Lan Switching part 1: Ethernet, Vlan & Trunk

Written on December 16, 2007 – 5:35 pm | by lessaid |

Notes takes during my layer 2 reading.
Switching suff

Cross-over cable ( T568A ) : KeY pins 1-3; 2-6

E1 Cable : Key pins : 1-2;4-5

Auto-MDX allow to use the wrong cable ( bot all cisco switch models)

Switches are on auto-negotiation mode ( auto-determine speed and duplex ), use FLP (Fast Link Pulse) to “detect speed”, if auto-nego is disabled, half-duplex will be used.

CSMA/CS define how to retransmit frame when collision happen.

1 port on a switch = 1 domain collision

802.3u IEEE standard for Fast-Ethernet (100 Mbps)

802.3z IEEE standard for Giga-Ethernet (1 Gbps) over copper or fiber

802.3ab IEEE standard for Giga-Ethernet (1 Gbps) over UTP

UTP cat 5/5e/6 = 1Gbps(4 pairs) and 100m

Funny thing is that when you try to translate “copper cabling”, on Google translator, to french, it show “Livebox” … ^^

Switches stuff :

Switches does :

Learn MAC address from examining the source Mac address.

Decide of Forwarding/Filtering frames based on destination Mac address

Loop-free on multiple bridged environment via the Spanning-Tree Protocol.

3 switch method :

Store n forward : Check the FCS for error-free frames.

Cut-through: Low latency mode, first bits send before the final bits get received.

Fragment-free: Like cut-through, but check the first 64 bytes before forwarding, prevent error due to collision.

Definition: Auto-negotiation, half duplex, cross-over cable, straight-through cable, unicast, multicast and broadcast address, loopback cour-circuitry, I/G Bits, U/L Bit, CSMA/CD

Lan Switching part 1: Ethernet, Vlan & Trunk Vlan/Trunk stuff

On 1 vlan = 1 broadcast domain.

Vlan is an administrative subnetting on the switches ports .

best practice 1 vlan with 1 subnet, but can multiples subnet within 1 vlan.

Forwarding frames on different Vlan ( inter-vlan routing) , use routers or MLS (Multi Layer Switch).

ISL vs 802.1Q : Both use 12 of the 15 bits VLAN ID Field ( ISL use to deal with 10bits ) so then support extended range ( 1006-4094 not stored vlan.dat), ISL Encapsule the entire frame (26 byte header + 4 byte CRC as tail), 802.1Q insert tag ( frame tagging , 4 bytes )

Trunk support vlan 1-4094

VTP: Cisco’s method to distribute VLAN configuration.

VTP Option: domain, password, mode, version, pruning, interface.

VTP Modes : Server, Client, Transparent and Off ( only on CatOS)

To use extended vlan ( 1006-4094 ), switches vtp configuration need to be in transparent mode.

Good reading for VTP here .

DTP, Dynamic Trunk Protocol : Detect trunk and witch protocol use. ( on desirable mode => ISL as default ) via the switchport mode command line

DTP modes:
Access : Forces the port into a permanent non-trunk mode.
Trunk : Forces the port into a permanent trunk mode and negotiates with the connected device on the other side to convert the link to trunk mode.
Auto : The port becomes a trunk port if the neighboring port is in a Trunk or Desirable mode. This is the default mode.
Desirable : The port attempts to become a trunk port if the neighboring port is in a Trunk , Desirable or Auto mode.
NoNegotiate : The port becomes a trunk port but does not use DTP

802.1Q-in-Q Tunneling : Allow to preserve 802.1q tags over a WAN.

Definition : VLAN, Broadcast domain, DTP, VTP Pruning, 802.1Q, ISL, Native VLAN, encapsulation, pVlan, Promiscuous port, community VLAN, isolated VLAN, 802.1Q-in-Q, Layer 2 protocol tunneling.

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Hey, My name is Lessaid, network engineer and wannabee a CCIE. More

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