Ethernet Errors and Problems

IST 228\Ch6:Virtual LANs (VLANs) 1

Ethernet Errors and Problems• Frame size errors

– Short frame– Long frame– Jabber

• Collision Errors– What percent collision rate is acceptable?– Late collision?– Why late collisions occur? How to prevent them?– Solutions to prevent collisions?

• Broadcasts– How much broadcast traffic is acceptable? – How to reduce the amount of broadcast traffic?


Ethernet Security Problems

• By default, all users can see all network devices located on a physical LAN.

• Anyone can observe all network traffic by plugging a network analyzer into a hub.


Solution(s)?

• Securing Switch Ports– limiting device connections

1900s2(config)#mac-address-table permanent <MAC Address> e0/7

– limiting communication1900s2(config)#mac-address-table restricted static <MAC

Address> e0/6 e0/12

– limiting the number of devices1900s2(config)#interface e/10

1900s2(config-if)#port secure max-mac-count 5


Solution(s)?• Switching Methods

– Cut-Through

– Store-and-Forward Switches

– Fragment-Free Switching

– Adaptive Cut-Through

1900s2(config)#switching-mode store-and-forward


Solution(s)? • Use bridges • Use switches• Use routers to

segment a network (Collapsed Backbone Approach)

• Advantages • Disadvantages


Virtual LANs (VLAN)• A VLAN is a logical

grouping of network devices connected to defined ports on switches.

• Each VLAN is a broadcast domain within a layer-2 switched network, created by assigning ports on switches to different subnetworks.


Benefits

• Simplified Network Management• Broadcast Control• Security• Flexibility and Scalability

– 20% to 40% of the workforce is on move every year.

– Implementing changes and moves constitutes 23% of the total network administration cost.


VLAN Membership

• Static VLANs– the most secure

• Dynamic VLANs– use an intelligent software to assign

ports.


Creating VLANs on Cisco Catalyst 19001900s2>enable

1900s2#configure terminal

1900s2(config)#vlan 2 name engineering

1900s2(config)#vlan 3 name marketing

: :

1900s2(config)#exit

1900s2#show vlan


Assigning Ports to VLANs1900s2(config)#interface e0/2

1900s2(config)#vlan-membership static 2

1900s2(config)#exit

1900s2(config)#interface e0/3

1900s2(config)#vlan-membership static 2

1900s2(config)#exit

: :

1900s2#show vlan

Use the show vlan [#] command to gather information about a specific vlan.


Link Types and Configuration• Access Links

– A device attached to an access link is unaware of a VLAN membership.

– Access-link devices cannot communicate with devices outside their VLAN unless...

• Trunk links– Switch-to-switch, switch-to-server, or switch-to-

router 100- or 1000Mbps links that can carry traffic from multiple VLANs.

– Five different states you can set for a trunk link:• Auto• Desirable• Nonegotiate• Off• On


Trunk Links


Configuration of Trunk links1900s2#configure terminal

1900s2(config)#interface f0/26

1900s2(config-if)#trunk on

By default, all VLANs are enabled to be routed over this newly configured trunk link.1900s2(config-if)#no trunk-vlan 2

You cleared VLAN 7 from the trunk. 1900s2#show trunk a allowed-vlans


VLAN Trunk Protocol (VTP)

• Layer 2 messaging protocol• Manages all changes to the VLANs

across networks.• Any changes made to a VLAN by an

administrator are automatically propagated by VTP to all VTP-enabled devices.

• No need for VTP if there is only a single switch or if all switches are in the same VLAN.


Using VTP• VTP devices are organized into domains.• Each domain must have at least one VTP

server. • Each switch can only be in one domain.1900s2(config)#VTP domain <domain name>

• Three different modes– Server– Client– Transparent

1900s2(config)#VTP client

• VTP Pruning: Reduces the number of VTP updates that traverse a link. By default, VTP pruning is disabled on all switches.

1900s2(config)#vtp pruning enable1900s2(config)#delete vtp


Nonswitching Hubs and VLANs• Considerations to keep in mind when

implementing hubs on a network that employs VLANs:– If you insert a hub into a port on a switch and then

connect several devices to the hub, all the system attached to that hub will be in the same VLAN

– If you must move a single workstation that is attached to a hub with several workstations, you will have to physically attach the device to another hub or switch ports in order to change its VLAN assignment

– The more hosts attached at individual switch ports, the greater the microsegmentation and flexibility the VLAN can offer


Routers and VLANs

• Routers are used with VLANs– to communicate each other– to increase security


VLAN Identification

• Inter-Switch Link (ISL)• IEEE 802.1Q• LAN emulation• 802.10 (FDDI)

Documents

Ethernet Errors and Problems