VLAN

Some facts:

• Breaks up broadcast domains in a layer 2 switch internetwork


• Inter-Switch Link (ISL) - it’s used for Fast Ethernet and Gigabit Ethernet links only


• Minimum speed for trunk – 100 Mbps


• ISL fully encapsulates the original Ethernet frame in a trunking header


• Configuring vlans on switch is possible in transparent and server mode


• Access link is only part of one VLAN and is referred to as the “native VLAN” of the port


• All switches must use the same native VLAN. Native VLAN is connected with a port, not with a switch


• “router on a stick.” – when router routes all vlans


• VLANs can greatly simplify adding, moving, or changing hosts on the network

VLAN Trunking Protocol (VTP) Layer 2

• Default VTP mode is server


• The link between the switches must be operating as a VLAN trunk (ISL or 802.1Q).


• The two switches’ case-sensitive VTP domain name must match.


• If configured on at least one of the switches, the two switches’ case-sensitive VTP password must match.


• VTP only learns about normal-range VLANs, with VLAN IDs 1 to 1005; VLANs with IDs greater than 1005 are called extended-range VLANs and they’re not stored in the VLAN database.


• VLAN IDs 1 and 1002 to 1005 are automatically created on all switches and can’t be removed.


• VTP gives you a way to preserve bandwidth by configuring it to reduce the amount of broadcasts, multicasts, and unicast packets. This is called pruning.


• Extended-range VLANs (VLAN IDs1006 to 4094) can’t be pruned


• VTP servers and clients also send periodic VTP messages every 5 minutes, in case any newly added switches need to know the VLAN configuration.


• The default VTP configuration on Cisco switches is VTP server mode with a null domain name.


• After configuring a domain name, that switch immediately starts sending VTP updates over all its trunks.


• If a switch that still has a (default) null domain name receives a VTP update—which by definition lists a domain name—and no password was used by the sending switch, the receiving switch starts using that VTP domain name.

Dynamic Tranking protocol (DTP)

• Dynamic Trunking protocol, port may be in auto, desirable or on state.

(interface) switchport mode dynamic [auto | desirable]

• on - ntunking is on and sending DTP packets.


• desireable - These links would like to become trunk links and will send DTP signals that attempt to initiate a trunk. They will only become trunk links if the other side responds to the DTP signal.


• auto – doesn’t send DTP packets, becomes trunk, if receives DTP packet.

Troubleshooting

R1#show vlan
R1#show vlan brief
R1#show interfaces switchport
R1#show interfaces trunk