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Figure 1223 Backhaul
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ISP However, cabbage s primary ISP s national provider, Dark Side, feeds Vegetable Valley from Dallas Traffic between cabbage and broccoli, therefore, goes via Seattle-Chicago-Dallas, or even further in the event of provider network failures or congestion ISPs and content providers in Vegetable Valley have banded together to form a local exchange in order to avoid sending local traffic halfway across the continent (see Figure 1224) Since broccoli is connected to two upstream providers as well as to the exchange, its router(s) probably need to be able to hold full routes
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Even when the connectivity is layer 2, there are two basic approaches to achieving it: central and distributed Centralized exchange points are more common At a centralized exchange point, each participant is responsible for installing a router and arranging connectivity from the router to the participant s main operations facility
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In a distributed exchange point (Figure 1225), there is either no central location or a central location with limited functionality The pioneering distributed exchange points use ATM and provide virtual circuits to the participants at the
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Figure 1224 Local exchange
participants own facilities There may be an ATM switch dedicated to the exchange service, or the ATM switching function may simply be a carrier capability The central location contains dedicated switches, route servers when they are used, and any value-added servers Local exchanges have been deployed with Frame Relay connectivity, in areas where the bandwidth requirements can be met with that speed It is reasonable to assume that metropolitan optical Ethernet is also an option for local exchanges
ISP1
ISP4
ISP2
ATM Cloud
ISP5
ISP3
ISP6
Figure 1225 Fully distributed exchange
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Partially Distributed Exchanges
Exchanges may have good operational requirements that there be more than one physical location where customers interconnect, but internal interconnections among those locations To the participants, the locations still appear to be a single layer 2 fabric (see Figure 1226) The exchange point operator can have a variety of motivations for physically extending the exchange point The London-based LYNX exchange and the Washington, DC, area MAE-EAST needed a period of parallel operation while they transitioned to larger physical locations Other exchanges, such as the French SPHYNX, have found it useful to provide cooperating exchange locations in different parts of their geographic service areas
Switches for the Ideal Large Exchange
Exchange point interconnection is more common in Europe than is private peering, so it is no surprise that the most intense work on exchange point requirements goes on in the European Exchange Point forum in RIPE, the European operational forum
ISP2
Site Layer 2 Switch
ISP5
ISP1
ISP4
Site Layer 2 Switch
Site Layer 2 Switch
ISP3
ISP6
OC-192 POS, etc WDM
10/100/1000 Ethernet
Figure 1226 Partially distributed exchange
The Provider-to-Provider Border
REALITY CHECK Can you think of a reason why an arbitrary user of the Internet should be able to send traffic directly to an interface in an exchange Remember that the address space of the exchange is registered and appears in reverse DNS, so it is quite identifiable in a traceroute I can t think of a good reason Yes, it is possible that a research organization might want to monitor exchange performance, but this is appropriately done with the knowledge and consent of the exchange point Indeed, many exchanges have explicit monitoring equipment frequently passive installed Would RFC 1918 space be appropriate for an exchange If it were used, how could you interpret a traceroute going through multiple exchanges
Topology Control
Exchanges assign exchange-specific, registered IP addresses to the components connected to their fabrics The prefixes containing these addresses are deliberately not advertised to the general Internet, although they will show up in traceroutes While IP addresses are assigned, much of the actual topology control and forwarding decisions use MAC addresses, not IP addresses Layer 2 topology enforcement is critical to most exchanges There are several aspects to such enforcement: at the specific address level and the shared medium topology level Enterprise switches, quite appropriately, use the IEEE 8021d spanning tree algorithm and protocol for their topology discovery They also make use of IEEE 8021q VLANs, essentially as a multiplexing method (Figure 1227) Spanning tree, however, has turned out to be less than desirable in most exchanges, where the exchange wants to control the information flow in a way that 8021d tries to automate Exchanges frequently disable spanning tree and configure everything manually Among the most basic problems is that a participant might leak 8021d information into the exchange point Exchanges want the ability to disable 8021d completely on a per-port basis or enable it with per-port filtering and possibly rate limiting Such filtering includes the ability to limit the number of MAC addresses learned at a given port and to replace previously learned addresses only on a last-out-first-in basis to prevent overwriting attacks Most exchanges that control this at all do so by periodic manual inspections of the number of MAC addresses per port Exchange point operators would like to examine alternatives to 8021d, but there is no specific proposal under active consideration other than the IEEE 8021w rapid spanning tree enhancements The special problems of spanning tree include slow convergence with no forwarding during