Cisco Systems BC-109 manual Following notes and caveats apply to all uses of SR/TLB

Configure Translation between SRB and Transparent Bridging Environments

Figure 50 Example of a Simple SR/TLB Topology

		Transparent
		bridging "ring"
		Transparent
Source-route	Router running	bridging domain
bridged domain	SR/TLB
Token
Ring

Frames lose RIFs in this direction

Frames gain RIFs in this direction

S1108a

Note The Spanning-Tree Protocol messages used to prevent loops in the transparent bridging domain are not passed between the SRB domain and the transparent bridging domain. Therefore, you must not set up multiple paths between the SRB and transparent bridging domains.

The following notes and caveats apply to all uses of SR/TLB:

•Multiple paths cannot exist between the source-route bridged domain and the transparent bridged domain. Such paths can lead to data loops in the network, because the spanning-tree packets used to avoid these loops in transparent bridging networks do not traverse the SRB network.

•Some devices, notably PS/2s under certain configurations running OS/2 Extended Edition Version 1.3, do not correctly implement the “largest frame” processing on RIFs received from remote source-route bridged hosts. The maximum Ethernet frame size is smaller than that allowed for Token Ring. As such, bridges allowing for communication between Ethernet and Token Ring will tell the Token Ring hosts, through the RIF on frames destined to the Token Ring, that hosts on the Ethernet cannot receive frames larger than a specified maximum, typically 1472 bytes. Some machines ignore this run-time limit specification and send frames larger than the Ethernet can accept. The router and any other Token Ring/Ethernet bridge has no choice but to drop these frames. To allow such hosts to successfully communicate across or to an Ethernet, you must configure their maximum frame sizes manually. For the PS/2, this can be done through Communications Manager.

•Any access filters applied on any frames apply to the frames as they appear on the media to which the interface with the access filter applies. This is important because in the most common use of SR/TLB (Ethernet and Token Ring connectivity), the bit ordering of the MAC addresses in the frame is swapped. Refer to the SR/TLB examples in the “SRB Configuration Examples” section of this chapter.

Caution Bridging between dissimilar media presents several problems that can prevent communication from occurring. These problems include bit order translation (or usage of MAC addresses as data), maximum transmission unit (MTU) differences, frame status differences, and multicast address usage. Some or all of these problems might be present in a multimedia bridged LAN and prevent communication from taking place. Because of differences in the way end nodes implement Token Ring, these problems are most prevalent when bridging between Token Rings and Ethernets or between Token Ring and FDDI LANs.

BC-120Bridging and IBM Networking Configuration Guide