Topic 2 - LAN Switching Technologies Flashcards Preview

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What address table an switch maintains?

MAC Table


what it is, when an entry ages out and is discarded from the MAC address table?

MAC Aging


Store-and-forward switches

Store the entire frame in internal memory and check the frame for errors before forwarding the frame to its destination.


Cut-through switching

the LAN switch copies into its memory only the destination MAC address, which is located in the first 6 bytes of the frame following the preamble.

The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards the frame on to its destination through the designated switch port.


Fragment-free switching

- Fragment-free switching can be viewed as a compromise between store-and-forward switching and cut-through switching.
- The reason fragment-free switching stores only the first 64 bytes of the frame is that most network errors and collisions occur during the first 64 bytes of a frame


Frame flooding

- Switches determine which port a frame must be sent out to reach its destination.
- If the address is known, the frame is forwarded only on that port.
- If the layer 2 MAC address is unknown, the frame is flooded to all ports except the one from which it originated.


Auto Negotiation

- Switch and the Pc auto negotiate at what speed the data needs to be transferred.
- should be enabled on only one end of the device


Interpret Ethernet frame format

1. Preamble – 7 bytes – used for synchronizing the Layer 2 hardware (indicates that a Ethernet Frame follows)

2. Start Frame Delimiter – 1 byte – Indicates that the next byte begins the Destination Mac Field

3. Destination Mac Address  – 6 bytes

4. Source Mac Address – 6 bytes

5. Length Or Type
Length – 2 bytes – defines the length of the data
field of the frame
Type – 2 bytes – defines the type of protocol
listed inside the frame

6. Date and Pad – 46 to 1500 bytes – Hold the PDU/IP packet

7. Frame Check Sequence (FCS) – 4 bytes – contains a CRC check of the frame – used to check the frame for errors.


Duplex and Speed?

- Duplex and speed should match on both ends or else you will have problems
- Traffic can still pass with mismatched duplex and speed, but you will experience re transmissions and reduced throughput


What port and protocol does the following use?

1 - SSH
2 - Telent

SSH - TCP & port 22 by default.
Telnet -TCP & port 23 by default.


What is the protocol used on Trunk port?



What is the proprietary trunking protocol?

ISL ( Inter - Switch link )


What is a Native VLAN?

The native VLAN is a way of carrying untagged traffic across one or more switches. which comes in untagged on a trunk port.


What is the default Native VLAN?



Spanning Tree protocol ( STP)

- STP Monitors the network to find all links, making sure that no loops occurs by shutting down any redundant link

- Adding redundant paths and prevent the loop/broadcast storms, unstable mac address table, duplicate frames

- Not only does spanning-tree blocks redundant paths but also re-opens them in case of a link failure.

-  All switches communicate using Bridge Protocol Data Units (BPDU) with each other.


STP Steps?

To obtain a loop-free topology STP uses three steps mentioned below
- Elect one Root Bridge
- Select one Root Port on the non-root bridges
- Select one Designated Port on each segment


Electing a Root Bridge:

Bridge with the Lowest Bridge ID is elected as the Root Bridge.

Bridge ID / System ID which is the burned in MAC address of the Switch.

Initially, all the switches with advertise themself as the Root bridge, once they receive the message from Superior BPDU, they start forwarding packets to them.


Bridge priority range?

From 0-65535, with 32768 being default.


Selecting Root Port ?

a port that has the lowest cost path towards the Root Bridge.

- This is a port that has the lowest cumulative path cost towards the Root Bridge.
- A Root Port is an upstream facing port that always points towards the Current Root Bridge.


Selecting Designated Port?

The Port that advertises the lowest Root Path Cost onto the segment is elected as Designated Port.)

Ports on the neighbor switch opposite the root port are designated port. - Simple.


What are the five STP Port states? and what do they do?

- Disabled -  All Ports when administratively shut down fall in the category of Disabled State.

- Blocked port - A blocked port is the port that, in order to prevent loops, will not forward any frames, however listen to frames.

- Listening - This port listens to the BPDU to make sure no loops occur on the network before forwarding data frames

- Learning - This port listens to BPDU and learns all the path in the network

- Forwarding - After another Forward Delay the port is transitioned to Forwarding state and now it can send and receive traffic and is a fully functional port.
Forward Delay (15 Seconds by Default


Types of STP?

STP: Standard and normal stp - Uses one STP for all VLANs in LAN

RSTP: Rapid Spanning Tree OR 802.1w - Improved STP for rapid convergence - Improved Convergence time ( few seconds ) to find if there is any loop in the network and start forwarding and uses one STP for all VLANS on LAN

MSTP: Multiple Spanning Tree Protocol (MSTP) - Enables grouping and mapping VLANs into different spanning tree instance for load balancing

PVST+ - Per VLAN Spanning Tree Plus - Uses a separate Spanning tree instance on every VLAN. This is default on cisco switchesAnd, problem is longer Convergence time.

RSVST+ - Rapid Per VLAN Spanning Tree Plus - Uses separate ST instance too and has imprived convergence time than PVST+


HSRP explain.

Hot Standby Router Protocol -

This should be configured to match spanning tree path,
Active router: The router that is currently forwarding packets for the virtual router

Standby router: The primary backup router

Standby group: The set of routers participating in HSRP that jointly emulate a virtual router.


3 Timers used by STP?

Hello ( Default - 2 Seconds )

Max Age ( Defaults 10 times hello )

Forward Delay ( Default 15 sec)


Command to check STP?

# Show spanning-tree


Command to set Priority on STP?

#Spanning-tree Vlan1 Priority 4096 (Can be changed as needed)


Command to set the root bridge?

#Spanning - tree vlan 1 root primary


Whats a port fast?

It can take up to 50 seconds for the spanning tree to transition a port to forwarding's state when it becomes active.

- a loop cannot be formed on ports where a single end host is plugged in.

- We can make port transition to a forwarding state immediately when it becomes active by disabling Spanning tree on the ports using Port fast command


Command to enable port fast?

#Spanning - tree portfast


BPDU Gaurd?

BPDU - bridge protocol data unit

we can enable BPDU on port fast port to guard against the loop happening.

IF BPDU is received the port will shut down.