Dec 16, 2007 0
Lan Switching part 1: Ethernet, Vlan & Trunk
Notes takes during my layer 2 reading.
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
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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