Tag: Cisco

Cisco related material

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Defining the Message Severity Level

You can set the severity level of messages according to the level of detail you need. You can also control the destinations of the messages.

Configure

The following command will set the logging to the console, this is the default.

Switch#configure terminal
Switch(config)#logging console
Switch(config)#end

If you want to send logs to a syslog server, you would send them to a host.

Switch#configure terminal
Switch(config)#logging host <ip address>
Switch(config)#end

In order to not be interrupted on the command line with messages, you can set the messages to be synchronized.

Switch#configure terminal
Switch(config)#line console 0
Switch(config-line)#logging synchronous
Switch(config-line)#end

Levels

  • Emergencies — level 0 — System unstable
  • Alerts — level 1 — Immediate action needed
  • Critical — level 2 — Critical conditions
  • Errors — level 3 — Error conditions
  • Warnings — level 4 — Warning conditions
  • Notifications — level 5 — Normal but significant condition
  • Informational — level 6 — Informational messages only
  • Debugging — level 7 — Debugging messages

By default, the console receives debugging messages (level 7) and numerically lower levels. For example, if you set the logging level to 4, you will recieve all warnings, errors, critical, alerts, and emergencies.

 

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Dynamic Trunking Protocol (DTP)

Dynamic Trunking Protocol is a Cisco proprietary protocol used to negotiate trunks between Cisco devices. To autonegotiate trunking, the interfaces must be in the same VLAN Trunking Protocol (VTP) domain. Trunk negotiation is managed by the Dynamic Trunking Protocol, which is a point-to-point protocol.

Configure

Switch# configure terminal
Switch(config)# interface interface-id
Switch(config-if)# switchport mode {access | dynamic {auto | desirable} | trunk}

Modes

  • access: Set the port to access mode. The port is set to access unconditionally and operates as a nontrunking, single VLAN interface that sends and receives nonencapsulated (non-tagged) frames. An access port can be assigned to only one VLAN.
  • dynamic auto: Set the interface trunking mode dynamic parameter to auto to specify that the interface convert the link to a trunk link. This is the default switchport mode.
  • dynamic desirable: Set the interface trunking mode dynamic parameter to desirable to specify that the interface actively attempt to convert the link to a trunk link.
  • trunk:  Set the port to trunk unconditionally. The port is a trunking VLAN Layer 2 interface. The port sends and receives encapsulated (tagged) frames that identify the VLAN of origination. A trunk is a point-to-point link between two switches or between a switch and a router.

Verify

Switch# show interfaces interface-id switchport
Name: interface-id
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On

Troubleshoot

The default switchport mode is dynamic auto, so if two switches are connected, they will not form a trunk.

Access mode will not trunk, period.

Trunk mode will change the interface to permanent trunking mode.

Dynamic auto will only trunk if the neighboring interface is set to trunk or desirable mode.

Dynamic desirable will trunk if the neighboring interface is set to trunk, desirable, or auto mode.

References

  1. https://networklessons.com/switching/cisco-dtp-dynamic-trunking-protocol-negotiation/
  2. https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/15-0_1_se/command/reference/cr_3560/cli3.html#marker-2357437
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Virtual Local Area Network (VLAN)

A Virtual Local Area Network or VLAN is a way to logically break up broadcast domains.

VLAN = Subnet = Broadcast Domain

When using multiple switches, you have to ensure that the VLAN is configured on any switch that will use the VLAN. Since a VLAN defines broadcast domains in a Layer 2 network, in order to pass traffic from one VLAN to another, you must use routers or Layer 3 switches.

Configure

A VLAN is configured in global configuration mode using the vlan keyword. VLAN configuration is stored in a vlan.dat file, not in the running-config or startup-config files.

Switch# configure terminal
Switch(config)# vlan vlan_ID
Switch(config-vlan)# end

VLAN Ranges

  • VLAN 1 is the Cisco default; it is created for you and cannot be deleted.
  • VLAN 2 – VLAN 1001 are used for Ethernet VLANs; these can be created and deleted
  • VLAN 1002 – VLAN 1005 are defaults for FDDI and Token Ring; these cannot be deleted.
  • VLAN 1006 – VLAN 4094 are considered in the extended range and are for Ethernet VLANs only. These will not be propagated by VLAN Trunking Protocol (VTP) and also will not be saved to the VLAN database unless in VTP transparent mode.

Verify

Switch# show vlan [id | name] vlan_name

Troubleshoot

Oddly enough, the Cisco exam objectives for ICND2 call out troubleshooting VLANs. While at this point there is not much for troubleshooting besides verifying that you have created VLANs, the troubleshooting objective will be necessary after further configurations have been made, such as after assigning an interface to a VLAN or with VLAN Trunking Protocol (VTP). There are no specific commands, per se, to aide in troubleshooting.