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ADSL WAN Connections
Routed Bridged Encapsulation
Routed Bridged Encapsulation
Some DSLAMs use routed bridged encapsulation (RBE) to route IP over
bridged Ethernet traffic. RBE is sometimes referred to as “half bridging,”
because it provides some of the advantages of bridging combined with some
of the advantages of routing.
With RBE, the ADSL service provider uses an aggregation device to establish
a bridge with the customer’s ADSL modem—in this case, the ProCurve Secure
Router. (See Figure 7-14.) With RBE, however, the router forwards packets
based on the Layer 3, or IP, header. In a pure bridging environment, the router
would use the Layer 2 header to forward packets.
When a device sends a packet that must be transmitted on the ATM subinter-
face, the router disregards the Layer 2 header and uses the Layer 3 header to
forward the packet. Likewise, when packets are received on the ATM subinter-
face, the ProCurve Secure Router examines the IP header. It then consults its
internal tables and identifies the MAC address associated with that IP address
and places that MAC address in the Layer 2 frame. If the ProCurve Secure
Router does not know the MAC address, it sends an Address Resolution
Protocol (ARP) frame to request that information.
ADSL service providers use RBE because it minimizes the configuration
required at the customer’s premises. It also provides better security than pure
bridging environments because each customer’s ADSL device establishes a
point-to-point connection with the service provider’s aggregation device. This
point-to-point connection also eliminates broadcast storms that are typical in
pure bridging environments.
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