13: VLANs and Trunks
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the switch-to-switch connection as an 8021Q trunk but set the native VLAN as the dark one, so that frames from this VLAN would go untagged across it and allow PC-G to process them One restriction placed on an 8021Q trunk configuration is that it must be the same on both sides In other words, if the dark VLAN is the native VLAN on one switch, the switch at the other end must have the native VLAN set to the dark VLAN Likewise, if the white VLAN is having its frames tagged on one switch, the other switch must also be tagging the white VLAN frames with 8021Q information With the 8021Q tagging method, the original Ethernet frame is modified A 4-byte field, called a tag field, is inserted into the header of the original Ethernet frame, and the original frame s FCS (checksum) is recomputed on the basis of this change The first 2 bytes of the tag are the protocol identifier For instance, an Ethernet type frame has a protocol identifier value of 0x8100, indicating that this is an Ethernet tagged frame The next 3 bits are used to prioritize the frame, which is defined in the IEEE 8021p standard The fourth bit indicates if this is an encapsulated Token Ring frame (Cisco no longer sells Token Ring products), and the last 12 bits are used for the VLAN identifier (number)
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Figure 13-7 shows the process that occurs when tagging an Ethernet frame by inserting the 8021Q field into the Ethernet frame header As you can see in this figure, step 1 is the normal, untagged Ethernet frame Step 2 inserts the tag and recomputes a new FCS value Below step 2 is a blow-up of the actual Tag field As you can see in this figure, the tag is inserted directly after the source and destination MAC addresses in 8021Q is a standardized the Ethernet header trunking method that inserts a 4-byte One advantage of using this tagging eld into the original Ethernet frame and mechanism is that since you are adding only recalculates the FCSThe 2950s and 2960s 4 bytes, your frame size will not exceed 1518 bytes, support only 8021Q trunking The native and thus you could actually forward 8021Q VLAN contains untagged frames, even on frames through the access-link connections of trunk connections switches, since these switches would forward the frame as a normal Ethernet frame
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The VLAN Trunk Protocol (VTP) is a proprietary Cisco protocol used to share VLAN configuration information between Cisco switches on trunk connections VTP allows switches to share and synchronize their VLAN information, which ensures that your network has a consistent VLAN configuration
13: VLANs and Trunks
Assume, for instance, that you have a network with two switches and you need to add a new VLAN This could easily be accomplished by adding the VLAN manually on both switches However, this process becomes more difficult and tedious if you have 30 switches In this situation, you might make a mistake in configuring the new VLAN on one of the switches, giving it the wrong VLAN identifier, or you might forget to add the new VLAN to one of the 30 switches VTP can take care of this issue With VTP, you can add the VLAN on one switch and have this switch propagate the new VLAN, via VTP messages, to all of the other switches in your layer 2 network, causing them to add the new VLAN also This is also true if you modify a VLAN s configuration or delete a VLAN VTP can verify that your VLAN configuration is consistent across all of your switches VTP can even perform consistency checks with your VLANs, to make sure that all the VLANs are configured identically For instance, some of these components of a VLAN include the VLAN number, name, and type If you have a VLAN number of 1 and a name of admin on one switch, but a name of administrator on a second switch for this VLAN, VTP can check for and fix these kinds of configuration mismatches VTP messages will propagate only across trunk connections, so you will need to set up trunking between your switches in order to share VLAN information via VTP VTP messages are propagated as layer 2 multicast frames Therefore, if a router separates two of your switches, the router will not forward the VTP messages from one of its interfaces to another In order for VTP to function correctly, you must associate your switch with a VTP domain A domain is a group of switches that have the same VLAN information applied to them Basically, a VTP domain is similar to an autonomous system, which some routing protocols use (autonomous systems and routing protocols are introduced in 15) A switch can belong only to a single domain Domains are given names, and when switches generate VTP messages, they include the domain name in their messages An incoming switch will not incorporate the VLAN changes in the VTP is a Cisco-proprietary received VTP message if the domain name protocol that traverses trunks It is used in the message doesn t match the domain to create a consistent VLAN con guration name configured on itself In other words, across all switches in the same domain: a switch in one domain will ignore VTP A VLAN can be added on one switch and messages from switches in other domains The propagated to other switches; however, following sections cover the components and switches will ignore VTP messages from messages that VTP uses, as well as some of the other VTP domains advantages that it provides, such as pruning