IXP1200 Network Processor Family ATM OC-3/12/Ethernet IP Router Example Design
40 Application Note

Modified on: 3/20/02,

4.4.2 DRAM Data Buffer Format

Packet payloads are stored in DRAM data buffers. Depending on if the data was received on an

ATM or Ethernet port, the payload will land in a different place within the data buffer..

Figure 29. Buffer Descriptor Format for Ethernet Transmit Destination Port

3
13
02
92
82
72
62
52
42
32
22
12
01
91
81
71
61
51
41
31
21
11
09876543210
0
1 RCV_PORT FL_ID START_BYTE END_BYTE ELE_COUNT
2-1
3
Entry Description
RCV_PORT Receive Port
FL_ID Free list ID
START_BYTE Frame start location in the buffer (zero-based)
END_BYTE Number of bytes in the last MPKT - minus 1 (e.g. 0 means 1 byte)
ELE_COUNT Number of 64-byte MPKTs in packet

Figure 30. DRAM Data Buffer Format - 12 Byte Offset (Received by ATM)

01234567891
01
11
21
31
41
51
61
71
81
92
0... Bytes ->
ATM
Header LLC/SNAP IP ... IP Pad AAL5 Trailer

Figure 31. DRAM Data Buffer Format - 6 Byte Offset (Received by ATM, Transmitted by

Ethernet)

01234567891
01
11
21
31
41
51
61
71
81
92
0... Bytes ->
Enet Dest Addr Enet Src Addr Typ IP ... IP

Figure 32. DRAM Data Buffer Format - 6 Byte Offset (Received by Ethernet, Transmitted by

ATM)

01234567891
01
11
21
31
41
5... Bytes ->
LLC/SNAP IP ... IP Pad AA L5 Trailer

Figure 33. DRAM Data Buffer Received by Ethernet

01234567891
01
11
21
31
4... Bytes ->
Enet DstAdr E net SrcAdr TYP IP ... IP