Upstream And Downstream Traffic - ZyXEL Communications MSC1000G User Manual

9.14.1 Upstream and Downstream Traffic

When the MSC forwards upstream traffic, it makes the following changes in the layer-2 header.
Table 119 IP Bridge: Layer-2 Header for Upstream Traffic
Source MAC address
Destination MAC address
VLAN ID
The original frame has the MSC's MAC address as the destination MAC address because the MSC,
not the device that really has the destination IP, responded to the ARP request for the destination
IP. (This is part of the ARP proxy feature for IP bridges.) Once the MSC receives the frame, it
updates the MAC addresses and VLAN ID and forwards it to the device that really has the
destination IP.
This is illustrated in the following example.
Figure 169 IP Bridge: Upstream Traffic Example
MAC a
IP: 192.168.1.1
VLAN: 100
Source IP: 192.168.1.1
Source MAC: a
Destination IP: 192.168.10.5
Destination MAC: x
VLAN: 100
Notice that the MSC does not change the IP packet header.
The process is reversed but otherwise similar for downstream traffic. The MSC learns how to
forward frames to the appropriate subscriber from one of the following sources.
• DHCP snooping. The IP-aware MSC snoops DHCP packets, so it knows what IP addresses have
been assigned to subscribers.
• ARP. The MSC uses ARP to find out which subscriber has a particular IP address.
• Static information. You should provide forwarding information manually for subscribers that have
static IP addresses and do not respond to ARP queries.
Management Switch Card User's Guide
ORIGINAL
Subscriber's MAC address
MSC's MAC address
Subscriber's VLAN ID
MAC x
Chapter 9 Subscriber Port Setup Screens
UPDATED
MSC's MAC address
Destination IP's MAC address
Destination IP's VLAN ID
MAC g
IP 192.168.10.5
VLAN: 200
Source IP: 192.168.1.1
Source MAC:
x
Destination IP: 192.168.10.5
Destination MAC:
g
VLAN:
200
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