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IXP1200 Network Processor Family ATM OC-3/12/Ethernet IP Router Example Design
18 Application Note
Modified on: 3/20/02,
syndrome is updated appropriately. The VC Table Entry also contains an AAL type field.
Currently, this example design supports only classical IP over ATM, where the AAL type can be
either 0 or 5. A value of 0 indicates that the VC is not open, so any cell received on that VC is
immediately discarded.
The LLC/SNAP field specifies the protocol type. Currently, the only valid value is 0x AA AA 03
00 00 00 08 00 (classical IP over ATM). While this implementation consumes and produces just
one valid LLC/SNAP pattern, this pattern is not hard-coded. The LLC/SNAP bits are included in
the IP route table entry, as well as the VC lookup table. This is to make it easy to modify the design,
not only support a different LLC/SNAP pattern, but also to be able to support different valid
patterns for each VC.
2.4.1.2 IP Lookup Table
Each PDU contains an IP header in its first cell. Therefore, a single IP lookup is performed for each
PDU, regardless of the number of cells in the PDU.
Figure 10. ATM to Ethernet Processing Steps
A9638-01
IP
Address
AAL
type
Cell N
(40 Bytes)
Cell 1
(48 Bytes)
Cell 0
(48 Bytes)
1
LLC
Hdr
ATM
Hdr
IP
Hdr
LLC/SNAP
header
IP
Lookup
ATM PDU on Rx Port
If end of PDU
VC Lookup Table Entry
SDRAM
Packet Buffer
Buffer
offset
VPI/VCI
Buffer base
address
CRC-32
Residue
Payload
Ether
Hdr
IP Payload
Receive
ATM Cell
Check
CRC
Check
length
Strip
AAL-5
trailer
ATM
Hdr
CRC
Port
number
Enet
header
Port
type
Route Table
2
3
4
Locate buffer & offset
5
Build MPKT,
add Ethernet
header on first
MPKT
IP look-up
on first cell
9
10
Check CRC
on AAL-5 PDU
Move
payload
to buffer
6
7
VC Look-up check
LLC/SNAP header
on first cell
Transmit
MPKT
8
7
Ethernet PDU on Tx Port
LENCLPPAD UU