Packing Your VLAN s Frames in a Trunk When Leaving the Switch


Packing Your VLAN's Frames in a Trunk When Leaving the Switch

So far, most of the examples about switching in this book showed just one switch. However, you can connect multiple switches so that devices on each switch can send frames to each other. Figure 7-7 shows the basic concept, with no VLANs configured.

Figure 7-7. Forwarding Frames Between Two Switches


The basic switch logic for each switch does not change when you use multiple switches. In this figure, both switches' address tables have been populated, and each switch knows where to forward the frames. Note that switch1's address table tells it to forward frames sent to Barney out port FastEthernet 23, to switch2. Switch2's table lists port E3 beside Barney's address, so it forwards the frame to Barney out port E3.

In the context of connecting multiple switches, the term trunk can be used a couple of different ways. For instance, the Ethernet cable segment between switch1 and switch2 can be called a trunk. In this case, trunk refers to the fact that the segment connects two switches. (The other use of the word "trunk" will be explained in the next section.)

How to Pack Your Trunk for the Trip to the Other Switch

To explain the other use of the term "trunk," Figure 7-8 adds VLANs to the network from Figure 7-7. As shown in previous figures, Fred and Barney are in one VLAN, and Betty and Wilma are in another VLAN. However, now the traffic in both VLANs must cross over the trunk between two switches. When a switch receives a frame over that trunk, it could be confused about which VLAN's MAC address table to use when deciding how to forward the frame. Figure 7-8 shows the sample network and the dilemma that switch2 faces upon receipt of the frame that Fred sent to Barney.

Figure 7-8. Switch2's Dilemma: Which VLAN?


Switch1 now has an address table for each VLAN. Its VLAN1 address table says to forward the frame for Barney (0200.2222.2222) out port Fa23. However, when switch2 receives the frame, it is confused. Is this frame part of VLAN1 or VLAN2? Should switch2 look in both address tables, hoping to find 0200.2222.2222? Should switch2 assume that the frame belongs in a particular VLAN? Because switch2 supports two VLANs, it simply needs more instructions.

To solve the problem, the switches use something called VLAN trunking. To perform VLAN trunking, before sending the frame over the Ethernet cable to the other switch, switch1 adds another header to the frame. That extra header identifies the frame as part of VLAN1. Switch2 expects frames coming in over port Fa24 to have that extra header, so it can now process the frame. Figure 7-9 shows the change in logic.

Figure 7-9. Trunking Header Tells Switch2 What to Do


Now, switch2 knows how to process each received frame. When it receives frames in port FastEthernet 24, it expects to see a trunking header, which identifies the VLAN. That tells switch2 which address table to use.

When switch2 needs to send frames out port Fa24 to switch1, switch2 also adds a VLAN trunking header, knowing that switch1 expects to be able to find the header to identify the right VLAN.

So, now to the other use of the term trunk, along with some related terms. The term VLAN trunk, or sometimes simply trunk, refers to a link between two switches over which the additional trunking header is added, as in Figure 7-9. The term VLAN trunking, or simply trunking, refers to the process of adding (when sending) and removing (upon receipt) the extra headers that identify the VLANs.

Tale of Two Trunking Protocols

It was the best of times, it was the worst of times…whoops, wrong tale! Once upon a time, no VLAN trunking existed. Cisco wanted something like trunking in its switches, but no standards body had created a standard protocol for trunking. So, Cisco created its own standard and named it Inter-Switch Link (ISL). ISL defines the type of header that should be added to the frame, including the field in which the VLAN can be numbered.

Later, the IEEE 802.1Q committee defined a standard for VLAN trunking called 802.1Q trunking, or simply "dot 1 Q." Unsurprisingly, 802.1Q trunking differs from ISL. As a result, when you connect two switches and you want to trunk, you need to pick between the two options. The switches on each end of the trunk must agree to which protocol to use, or trunking will not work.




Computer Networking first-step
Computer Networking First-Step
ISBN: 1587201011
EAN: 2147483647
Year: 2004
Pages: 173
Authors: Wendell Odom

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