Jaycee's Networking

January 9, 2009


Filed under: IOS, VLAN, VTP — Tags: , — Jaycee @ 5:53 pm

1. A trunk — an interface or link that can carry frames for multiple VLANs at once.
=> switches are connected at layer 2 using trunks.

2. How Trunks Work

(1) IP packets have no concept of VLANS, and nor does TCP, UDP, ICMP, or any other protocol above layer 2.
(2) A VLAN is a layer-2 concept, so if there were to be any mention of a VLAN, it would happen at the data-link layer.
(3) The protocols for trunking:

a. ISL — Cisco’s Inter-Switch Link
b. 802.1Q — IEEE standard

(4) To accomplish the tagging of frames to be sent over a trunk, both sides must agree to a protocol.
=> may configure ISL and 802.1Q trunks on the same switch and in the same network.

(5) Cisco 2950 and 4000 only support 802.1Q.
(6) To determine whether a switch can use a specific trunking protocol:

a. IOS-Switch# show interface capabilities
b. Catos-Switch# show port capabilities

(7) ISL differs from 802.1Q:

a. ISL is a Cicso proprietary protocol; 802.1Q is an IEEE standard.
b. ISL encapsulates Etherenet frames within an ISL frame; 802.1Q alters existing frames to include VLAN tags.
c. ISL is only capable of supporting 1,000 VLANs; 802.1Q is capable of supporting 4,096.

3. ISL

(1) If an Ethernet frame has been created at the maximum size of 1,518 bytes, ISL will add an additional 30 bytes, for a total frame size of 1,548 bytes.
(2) These frames may be counted as “giant” frame errors. Cisco Equipment has no problem accepting them.

4. 802.1Q

(1) 802.1Q inserts 4-byte tag field into existing headers b/w the Source Address and Type/Length fields.
(2) FCS (Frame Check Sequence) of the frame is altered to relfect to the change.
(3) The maximum size for an 802.1Q frame is 1,522 bytes.
(4) It may result in “baby giant” frame errors. The frames will still be supported on Cisco devices.

5. Which Protocol to Use

(1) Catalyst 4000 only support 802.1Q
(2) 10-Gb blade available for the Catalyst 6509 only supports 802.1Q
(3) Catalyst 6509 switch supports both 802.1Q and ISL
(4) System Requirements to Implement Trunking
(5) The trunking protocol is local to each individual trunk.

6. Trunk Negotiation — DTP (Dynamic Trunking Protocol)

(1) includes the VTP domain name in the process
=> both switches must have the same VTP domain name

(2) DTP (Dynamic Trunking Protocol)

a. determine what trunking protocol are supported on each side
b. establish a trunk if possible
c. switchport nonegotiate disable DTP

(3) Possible switch port modes related to trunking:

switchport mode

a. mode access — the port will never be a trunk
b. mode trunk — the port will be a trunk regardless of any other settings
c. mode dynamic desirable — the port will attempt to convert the link to a trunk
d. mode dynamic auto — (default mode) the port will become a trunk if the other side is configured to be a trunk. It’ll not attempt to convert a link to a trunk.

i. dynamic — means the port may become a trunk
ii. desirable — indicates the port will initiate negotiations and try to make the link a trunk
iii. auto — indicates the port will listen for DTP but will not actively attempt to become a trunk

(4) Switchport Mode

7. Configuring Trunks on IOS switch:

(1) Configuring a trunk determine:

a. what port will be a trunk
b. what protocol the trunk will run
c. whether and how the port will negotiate
d. what VLANs are allowed on the trunk link

(2) On an IOS switch capable of both ISL and 802.1Q, you must specify a trunk encapsulation before you can configure a port as a trunk.

3550-IOS(config-if)# switchport mode encapsulation dot1q
3550-IOS(config-if)# switchport mode trunk

(3) Remove trunking for the interface, the command to do so is switchport mode access.

(4) By default all VLANs on a switch are included in a trunk.
(5) Broadcasts from all allowed VLANs will be sent on every trunk port, excluding unneeded VLANs can save a lot of bandwidth on the trunk link.
(6) Specify which VLANs are able to traverse a trunk with the switchport trunk allowed command.
(7) More options for switchport trunk allowed vlan:

3550-IOS(config-if)# switchport trunk allowed vlan ?
WORD VLAN IDs of the allowed VLANs when this port is in trunking mode
add add VLANs to the current list
all all VLANs
except all VLANs except the following
none no VLANs
remove remove VLANs from the current list

(8) Allow only one VLAN (VLAN 100) on a trunk:

3550-IOS(config-if)# switchport trunk allowed vlan 100

(9) Show trunk port:

3550-IOS(config-if)# sho int trunk

(10) Allow all VLANs except VLAN 100:

3550-IOS(config-if)# switchport trunk allowed vlan except 100

(10) Remove VLANs 200 and 300:

3550-IOS(config-if)# switchport trunk allowed vlan remove 200
3550-IOS(config-if)# switchport trunk allowed vlan remove 300



  1. cisco waas…

    Cisco WAAS seems good enough if you only have a few sites…

    Trackback by cisco waas — January 15, 2009 @ 4:12 am

  2. thanks for the lesson. wish we can study together.

    Comment by ronky — January 18, 2009 @ 12:48 am

    • Knowledge is unlimited…. Let’s all study together, and learn from each other!

      Comment by Jaycee — January 18, 2009 @ 3:36 am

  3. Whats up? New here and just figured that I should post and say hi.

    Comment by snustispany — May 30, 2010 @ 10:28 am

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