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In this example, we’ve got 2 switches and 4 routers
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which are acting as PCs in the topology
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we also got a hub which I'm going to use to capture traffic using Wireshark.
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These devices have just booted up so that they have no configuration.
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on the first switch, I’ll give this switch a hostname of S1
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and on the second switch give it a hostname of S2.
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sh vlan at the moment we only have 1 Ethernet VLAN configured VLAN 1.
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All ports on the switch are configured in that VLAN
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Cisco switches also have some older VLANs by default 1002, 1003, 1004 and 1005
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which were used previously for FDDI and token ring
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those VLANs are not used anymore today.
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Switch 2 shows something similar so sh vlan brief
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we have VLAN 1 all ports are configured in VLAN 1.
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Here’s router 1 which is acting as our PC
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I’m going to configure the FastEthernet interface
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with an IP address of 10.1.1.1/24 mask
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and I’ll no shut the interface on router 2
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do something similar
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no shut the interface given an IP address of 10.1.1.2
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so without any further configuration
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we should be able to ping between router 1 and router 2
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notice router 2 can ping router 1 and router 1 can ping router 2.
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The switch just has a default config
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all ports on the switches are in VLAN 1
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we can see that by using the command sh int g0/0 switchport
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this port gigabit 0/0 is using administrative mode dynamic auto
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operation mode at the moment is static access
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negotiation of trunking is on, in other words DTP is enabled
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but at the moment because there is no switch to negotiate with
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the port is in VLAN 1, the default VLAN.
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Now we can see the traffic by doing a capture using Wireshark.
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Various traffic is seen here we’ve got a Spanning Tree BPDU being sent
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by the switch, we can see Cisco discovery protocol messages
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being sent by the switch as well
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showing the switch port, the platform and other information.
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But what I’d like you to see is all traffic is Ethernet frames.
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There is no 802.1Q tagging on this frames.
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Let’s do a search for ICMP traffic.
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At the moment there's is no ICMP traffic.
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I'll get router 1 to ping router 2 and notice we see the ICMP traffic
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what you’ll notice is we have the source MAC address of router 1 pinging router 2
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protocol type is IPv4 source IPv4 address is 10.1.1.1
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destination IP address is 10.1.1.2
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it’s a standard Ethernet frame encapsulating IP and ICMP.
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In this case it’s an echo and the reply is in echo reply.
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There is no Ethernet tagging on this frames at all.
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It's a standard Ethernet frame, there is no concept of VLAN either
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no VLAN information is transmitted on the frames.
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Even the other frames such as CDP don’t show any VLAN information whatsoever.
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I'll stop that capture.
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this is an access port and this is an access port by default
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the same will be true on switch 2
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I'll configure router 3 with an IP address of 10.1.1.3
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no shut the interface and I'll do something similar on router 4
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no shut IP address 10.1.1.4
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Can router 4 ping routers 3 acting as host 3?
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Yes it can.
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Start capture here.
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We can see various traffic types being captured
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including once again CDP on router 3, I'll ping router 4 ping succeeds.
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I’ll filter for ICMP
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and once again you can see that this is a standard Ethernet frame.
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There is no information about VLANs contained in this frames.
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Just standard Ethernet frames.
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So the PCs in the topology are unaware of any VLAN information.
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they're sending untagged frames
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in other words, standard Ethernet frames to the switches
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and the switches are sending untagged frames to the PCs.
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Now let’s concentrate on switch 1 for the moment
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I’m gonna set the VTP mode of this device to transparent
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which means it won’t try and synchronize VLAN information with the other switch.
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sh vlan once again show us that we have 1 Ethernet VLAN configured.
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Let's create VLAN 2, sh vlan brief
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we now have 2 Ethernet VLANs but no ports are configured in VLAN 2.
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