User`s guide

Introduction to the HotWire DSLAM
1-10
8000-A2-GB21-10
June 1997
— Packets flowing from the NSP network to the subscribers are routed
within the NAP network based on the destination IP address (of the
subscriber) as is most common for IP-routed networks.
 If WAN-C is a VLAN switch, the WAN must be a layer 2 switching network
supporting a Virtual LAN overlay interconnected via a point-to-point network,
a frame relay switching network, or an ATM switching network.
In this case:
— Each NSP would be a member of a different Virtual LAN.
— The VLAN switch at the wiring center would support either port-based
VLAN switching (i.e., switching all MAC frames received on a specific
port to a specific NSP VLAN on the WAN) or port-based VLAN switching
with MAC-based attributes (i.e., switching frames received on a specific
port to a specific NSP VLAN on the WAN based on the destination MAC
address) for packets sent from the DSLAMs.
— The router at the NSP premises would either be front ended by a VLAN
switch or have an integrated VLAN card that supports protocols
consistent with the wire center VLAN switch (e.g., ATM Forum LAN
Emulation Protocol).
— The routing tables in the DSLAM are configured such that the next-hop
router points to the IP address of the NSP premises router for all
authorized subscriber IP source addresses associated with the address
domain of that NSP. (See the discussion on source-based routing in
Chapter 6,
IP Routing .)
— A different next-hop router is specified for each NSP address domain in
contrast to the routed network case where a single next-hop router was
specified for all NSP domains. If the DSLAM at that instance does not
know the MAC address of the NSP premises router, it uses ARP to obtain
its MAC address from the NSP premises router (i.e., the wire center
VLAN switch forwards an ARP request over the WAN to the NSP router)
prior to forwarding the packet.
— Packets flowing from the NSP network to the subscribers are routed to
the subscriber based on the destination IP address (of the subscriber) as
is most common for IP-routed networks. In this case, the LAN on which
the DSLAM resides appears to be part of a local subnet connected
directly to the NSP premises router. If the NSP router at that instance
does not know the MAC address of the subscriber, it uses ARP to obtain
the MAC address from the DSLAM that acts as a proxy for the
subscriber. (See the discussion on proxy ARP in Chapter 2,
Customer
Domain Features
.)