Enterasys Networks Security Router X-PeditionTM - Manual (Page 122)

IP Routing Protocols
5-20 Configuring IP
Figure 5-3 Topology of Ethernet/PPPoE/VLAN/PPPoE over VLAN
VLAN Processing Over the XSR’s Ethernet Interfaces
The VLAN routing process, shown in Figure 5-4 , works as follows on the XSR. The following steps
are reflected in the graphic below.
Figure 5-4 XSR’s VLAN Processing
1. When a VLAN-tagged Ethernet frame is received, it is stripped and the tag’s VLAN ID used to
map the frame to a sub-interface of the Ethernet port. Priority bits are kept internally with the
frame. If a PPPoE session header exists behind the tag, the frame is further de-muxed. The
matched sub-interface is marked as the incoming port. If no match is found for the VLAN ID,
the frame is dropped.
2. The Ethernet frame’s IP packet is routed based on the XSR’s forwarding table which chooses
the next hop and outgoing interface.
3. If the outgoing interface is associated with a VLAN by a VLAN ID, a VLAN tag including the
VLAN ID is created and inserted into the outgoing Ethernet frame. If priority bits are stored
with the frame, they are marked in the VLAN tag.
VLAN Processing: VLAN-enabled Ethernet to Standard LAN Interfaces
In this scenario, illustrated in Figure 5-5 , the XSR does not insert a VLAN tag since there is no
VLAN associated with the outgoing interface.
G3.1
IP
PPPoE
VLAN 100
Ethernet
VLAN 200 VLAN 300
PPPoE PPPoE
IP IP IP
G3.2 G3.3 G3.4
1
2
3
Incoming VLAN tagged frame
F1.1
VLAN 1200
IP 1.2.3.4/24
Ethernet VLAN Tag IP: 9.9.9.1
Ethernet VLAN Tag IP: 9.9.9.1
Priority CFI VLAN: 1300
F2.2
VLAN 1400
IP 3.3.2.4/24
F2.1
VLAN 1300
IP 2.2.3.4/24
IP Routing Table
1.2.3.0/24 F1.1
2.2.3.0/24 F2.1
3.3.2.0/24 F2.2
9.9.9.0/24 F2.1 (Static)
Priority CFI VLAN: 1300
Outgoing VLAN tagged frame