room lock so that un-authorized persons
do not have access to the network devices.
b. Use firewall so that outsiders cannot access network devices from outside the network
c. Enable SSH so that passwords are transmitted in encrypted form
30. The four layers of DoD model from Top to Bottom are:
1. Process Layer: Responsible for implementation of user-level applications
such as mail delivery, file transfer, and remote login.
2. Host-to-Host Layer: Responsible for flow control, and retransmission of lost packets. TCP, and UDP protocols work at this layer.
3. Internet Layer: Responsible for routing data across a network of networks. Routed protocols such as IP work at this layer. Routing protocols are also associated with this layer.
4. Network Access Layer: Responsible for the delivery of data segments over a given particular hardware media. Different protocols such as CSMA/CD work at this layer.
31. MAC sub-layer:
· The Mac sublayer specification 802.3z describes gigabit Ethernet over fiber or shielded copper.
· The Mac sublayer specification 802.3ab (1000BaseT) describes gigabit Ethernet over Category 5 UTP.
32. The MAC address contains 48 bits expressed as 12 hexadecimal digits.
Note that 2 hexadecimal digits represent a byte. Therefore, a MAC address
contains 6 bytes. The first 3 bytes represent the manufacturer identification
code. The next 3 bytes represent the interface number unique to a given
manufacturer. MAC address represents the hardware address and is usually
burned into the ROM. A typical MAC address looks like:
xx-xx-xx-xx-xx-xx, where x represents a hexadecimal digit.
Here, 00-00-0c represents the vendor address, and 12-14-33 typically represents the interface serial number. The last 6 hex digits are administered by respective vendors, and unique to a given vendor.
Traditionally, the router interface on a serial link is a 60-pin D-shell connector. The CSU/DSU may have EIA/TIA 232 interface to the router. The CSEU/DSU interface to the service provider could be RJ-11, RJ-45 etc.
33. Transport Layer is responsible for multiplexing the packets received from upper layers. It also makes the network transparent to the upper layers (Session, Presentation, and Application layers).
34. When designing an ESS WLAN, APs with overlapping coverage areas should be configured to use non-overlapping channels. This is true whether the APs are using DSSS (Direct Sequence Spread Spectrum) or FHSS (Frequency Hopping Spread Spectrum) technology.
35. While a packet travels through an Internetwork, it usually involves multiple hops. It is important to know that the logical address (IP address) of source (that created the packet) and destination (final intended destination) remain constant, the hardware (Interface) addresses change with each hop.
36. Windowing, Buffering, and Congestion avoidance are three different
types of flow controls used frequently.
You can use "show controllers serial [port number]" to see if the interface detects a DCE or DTE cable.
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