The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

2.4. Bridge Performance

A bridge performs two performance-intensive operations:

• It implements the decision-making algorithms-table lookup, fil-ter/forward decision, and table maintenance.

• It moves data among its ports (data flow).

A deficiency in either element can reduce the bridge's performance below the theoretical best case.

2.4.1. What Does It Take to Be the Best?

You cannot ask a bridge to do more than filter and/or forward 100 percent of the arriving traffic. The decision-making algorithms of a bridge are performed on a frame basis; that is, the amount of work required of the bridge is the same regardless of whether the frames received are short or long. The worst-case situation from the decision-making perspective is therefore to have every port bombarded with the maximum number of frames theoretically possible in a given period of time. This generally occurs with minimum-length frames, minimum interframe spacing, and negligible propagation delays. For a given LAN port, the maximum frame arrival rate is a function of the data rate and the LAN technology employed. Table 2-2 lists the maximum frame arrival rates for some popular LAN technologies.

Table 2.2. Maximum Frame Arrival Rates

TECHNOLOGY	DATA RATE (MB/S)	MAXIMUM RATE (SECOND S/FRAME)	MAXIMUM RATE (FRAMES/SECOND)
Ethernet	10	67.2μs	14,881.0
	100	6.72μs	148,809.5
	1000	672.0 ns	1,488,095.2
Token Ring	4	50.0μs	20,000.0
	16	14.5μs	68,965.5
	100	2.88μs	347,222.2
FDDI	100	2.0μs	500,000.0
[]

^[] Note: Figures for Token Ring and FDDI assume LLC-1, zero ...