Term
What forwarding decision does a switch (bridge) make on receiving a broadcast or multicast frame? |
|
Definition
|
Flood on all ports except the one it was received on.
|
|
|
Term
| What forwarding decision does a switch (bridge) make on receiving a unicast frame? |
|
Definition
|
If the destination is in the switch CAM table, then forward only to the associated port. If destination is not known, broadcast (flood) on all ports except the originating one.
|
|
|
Term
| What forwarding decision does a switch (bridge) make on receiving a unicast frame where the source and destination hosts are associated with the same port on the switch? |
|
Definition
|
Frame is filtered and not forwarded.
|
|
|
Term
| What are the differences/advantages of a switch (bridge) over a hub? |
|
Definition
|
Hubs will flood all traffic on all ports in one large collision domain. Bridges/switches separate collision domains and will forward frames more intelligently/efficiently. Bridges/switches can also segment LANs using VLAN.
|
|
|
Term
| What are the differences between bridges and switches? |
|
Definition
|
Switches switch in hardware; bridges in software Switches can be full duplex; bridges are half duplex Bridges can only use store and forward mode Generally: Switches have more ports/higher port density Bridges are generally slower
|
|
|
Term
| What are the three switching modes? |
|
Definition
|
Store and Forward Cut Through Fragment Free
|
|
|
Term
| What are the characteristics of the store and forward switching mode? |
|
Definition
|
Only mode that bridges can also use. The entire frame is buffered and CRC/FCS is run to ensure validity.
|
|
|
Term
| What are the characteristics of the cut through switching mode? |
|
Definition
|
High speed at cost of reliability:
Switch makes forwarding decision after receiving the first 6 bytes (destination MAC) after preamble of frame, and immediately begins to forward frame. CRC/FCS not checked; though switch can be set to fall back to Store & Forward mode if bad CRC/FCS threshold is hit.
|
|
|
Term
| What are the characteristics of the fragement free switching mode? |
|
Definition
|
Switch buffers the first 64 bytes of the frame, updates the source MAC/port if needed, then forwards the frame. Frames smaller than 64 bytes (runts) are discarded. Speed vs. reliability in between the store & forward and cut through modes.
|
|
|
Term
| What are the characteristics of a half duplex, and full duplex connection. |
|
Definition
|
half duplex: Only one device can use the wire at a time. Connections are considered multipoint.
full duplex: Devices can send and receive at the same time. Connections are considered point-to-point.
|
|
|
Term
| What are the three major problems caused by switching loops? |
|
Definition
|
Broadcast storms - multiple copies of a signal broadcast perpetually cycling the loop MAC Table instability - single MAC address appears to be reachable on multiple ports of a switch; switch constantly updates the MAC table Duplicate frames - single frame flooded out on multiple paths to a single destination.
|
|
|
Term
| What is microsegmentation? |
|
Definition
|
When all devices connecting to a switch is in full duplex mode, it creates a zero-collision environment, because every device has been segmented into its own collision domain.
|
|
|
Term
| What happens to collision detection on a switch port when a device is connected to it at full duplex? |
|
Definition
|
The collision detection circuit is disabled.
|
|
|
Term
| What is the purpose of STP |
|
Definition
|
Prevent L2 loops in switched networks with redundant paths
|
|
|
Term
| In STP, how is the root bridge elected? |
|
Definition
|
Switch with lowest Bridge ID is Root Bridge ID = [ Admin Priority | MAC ]
|
|
|
Term
What are the packets STP uses called? What does it stand for? What IEEE spec describes it? |
|
Definition
|
BPDU Bridge Protocol Data Unit 802.1d
|
|
|
Term
| In STP, how is the Root Port of a switch determined? |
|
Definition
|
Root port is the port on the switch with the lowest cost path to the root switch. If tiebreak - go with lowest MAC Lower cost = Higher link speed
|
|
|
Term
| In STP, how is the Designated port determined? |
|
Definition
|
Port that forwards least cost path to switches on same segment. If tiebreak - go with lowest MAC
|
|
|
Term
| In STP, how is a Blocked port determined and what are its characteristics? |
|
Definition
|
Blocked ports are ports that were not selected to be root ports or designated ports. Blocked ports do not sent any data, but does listen for BPDU's.
|
|
|
Term
| What are the characteristics of a converged STP system? |
|
Definition
|
All ports are either blocked or forwarding. All ports are determined to be root, desginated, or blocked ports.
|
|
|
Term
What are the four port states in STP? What are their characteristics? |
|
Definition
|
1. Blocking - switch listens but does not send BPDUs 2. Listening - when blocked port begins to send its own BPDUs 3. Learning - when switch is still populating the MAC address table; no frames are forwarded 4. Forwarding - normal operation for a non-blocked port.
|
|
|
Term
| In STP, what states make up the Forward Delay interval and how long do they last by default? |
|
Definition
|
Listening + Learning states 15 + 15 = 30 seconds by default This is the interval between blocked and forwarding
|
|
|
Term
| Under what circumstances are ports in blocked state? |
|
Definition
|
1. When the port is not a root or desginated port after STP convergence 2. When a switch first boots up, all ports start in blocking state 3. When a Topology Change Notification (TCN) is received, all ports of a switch go into blocking state
|
|
|
Term
| What does TCN stand for and when do they occur? |
|
Definition
|
Topology Change Notifications are sent when a new link is added or removed
|
|
|
Term
What determines the time interval between an link failure and the start of STP re-convergence? How long is it by default? |
|
Definition
|
After receiving TCN, the time between Forward Delay and STP reconvergence is determined by the Max Age Timer By default it is 20 seconds (which is also 10 missed BPDUs)
|
|
|
Term
| What are the port states for Rapid STP and how to they equate to STP? |
|
Definition
|
RSTP port states (...STP port states)
Discarding (= Blocking/Listening) Learning == Learning Forwarding == Forwarding
|
|
|
Term
| For Rapid STP, how long is the delay before re-convergence? |
|
Definition
|
6 seconds = 3 missed BPDUs
|
|
|
Term
What are the different port types under RSTP? What are the characterstics of the ports that differ from STP? |
|
Definition
|
Root Desginated Alternate - will become root port if the primary root port fails Backup - will become the desginated port if the desginated port fails
|
|
|
Term
| What is RSTP and what are its main differences/enhancements compared to STP? |
|
Definition
|
Rapid Spanning Tree Protocol - converges faster than STP due to less port states - faster reconvergence due to lower timer delays - less need for reconvergence due to more port types for backing up forwarding ports
|
|
|
Term
|
Definition
|
VLANs logically divide - a switch into multiple independent switches at L2 - broadcast domains (each should be a domain) - subnets (each VLAN should be a subnet)
|
|
|
Term
| What are the advantages of VLANs: |
|
Definition
|
- increase number of broadcast domains while reducing their size - additional security - increase flexibility of equipment - allow logical grouping of users by function (not location) - make adding/editing users easier
|
|
|
Term
| What are the main steps to implementing a VLAN? |
|
Definition
|
1. Create the VLAN switch(config)#vlan n
2. Name the VLAN 3. Assign switch ports to the VLAN switch(config-if)#switchport access vlan n
|
|
|
Term
| What does Dynamic VLAN Membership do and what service does it require? |
|
Definition
|
Dynamic VLAN Membership automatically assigns switchports to a VLAN based on the MAC of the connected host Requires VMPS (VLAN Membership Policy Server) to run on the switch
|
|
|
Term
| What are the characteristics of a trunk port? |
|
Definition
|
Trunk ports connect multiple VLANs with a single link between switches (by default, all VLANs are allowed over a trunk)
|
|
|
Term
What are the two supported VLAN frame tagging trunk encapsulations? What are their main characteristics and which is more common? |
|
Definition
|
ISL - Cisco proprietary; adds 26 byte header & 4 byte trailer; re-encapsulates original frame 802.1Q - IEEE standard; inserts 4 byte tage into header; more common
|
|
|
Term
What are the characteristics of the five switchport modes? 1. off 2. on 3. NoNegotiate 4. (Dynamic) Desirable 5. (Dynamic) Auto
|
|
Definition
|
1. off - access port that does not trunk 2. on - trunks unconditionally if neighbor can trunk 3. NoNegotiate - trunks even if neighbor disagrees 4. Desirable - solicits trunk with neighbor 5. Auto - only trunks if solicited by neighbor
|
|
|
Term
What switch port mode on the neighboring port is required to trunk for the following port modes: 1. On 2. NoNegotiate 3. Desirable 4. Auto
|
|
Definition
|
1. On - On, Auto, Desirable, NoNegotiate 2. NoNegotiate - On, NoNegotiate 3. Desireable - On, Auto, Desirable 4. Auto - On, Desirable
|
|
|
Term
What are the commands to configure the following port modes: 1. off 2. on 3. NoNegotiate 4. (Dynamic) Desirable 5. (Dynamic) Auto
|
|
Definition
|
1. switchport mode access 2. switchport mode trunk 3. switchport nonegotiate 4. switchport mode dynamic desirable
5. switchport mode dynamic auto
|
|
|
Term
| What is the command to set the trunk encapsulation to ISL or 802.1Q? |
|
Definition
|
switchport trunk encapsulation isl switchport trunk encapsulation dot1q
|
|
|
Term
| What does VTP stand for and what does it do? |
|
Definition
|
VLAN Trunking Protocol takes care of creating and naming VLANs on all switches in the system
|
|
|
Term
What are the three VTP switch modes? What do they do? |
|
Definition
|
1. server mode - the switch in charge of VLAN info for the VTP domain and sends to clients 2. client mode - switches get VLAN info from server but does not save to NVRAM; commands to add/edit VLAN are disabled 3. transparent mode - switch forwards but does not accept changes from server; VLAN commands take effect locally
|
|
|
Term
| What are the four elements required for a switch to properly communicate with VTP? |
|
Definition
|
1. All switches connected by working trunk links
2. Unique domain name configured identically on all switches in the VTP system (vtp domain [name])
3. One server mode switch 4. Same encapsulation / tagging ID (ISL, dot1q)
|
|
|
Term
What is the purpose of inter-VLAN routing? What devices are required? How does it work? |
|
Definition
|
Route traffic from one VLAN to another Requires a L3 device to route (router) Trunk link connecting router to the switch. Then subinterfaces are configured on the router interface for each VLAN.
|
|
|
Term
| What kind of physical connections can be used for trunking? |
|
Definition
|
100 Mbps Ethernet (FastEthernet) 1 Gigabit Ethernet 10 Gigabit Ethernet
|
|
|
Term
| What does portfast do and where is it typically used? |
|
Definition
|
Immediately brings up access or trunk link from blocked to forwarding state. Normally used on ports connecting to an end host, or to trunks.
|
|
|
Term
| How is inter-VLAN routing configured on the router subinterfaces? |
|
Definition
For each subinterface:
Router(config-sub-if)#encapsulation dot1q 1 native or Router(config-sub-if)#encapsulation dot1q [vlan#]
|
|
|
Term
| What kind of interfaces are displayed in the "show vlan" output? |
|
Definition
|
access mode (non-trunk) ports
|
|
|
Term
| What is the command to desginate a VLAN for all untagged (default) traffic? |
|
Definition
|
switchport trunk native <vlan>
|
|
|
Term
| What is the command to set the VTP mode? |
|
Definition
|
vtp mode [server | client | transparent]
|
|
|
Term
| What is the purpose of the VTP revision number? |
|
Definition
|
Indicates how current the update is. VTP client will only copy changes from a higher revision update.
|
|
|
Term
| What is the difference between configuring a VLAN and a VLAN interface? |
|
Definition
|
VLAN divides a switch at L2 to separate broadcast domains, and have switchports assigned. VAN interface is a virtual interface that is the only place on a L2 switch that will accept an IP address/mask. VLAN 1 interface is default and used for managing switch remotely.
|
|
|
Term
| What is the command to desginate a VLAN for all untagged (default) traffic? |
|
Definition
|
switchport trunk native <vlan>
|
|
|
