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Cisco IOS software creates these rings on behalf of the media controllers and then manages them jointly with
the controllers. Each interface has a pair of rings: a receive (RX) ring for receiving packets and a transmit
(TX) ring for transmitting packets.
Receive rings have a constant number of packet buffers allocated to them that equals the size of the ring. The
show controllers command below displays the size and the location of the receive and transmit rings:
router#show controllers ethernet 0
QUICC Ethernet unit 0 using SCC1, Microcode ver 3
Current station address 0060.5cbc.3d41, default address
0060.5cbc.3d41
idb at 0x2AFE0EC, driver data structure at 0x2AEF820
SCC Registers:
General [GSMR]=0x0:0x1088003C, Protocol−specific [PSMR]=0x80A
Events [SCCE]=0x0000, Mask [SCCM]=0x001F, Status [SCCS]=0x0002
Transmit on Demand [TODR]=0x0, Data Sync [DSR]=0xD555
Interrupt Registers:
Config [CICR]=0x00368461, Pending [CIPR]=0x0100C402
...
RX ring with 16 entries at 0xFF00420, Buffer size 1524
Rxhead = 0xFF00458 (7), Rxp = 0x2AEF858 (7)
00 pak=0x2B00F34 buf=0x2D8A48C status=9000 pak_size=0
01 pak=0x2B02B24 buf=0x2D8F55C status=9000 pak_size=0
02 pak=0x2B01D2C buf=0x2D8CCF4 status=9000 pak_size=0
03 pak=0x2B00CE0 buf=0x2D89DD0 status=9000 pak_size=0
...
TX ring with 4 entries at 0xFF004A0, tx_count = 0
tx_head = 0xFF004A0 (0), head_txp = 0x2AEF898 (0)
tx_tail = 0xFF004A0 (0), tail_txp = 0x2AEF898 (0)
00 pak=0x0000000 buf=0x0000000 status=0000 pak_size=0
01 pak=0x0000000 buf=0x0000000 status=0000 pak_size=0
02 pak=0x0000000 buf=0x0000000 status=0000 pak_size=0
03 pak=0x0000000 buf=0x0000000 status=2000 pak_size=0
...
The highlighted entries are explained as follows:
RX ring with 16 entries at 0xFF00420, Buffer size 1524 : The size of the receive ring is 16 and it begins at
the address 0x0xFF00420 in I/O memory. The size of the buffers for the Ethernet interface is 1524.
TX ring with 4 entries at 0xFF004A0, TX_count = 0: The size of the transmit ring is 4, it begins at the
address 0xFF004A0 in I/O memory and there are no packets awaiting transmission on this interface.
Switching Paths
The description below is based on the book Inside Cisco IOS Software Architecture, Cisco Press
1
.
1 − Receiving the packet
Cisco 1600 Series Router Architecture