Zyxel IES-6000 [268/1156] Upstream and downstream traffic

Zyxel IES-5112 [268/1156] Upstream and downstream traffic
Chapter 10 Subscriber Port Setup Screens
Management Switch Card User’s Guide
268
10.14.1 Upstream and Downstream Traffic
When the MSC forwards upstream traffic, it makes the following changes in the layer-2 header.
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 172 IP Bridge: Upstream Traffic Example
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.
Table 123 IP Bridge: Layer-2 Header for Upstream Traffic
ORIGINAL UPDATED
Source MAC address Subscriber’s MAC address MSC’s MAC address
Destination MAC address MSC’s MAC address Destination IP’s MAC address
VLAN ID Subscriber’s VLAN ID Destination IP’s VLAN ID
MAC g
MAC x
MAC a
Source IP: 192.168.1.1
Source MAC: x
Destination IP: 192.168.10.5
Destination MAC: g
VLAN: 200
Source IP: 192.168.1.1
Source MAC: a
Destination IP: 192.168.10.5
Destination MAC: x
VLAN: 100
IP 192.168.10.5
IP: 192.168.1.1
VLAN: 100
VLAN: 200

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