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Configuring Bridging Services

1-8

117351-B Rev. 00

Figure 1-7. AppleTalk ARP (Originating on FDDI) to 802.3 Translation

Spanning Tree Algorithm

The spanning tree algorithm ensures the existence of a loop-free topology in

networks that contain parallel bridges. (See Source Routing Appendix to IEEE

Standard 802.1d Media Access Control (MAC) Bridges for details on the spanning

tree algorithm.) A loop occurs when there are alternate routes between hosts. If

there is a loop in an extended network, bridges may forward traffic indefinitely,

which can result in increased traffic and degradation in network performance.

Figure 1-8

shows an example of a network containing a loop: two parallel bridges,

bridge 1 and bridge 2, connect LANs A and B.

DA SA

TYPE

DATA FCS

SNAP LLC OUI

AA AA 03 = 00 00 00

AppleTalk ARP frame originating on FDDI LAN

Through bridge B to 802.3 LAN

=80F3

Bridge B

• Extracts addressing information from the FDDI MAC header.

• Incorporates address information into newly generated 802.3 header.

• Adds a length field.

• Encapsulates FDDI data according to RFC 1042.

• Recalculates FCS.

DA SA

TYPE

DATA

FCS

SNAP LLC OUI

AA AA 03 = 00 00 00

=80F3

LEN

BRG0005A

1 2 ... 27 28 29 30 31 32 33 34 35 36 37 ... 259 260

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Avaya Configuring Bridging Services Manual de usuario Pagina 32