November 27, 2007

Switchport Configurations Explained

It always helps me to think of the English translation when trying to memorize and understand some the Cisco IOS settings I think are important.

Here are some Cisco IOS switchport configurations translated into English:

‘switchport mode trunk‘ says: “Always trunk on this end, and I will send DTP to attempt to negotiate a trunk on the other end.”

‘switchport nonegotiate‘ says: “Do not send or respond to DTP from this end. Disable all DTP on this port.” (Best used on user access ports, when trunking to non-Cisco switches, when trunking to a router1, or if you are paranoid about fast convergence2)

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Filed Under: CCIE, Switching Tagged With: 6 Comments
November 27, 2007

VLAN Trunking using IEEE 802.1Q

IEEE 802.1Q (sometimes referred to as 1Q or DOT1Q) is a industry standards based implementation of carring traffic for multiple VLANs on a single trunking interface between two Ethernet switches. 802.1Q is for Ethernet networks only.

Unlike ISL , 802.1Q does not encapsulate the original Ethernet frame.

For Ethernet V2 frames, 802.1Q inserts a new 4-byte field between the SA and TYPE fields of the original Ethernet frame. Since the addition of this new DOT1Q field changes the original frame, 802.1Q also recomputes and rewrites the original 4-byte FCS at the end of the frame.
802.1Q Header Insertion into Ethernet II frame

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Filed Under: CCIE, Switching Tagged With: 1 Comment
November 26, 2007

VLAN Trunking using ISL

Inter-Switch Link (ISL) is a Cisco specific implementation of trunking multiple VLANs between two Cisco switches where a single interface will carry traffic for more than one VLAN. ISL was designed to work with Ethernet, FDDI, Token Ring, and ATM.

ISL completely encapsulates the original Ethernet frame by adding a new 26 byte header and new 4 byte FCS trailer. The new ISL header contains a VLAN ID field and QoS priority information (among other things).

ISL Header (click image to enlarge)

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Filed Under: CCIE, Switching Tagged With: 3 Comments
November 21, 2007

Things to know about VTP

Some notes about VTP (VLAN Trunking Protocol):

  • Cisco switches running Cisco IOS store VTP and VLAN information in a separate database stored in Flash, in file called vlan.dat.
  • Cisco switches running CatOS store VTP and VLAN information in the main switch configuration file, stored in NVRAM.
  • VTP information is only transmitted over trunk ports.
  • A VTP client does not need a VTP domain name to be configured to learn VLANs. If the domain name is left blank it will configure itself with the domain name learned from the first advertisement from the VTP server.
  • A VTP advertisement contains the VLANs configured in the domain that are allowed on the trunk, the VTP domain name, and the VTP configuration revision number.
  • If the revision number received in a VTP advertisement is greater than the current stored revision number, the switch will accept the new configuration and overwrite its existing vlan.dat file with the newly received VLAN configuration.

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Filed Under: CCIE, Switching Tagged With: 2 Comments
November 21, 2007

Identifying Ethernet Multicast

Just like there are 3 different Ethernet header types, there are also 3 different types of Ethernet addresses:

  • Unicast
  • Broadcast
  • Multicast

A unicast frame contains the unique MAC address of the destination receiver. A broadcast frame contains all binary 1′s as the destination address (FFFF.FFFF.FFFF). A multicast frame contains the unique multicast MAC address of an application, protocol, or datastream.

Why is it important to be able to distinguish between the 3 types of Ethernet address type? In an Ethernet switch, each of the three are treated differently.

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Filed Under: CCIE, Switching 7 Comments
November 19, 2007

Identifying Ethernet Header Types

There are 3 different Ethernet Header types defined by the IEEE and in use today. So, one question comes to mind: When a Ethernet receiver receives a frame, how does it know what kind of header it is? After all, if a receiver is unable to properly recognize the header type, it will not be able to properly extract the frame data and send it to the proper upper layer protocol or application.

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Filed Under: CCIE, Switching 4 Comments