L-ET4148-50C-DB LSI, L-ET4148-50C-DB Datasheet - Page 36

• Manufacturer Part Number: L-ET4148-50C-DB
• Manufacturer: LSI

Specifications of L-ET4148-50C-DB

• Lead Free Status / RoHS Status: Supplier Unconfirmed

ET4148-50
Preliminary Data Sheet
Single-Chip 48 x 1 Gbit/s + 2 x 10 Gbits/s Layer 2+ Ethernet Switch
April 2006
Functional Description
(continued)
VLAN Encapsulation
(continued)
The VLAN encapsulation process operates in one of two fundamentally different ways depending on whether the
transmit port is configured as a trunk port or an access port.
Access ports may only transmit packets that have no VLAN tags. Hence, if the packet was received with a VLAN
tag, it must be removed from the packet prior to transmission.
The behavior of a trunk port is a bit more complex. An optional mode is available for the ET4148-50 whereby pack-
ets are transmitted on a trunk port without a VLAN tag if the VLAN that the packet belongs to matches the default
VLAN ID of the transmit (trunk) port. If this mode is not in use, all packets transmitted by a trunk port are transmit-
ted with a VLAN tag.
If the packet was received without a VLAN tag, then one must be added. If the packet was received with a VLAN
tag, then its current tag is replaced with a new one. For typical configurations, the replacement VLAN tag is the
same as the one that the packet was received with and the replacement of the tag has no effect on the contents of
the packet.
Finally, if the transmit packet is an IP packet, its DSCP value is updated prior to transmission. If the packet was
marked for priority demotion during ingress policing, the new priority value reflects a reduction in priority. The
nature of this reduction in priority depends on the configuration of the priority decode table.
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Agere Systems Inc.
Agere Systems - Proprietary