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28 IBM 9077 SP Switch Router: Get Connected to the SP Switch
1. Normally, all media cards have a 4 MB send buffer and a 4 MB receive
buffer, except the SP Switch Adapter card, which has a 16 MB buffer
size for each buffer. See also Section 2.3.5, “Characteristics of GRF
Media Cards” on page 36 and Figure 19 on page 37.
2.
Quick Branch Routing Technology
(QBRT) is a hardware-assisted
route table lookup. Route lookup times range from 1 - 2.5 µs with up to
150,000 next-hop routes in the table. Not all media cards use QBRT.
Cards that do not use QBRT use a microcode lookup.
The benefit of this architecture is that the entire route table can be
stored locally on the media card and searched quickly. In the traditional
cached route table method, a small number of routes can be stored and
searched locally. However, when a large number of routes is desired,
or the kind of traffic one would see on the Internet backbone arises,
caching is inadequate. Inevitably, cache misses occur, and route table
lookups are performed at a limited, central, shared resource.
Performance is enhanced even further with parallel processing of table
lookups occurring on each media card, which is another technique that
helps assure linear scalability. The router manager on the controller
board, which also contains the switch fabric, maintains the master
route table and distributes updates simultaneously to all installed
media cards, even as the cards continue their forwarding functions.
3. On-Board Processor
4. Router management takes place on the IP Switch Control Board (see
Section 2.3.3.2, “IP Switch Control Board Components” on page 33),
based on a 166 Mhz Pentium processor. It is responsible for system
monitoring, configuration management and the user interface.
5. The GRF
communications bus (Combus)
is an "out-of-band" data path
for configuration, control and monitoring of media cards. The Combus
connects the IP Switch Control Board to the media cards independent
of the switch connection to each card. It is not used for routed data
between the cards. Route update packets received on any media card
are also sent across the Combus to the Route Manager and, therefore,
do not have to compete with normal IP traffic. The Combus is a serial
bus with a transferrate of 80 Mb/s and is FIFO-buffered. The Arbitration
Logic is on the IP Switch Control Board.
2.3.2.1 Data Packet Processing
With the knowledge about the local routing functions of the media cards, we
now look at Figure 13 on page 29 to see how a data packet is transferred
from one media card to another.