Introduction to the HotWire DSLAM
1-15
8000-A2-GB21-20
November 1997
If WAN-C is a VLAN switch, the WAN must be a layer 2 switching network
supporting a Virtual LAN overlay.
In this case:
— Each NSP would be a member of a different Virtual LAN.
— The VLAN switch at the wire 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
address field points to the IP address of the NSP premises router for all
authorized subscriber IP source addresses.(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 does not know the MAC
address of the NSP premises router, it uses ARP to obtain the MAC
address from the NSP premises router prior to forwarding the packet (i.e.,
the wire center VLAN switch forwards an ARP request over the WAN to
the NSP router).
— 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 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,
Service Domain Features
.)
