- native VLAN mismatches - trunk mode mismatches - VLANs and IP Subnets - allowed VLANs on trunks

- grouping users by department instead of physical location - segmenting devices into smaller LANs to reduce processing overhead for all devices on the LAN - reducing the workload of STP by limiting a VLAN to a single switch - enforcing better security by isolating sensitive data to separate VLANs - separating IP voice form IP data

Switching Flashcards by Daniel Schaaff

Features of access layer switches

Port security
VLANs
Fast/gigabit Ethernet
PoE
Link aggregation
QoS

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Features of distribution layer and core switches

Layer 3 support
High forwarding rate 
Gigabit/10 gigabit Ethernet 
Redundant components 
Security policies/ACLs
Link aggregation
QoS

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5 basic LAN switch functions

Learning
Aging
Flooding
Selective forwarding
Filtering

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Data VLAN

Configured to carry only user-generated traffic, ensuring that voice and management traffic is separated from data traffic

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Default VLAN

All the ports on a switch are members of the default VLAN when the switch is reset to defaults. The default VLAN for Cisco is VLAN 1.

It is best practice to restrict VLAN 1 to serve as a conduit only for layer 2 control traffic (CDP, VTP, etc.) and no other traffic

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Black hole VLAN

Security best practice is to define a black hole VLAN to be a dummy VLAN distinct from all other VLANs defined in the LAN. All unused switch ports are assigned to the black hole VLAN

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management VLAN

a VLAN defined by the network admin as a means to access the management capabilities of the switch

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voice VLAN

separates voice traffic from data traffic

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how are VLANs identified?

IEEE 802.1q frame tagging

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VTP

cisco’s VLAN trunking protol
-layer 2 messaging protocol that maintains VLAN config consistency by managing the additions, deletions, and name changes of VLANs across networks

-VTP domain boundaries are by created by routers or layer 3 switches

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3 VTP device modes

Server-where VLANs are created, deleted, or renamed and changes advertised

client-receives config from server, can’t create its own

transparent-doesn’t participate, simply forwards VTP messages

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STP

Spanning Tree Protocol

IEEE 802.1D
creates a tree that ensures there is only one one to each network segment at any one time.

If any segment experiences a disruption in connectivity, STP rebuilds a new tree by activating the previously inactive, but redundant, path.

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BPDU

bridge protocol data unit

-message format used by STP

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BID

bridge ID

unique to each switch and contains a priority

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root bridge

bridge with lowest priority number

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troubleshooting STP

1) determine root switch
2) for each non root switch, determine its root port and cost to reach the root switch thru that port
3) for each segment, determine the designated port and the cost advertised by the DB of that segment

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configure VLAN in global config mode

VLAN vlan-id name vlan-name

apply to interface

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common VLAN errors

native VLAN mismatches
trunk mode mismatches
VLANs and IP Subnets
allowed VLANs on trunks

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VTP troubleshooting step 1

confirm switch names, topology, and VTP modes
identify sets of 2 neighbor switches that should be either VTP clients or servers or VLAN databases differ
on those switches verify trunk between them, same VTP domain, same VTP password
fix configuration issues

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reasons for VLANs

grouping users by department instead of physical location
segmenting devices into smaller LANs to reduce processing overhead for all devices on the LAN
reducing the workload of STP by limiting a VLAN to a single switch
enforcing better security by isolating sensitive data to separate VLANs
separating IP voice form IP data

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data VLAN

configure to carry only user-generated traffic, ensuring that voice and management traffic separated from data traffic

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default VLAN

all the ports on a switch are members of the default VLAN when the switch is reset to defaults

the default VLAN for cisco is VLAN 1.

It is best practice to restrict VLAN 1 to serve as a conduit only for layer 2 control traffic (CDP, VTP, etc.) and no other traffic

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black hole VLAN

security best practice is to define a black hole VLAN to be a dummy VLAN distinct from all other VLANs established in the LAN.

all unused switch ports are assigned to the black VLAN

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How often are VTP advertisements sent?

5 mins

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When viewing the show interface output on a catalyst switch, you notice a large number of runts, what could this indicate?

There is a possible problem with the duplex settings of the port

A catalyst switch is running the default STP type. What type of STP is this?

PVST+

All possible switch port modes for DTP

Trunk, access, dynamic desirable, dynamic-auto, no-negotiate

Which protocol allows switches running RSTP to configure a group of VLANs into a single instance of STP?

MSTP multiple spanning tree protocol

Where do you want the root bridge to ideally be located?

At the center of the network in order to reduce STP convergence times Changing the default priority of the switch is the best way to accomplish this

Designing switched network for fast STP convergence

Use hierarchal design, make core switch the root bridge

Portfast

Used to make a switch port move directly to forwarding mode. Usually used for ports with servers that can't experience downtime during convergence

BPDU gaurd

Puts a switch port into an error disabled state if it receives a BPDU with port fast enabled. This prevents you from plugging a switch or hub into a port fast port and creating a loop

BPDUfilter

Immediately takes a port out of port fast mode if a BPDU is received, leaves the port up unlike BPDUGuard

Command to see how many VLANs a switch will support

Show VTP status

How to configure voice VLAN

Use mls qos command to set quality of service Set qos trust settings for port Switch port voice VLAN dot1p

switchport dynamic desirable mode

Triggers the port to negotiate the link from nontrunk to trunk mode. The port negotiates to a trunk port if the connected device is in trunk state, desirable state, or auto state. Otherwise, the port becomes a nontrunk port.

switchport dynamic auto mode

Enables a port to become a trunk only if the connected device has the state set to trunk or desirable. Otherwise, the port becomes a nontrunk port.

command to determine current dtp mode

show dtp interface

which command is used to verify trunk establishment?

show interfaces trunk

PVST+

a Cisco enhancement of STP that provides a separate 802.1D spanning-tree instance for each VLAN that is configured in the network.

RSTP

802.1w an evolution of STP that provides faster STP convergence. This version addresses many convergence issues but, because it still provides a single instance of STP, it does not address the suboptimal traffic flow issues

Rapid PVST+

a Cisco enhancement of RSTP that uses PVST+. It provides a separate instance of 802.1w per VLAN.

MSTP

MSTP is an IEEE standard inspired by the earlier Cisco proprietary MISTP implementation. To reduce the number of required STP instances, MSTP maps multiple VLANs that have the same traffic flow requirements into the same spanning-tree instance

What is the default spanning tree mode on Catalyst switches?

PVST+

command to configure switch as root for a specific vlan

spanning-tree vlan# root primary

command to configure switch as secondary root for specific vlan

spanning-tree vlan# root secondary

command to set the spanning-tree type or mode on a switch

spanning-tree mode *modetype

stages a switchport goes through before forwarding (spanning-tree)

blocking listening learning forwarding

commands to configure portfast as default on all on trunk ports and enable BPDU gaurd on all ports

SwitchX(config)# spanning-tree portfast bpduguard default | SwitchX(config)# spanning-tree portfast default

PAgP

Port Aggregation Protocol. A Cisco proprietary protocol that enables ports with similar characteristics to form an EtherChannel through dynamic negotiation with adjoining switches

LACP

Link Aggregation Control Protocol. An IEEE standard (802.3ad) that enables ports with similar characteristics to form an EtherChannel through dynamic negotiation with adjoining switches

command to form an etherchannel

channel-group channel-group-number mode { active | on | auto | desirable | passive }

PAgP modes

auto | desirable

LACP modes

Active | Passive

how to modify port settings for an ether channel

use etherchannel config mode, not individual interface config mode ie interface port-channel 1

command to display detailed info about port channel

show etherchannel port-channel

STP cost of 10 Mb/s link

100

STP cost of 100 Mb/s link

STP cost of 1000 Mb/s link

STP cost of 10,000 Mb/s link