Ethernet Bridging in Objective-C

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Ethernet Bridging
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that each customer is assigned to a different VLAN and that VLAN s ID is the PVID for each port connected to that customer The port is configured to allow only that VID to pass in or out When frames belonging to different customers pass over the same trunk between provider bridges, those frames may have both a C-tag and an S-tag This is the Q-in-Q format shown in Figure 134c Whether the C-tag is present is the choice of the customer; it makes no difference to the provider The S-tag is necessary for the provider to keep each customer s traffic separate from the other customers Figure 135 points out one limitation of the Q-in-Q model In Figure 135, there are three customer VLAN bridges, X, Y, and Z, connected via four provider bridges, P, Q, R, and S For clarity, no other customers ports or equipment is shown The customer is using two C-VLANs, the solid and the dotted lines The provider is carrying the customer s traffic in a single S-VLAN, shown as a wide gray band in the illustration Each of the customer s five stations, of which two are routers and three are workstations, are labeled with their MAC addresses, A through D Every Ethernet station is built with a globally unique MAC address A network can, however, be operated by overriding that globally unique MAC address with a locally administered MAC address These addresses are defined in IEEE Std 802-2006 If the next-to-low-order bit in the first byte of the MAC address (02-00-00-00-00-00) is set, then the network administrator is free to use any value in the remaining 46 bits (the low bit of the first byte is the multicast/unicast indication) This technique is not commonly used today because of the likelihood that a configuration error will result in unintentionally duplicated MAC addresses There is, unfortunately for bridges, a far more likely reason for duplicate MAC addresses IETF RFC 3768 defines the Virtual Router Redundancy Protocol (VRRP) Running this protocol can result in two (or more) routers having the same MAC address, though on different VLANs In Figure 135, two routers are circled One is on the customer s dotted VLAN and one on the customer s solid VLAN, but both have the same MAC address, A
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D o e, tw wir s Customer One VLAN bridge X C A
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Customer bridge Y
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Trunk wires, two C-VLANs carried inside one S-VLAN Provider bridges P, Q, R, S
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Figure 135 Duplicate MAC address problem
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Customer bridge Z
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This duplicated MAC address is no problem for the three customer bridges, X, Y, and Z As shown in Figure 135, each of the customer bridges is aware of the two VLANs and keeps the MAC addresses in the two VLAN separate in its filtering database Each learns that dotted A is on one port and solid A is on another port Some of the provider bridges, particularly bridge R, have a problem, however Once the customer s frames enter the provider s network, they have an outer S-tag applied The provider bridges forward frames based on the S-VID and the MAC addresses in the frame They do not look inside the frame at the C-tags (See Proper Layering next) Bridge R sees a frame with source MAC address A on the gray S-VLAN coming from both directions and cannot tell in which direction to send frames from D bound for (solid) A Note that there is no confusion in the provider s network among different customers MAC addresses in the gray S-VLAN are kept completely separate from any other customers MAC addresses The problem is strictly one of duplicated MAC addresses within a single S-VLAN Fortunately, this situation is relatively easy for the customer to avoid One way is for the customer to configure VRRP so the two routers do not share a common MAC address The other is for the customer to purchase two separate EVCs in two separate S-VLANs and make sure no address is duplicated in either S-VLAN In this case, the dotted C-VLAN could stay in the same S-VLAN, and the solid C-VLAN could be moved to a new S-VLAN If the customer is unaware of this possibility, however, it can take some effort to resolve and can involve some avoidable finger pointing Although this problem condition is easy for a customer to avoid, it does point out why you cannot easily build a Q-in-Q-in-Q bridge (or a 4-Q or a 5-Q bridge) One customer can adjust his or her configuration to avoid this problem But, adding a third VLAN tag means that two or more different customers EVCs are packed inside a single outermost VLAN tag A provider cannot expect those customers, perhaps all running VRRP, to cooperate in their network configurations in order to avoid this duplicate address problem This is especially true since, if one of those customers network administrators makes a configuration error, all of the customers will suffer from the consequent misdirection of frames Why can t a bridge simply look at two tags and differentiate between the two routers The answer is layering Protocol layering is the most fundamental principle in networking, and its use is precisely why you do not need to upgrade every piece of network equipment in the world every time one carrier or another adds a new feature to its network Layering means that a new LAN can be substituted for Ethernet, as long as it looks to the upper layers like Ethernet VLANs were added to bridges without disrupting the operations of stations because the same service was offered to the stations as was offered before VLANs were introduced When protocols are properly layered, protocol entities in different systems communicate with each other as peers, utilizing the services provided by lower layers and offering some service to the higher layers They can be stacked ad infinitum Although this sounds trivial, it has a direct effect on tags added to frames If one entity adds a tag (eg, a C-tag) as a frame leaves a port, then its peer entities, and only its peer entities,
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