RFC 1220 (rfc1220) - Page 2 of 18
Point-to-Point Protocol extensions for bridging
Alternative Format: Original Text Document
RFC 1220 Bridging Point-to-Point Protocol April 1991
reliability, but message sequence issues are solved by the
transmitting end.
3. General Considerations
3.1. Link Quality Monitoring
It is strongly recommended that Point-to-Point Bridge Protocol
implementations utilize Magic Number Loopback Detection and Link-
Quality-Monitoring. This is because the 802.1 Spanning Tree
protocol, which is integral to both Transparent Bridging and Source
Routing (as standardized), is unidirectional during normal operation,
with HELLO PDUs emanating from the Root System in the general
direction of the leaves, without any reverse traffic except in
response to network events.
3.2. Message Sequence
The multiple link case requires consideration of message
sequentiality. The transmitting station must determine either that
the protocol being bridged requires transmissions to arrive in the
order of their original transmission, and enqueue all transmissions
on a given conversation onto the same link to force order
preservation, or that the protocol does NOT require transmissions to
arrive in the order of their original transmission, and use that
knowledge to optimize the utilization of the several links, enqueuing
traffic to links to minimize delay.
In the absence of such a determination, the transmitting station must
act as though all protocols require order preservation; many
protocols designed primarily for use on a single LAN in fact do. A
protocol could be described to maintain message sequentiality across
multiple links, either by sequence numbering or by fragmentation and
re-assembly, but this is neither elegant nor absolutely necessary.
3.3. Maximum Receive Unit Considerations
Please note that the negotiated MRU must be large enough to support
the MAC Types that are negotiated for support, there being no
fragmentation and re-assembly. Even Ethernet frames are larger than
the default MRU of 1500 octets.
3.4. Separation of Spanning Tree Domains
It is conceivable that a network manager might wish to inhibit the
exchange of BPDUs on a link in order to logically divide two regions
into separate Spanning Trees with different Roots (and potentially
different Spanning Tree implementations or algorithms). In order to
Point-to-Point Protocol Extensions Working Group