metrotek Bercut-MMT, Operation Manual

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11. Loopback
In order to perform measurements an certification according to RFC 2544,
Bercut-MMT supports loopback at different levels.
This mode can be used to check functioning of software components that
are involved in TCP/IP family protocols implementation. Testing can be
implemented at different OSI model layers.
11.1 Loopback Modes in Ethernet Networks
11.1.1 First Layer Loopback
At the physical layer (PHY), all incoming traffic including packets with errors
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, are redirected back without any changes (Figure 11.1).
Figure 11.1. First Layer Loopback Connection
11.1.2 Second Layer Loopback
Incoming traffic is redirected back at the link layer (defective packets are not
transmitted), with this sender and recipient MAC addresses are switched,
VLAN tags can be replaced, other (user defined) MAC addresses may be
inserted.
Figure 11.2, page 64 schematically shows device connection to the physical
network using one port.
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Packets with defective header, wrong CRC, oversized data field and other errors.
Bercut-MMT: Ethernet Networks Analysis