Computer Networks Chapter 1

Questions and Answers

What is the primary function of the Bridge ID?

To keep track of all switches in the network (correct)

To calculate the path cost to the root bridge

To determine the root port of a bridge

To exchange BPDUs with other bridges

What is the main purpose of BPDUs?

To determine the designated port on a network segment

To forward frames to their destination

To block frames from being sent to the root bridge

To exchange information about ports, costs, and priorities between bridges (correct)

What is the state of all ports on bridges and switches after convergence?

All ports are in forwarding mode

All ports are in blocking mode

All ports are in a disabled state

Some ports are in forwarding mode, while others are in blocking mode (correct)

What is the root port of a non-root bridge?

The port with the best path to the root bridge Signup and view all the answers

What is the purpose of the designated port?

To have the lowest cost to get to a network segment Signup and view all the answers

What is the main difference between the original STP (802.1d) and RSTP (802.1w)?

RSTP has a faster convergence time than STP Signup and view all the answers

What is the term used in RSTP to describe the blocking state?

Alternate port Signup and view all the answers

What is the path cost in a spanning tree network?

The sum of the costs of multiple links to the root bridge Signup and view all the answers

What is the function of a non-designated port?

To have the highest cost to get to a network segment Signup and view all the answers

What is the purpose of a backup port?

To act as a redundant link on a LAN segment Signup and view all the answers

Study Notes

Reliability and Flow Control

Reliability in transport ensures segments are delivered and acknowledged, with retransmission of unacknowledged segments and correct sequencing at the destination.
It also provides congestion avoidance and control.
Flow control avoids buffer overflow at the receiving host by managing data transmission.

Acknowledgment

Positive acknowledgment requires the recipient to send an acknowledgment message to the sender upon receiving data.
The sender keeps a record of each data packet sent and expects an acknowledgment.

TCP and UDP

TCP is a reliable, connection-oriented protocol used by:
- FTP (File Transfer Protocol)
- HTTP (Hypertext Transfer Protocol)
- SMTP (Simple Mail Transfer Protocol)
- Telnet
UDP is a connectionless protocol used by:
- TFTP (Trivial File Transfer Protocol)
- SNMP (Simple Network Management Protocol)
- DHCP (Dynamic Host Control Protocol)
- DNS (Domain Name System)

TCP and UDP Ports

Both TCP and UDP use port numbers to pass information to upper layers.
Well-known port numbers are below 1024, while dynamically assigned port numbers are above 1024.
Registered port numbers are used for vendor-specific applications, mainly above 1024.

File Transfer Protocols

FTP is a reliable, connection-oriented service that uses TCP to transfer files between FTP-supporting systems.
TFTP is a connectionless service that uses UDP, used for transferring configuration files and Cisco IOS images on routers.

SNMP

SNMP is an application layer protocol that facilitates the exchange of management information between network devices.

Access Control Lists (ACLs)

Types of Access Lists

Standard ACL: access-list number 1-99, can block a network, host, or subnet, stops two-way communication, and is implemented closest to the destination.
Extended ACL: access-list number 100-199, can block a network, host, subnet, and service, stops one-way communication, and is implemented closest to the source.
Named ACL: uses names instead of numbers, allows editing, and has no number limitations.

Wild Card Mask

Tells the router which addressing bits must match in the ACL statement.
It's the inverse of the subnet mask, also called the inverse mask.
A bit value of 0 indicates MUST MATCH, while a bit value of 1 indicates IGNORE.

Creation and Implementation of Access Lists

Standard Named ACL: ip access-list standard and interface commands.
Extended Named ACL: ip access-list extended and interface commands.
Standard ACL: access-list command.
Extended ACL: access-list command with destination wildcard mask.

Verification of Access-List Configuration

show access-list command displays all access lists and their parameters configured on the router.

Spanning Tree Protocol (STP)

Bridge ID: determined by bridge priority and MAC address, used to keep track of all switches in the network.
Non-root bridges: exchange BPDUs with all bridges and update STP topology database.
Port Cost: determined by link bandwidth.
Path Cost: sum of port costs to the root bridge.
BPDU: data messages exchanged between switches containing information about ports, costs, priorities, and bridge ID.
Convergence: all ports on bridges and switches have transitioned to either forwarding or blocking modes.

Bridge Port Roles

Root Port: the port with the best path to the root bridge, placed in forwarding state.
Designated Port: has the best cost to get to a network segment, placed in forwarding state.
Non-designated Port: has the highest cost and is kept blocked.
Forwarding Port: forwards frames, can be either a root port or a designated port.
Blocked Port: only receives BPDU frames from other switches.
Alternate Port: corresponds to the blocking state of 802.1d, used with RSTP (802.1w).
Backup Port: connected on a LAN segment with another port on the same switch acting as the designated port.