As per Relevance of the word spanning, we have this rfc below:
Network Working Group M.
Request for Comments: 2878
Obsoletes: 1638 F.
Category: Standards Track
July 2000
PPP Bridging Control Protocol (BCP
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
The Point-to-Point Protocol (PPP) [6] provides a standard method
transporting multi-protocol datagrams over point-to-point links.
defines an extensible Link Control Protocol, and proposes a family
Network Control Protocols for establishing and configuring
network-layer protocols
This document defines the Network Control Protocol for
and configuring Remote Bridging for PPP links
This document obsoletes RFC 1638, which was based on the
802.1D-1993 MAC Bridge[3]. This document extends that
by including the IEEE 802.1D-1998 MAC Bridge[8] and IEEE 802.1
Virtual LAN (VLAN)[9] standards. This document also improves
protocol in order to support high-speed switched LANs
Higashiyama & Baker Standards Track [Page 1]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Table of
1. Historical Perspective ................................ 3
1.1 Requirements Keywords ........................... 3
2. Methods of Bridging ................................... 3
2.1 Transparent Bridging ............................ 3
2.2 Remote Transparent Bridging ..................... 4
2.3 Source Routing .................................. 5
2.4 Remote Source Route Bridging .................... 6
2.5 SR-TB Translational Bridging .................... 7
3. Traffic Services ...................................... 7
3.1 LAN Frame Checksum Preservation ................. 7
3.2 Traffic having no LAN Frame Checksum ............ 7
3.3 Tinygram Compression ............................ 8
3.4 Virtual LANs .................................... 8
4. A PPP Network Control Protocol for Bridging ........... 9
4.1 Sending Bridge Frames ........................... 10
4.1.1 Maximum Receive Unit Considerations ............. 11
4.1.2 Loopback and Link Quality Monitoring ............ 11
4.1.3 Message Sequence ................................ 11
4.1.4 Separation of Spanning Tree Domains ............. 12
4.2 Bridged LAN Traffic in IEEE 802 Untagged Frame .. 12
4.3 Bridged LAN Traffic in IEEE 802 Tagged Frame .... 16
4.4 Bridge management protocol data unit ............ 21
5. BCP Configuration Options ............................. 21
5.1 Bridge-Identification ........................... 22
5.2 Line-Identification ............................. 23
5.3 MAC-Support ..................................... 25
5.4 Tinygram-Compression ............................ 26
5.5 MAC-Address ..................................... 27
5.6 Spanning Tree Protocol (old formatted) .......... 28
5.7 IEEE-802-Tagged-Frame ........................... 30
5.8 Management-Inline ............................... 30
6. Changes From RFC 1638 ................................. 31
7. Security Considerations ............................... 32
8. Intellectual Property Notice .......................... 32
9. IANA Considerations ................................... 33
10. Acknowledgments ....................................... 33
APPENDICES ................................................... 34
A. Spanning Tree Bridge PDU (old formatted) ........... 34
B. Tinygram-Compression Pseudo-Code ................... 35
References ................................................... 36
Authors' Addresses ........................................... 37
Full Copyright Statement...................................... 38
Higashiyama & Baker Standards Track [Page 2]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
1. Historical
Two basic algorithms are ambient in the industry for Bridging
Local Area Networks. The more common algorithm is
"Transparent Bridging", and has been standardized for Extended
configurations by IEEE 802.1. The other is called "Source
Bridging", and is prevalent on IEEE 802.5 Token Ring LANs
The IEEE has combined these two methods into a device called a
Routing Transparent (SRT) bridge, which concurrently provides
Source Route and Transparent bridging. Transparent and SRT
are specified in IEEE standard 802.1D-1998 [8].
Although IEEE committee 802.1G is addressing remote bridging [2],
neither standard directly defines the mechanisms for
remote bridging. Technically, that would be beyond the IEEE 802
committee's charter. However, both 802.1D and 802.1G allow for it
The implementor may model the line either as a component within
single MAC Relay Entity, or as the LAN media between two
bridges
The original IEEE 802.1D is augmented by IEEE 802.1Q [9] to
support for Virtual LAN. Virtual LAN is an integral feature
switched LAN networks
1.1 Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in
document, are to be interpreted as described in [12].
2. Methods of
2.1. Transparent
As a favor to the uninitiated, let us first describe
Bridging. Essentially, the bridges in a network operate as
entities, largely unaware of each others' presence. A
Bridge maintains a Forwarding Database consisting
{address, interface
{address, interface, VLAN ID
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records, by saving the Source Address of each LAN transmission
it receives, along with the interface identifier for the interface
was received on. Bridges which support Virtual LANs
keep the Virtual LAN ID in their forwarding database. It goes on
check whether the Destination Address is in the database, and if so
either discards the message when the destination and source
located at the same interface, or forwards the message to
indicated interface. A message whose Destination Address is
found in the table is forwarded to all interfaces except the one
was received on. This behavior applies to Broadcast/Multicast
as well
The obvious fly in the ointment is that redundant paths in
network cause indeterminate (nay, all too determinate)
behavior to occur. To prevent this, a protocol called the
Tree Protocol is executed between the bridges to detect and
remove redundant paths from the network
One system is elected as the "Root", which periodically emits
message called a Bridge Protocol Data Unit (BPDU), heard by all
its neighboring bridges. Each of these modifies and passes the
on to its neighbors, until it arrives at the leaf LAN segments in
network (where it dies, having no further neighbors to pass
along), or until the message is stopped by a bridge which has
superior path to the "Root". In this latter case, the interface
BPDU was received on is ignored (it is placed in a Hot
status, no traffic is emitted onto it except the BPDU, and
traffic received from it is discarded), until a topology
forces a recalculation of the network
To establish Virtual LANs in an environment of multiple bridges,
(GARP VLAN Registration Protocol) is executed between bridges
exchange Virtual LAN information. GVRP provides a mechanism
dynamically establish and update their knowledge of the set
Virtual LANs that currently have active members
To reduce unnecessary multicast flooding in the network,
exchange group MAC addresses using the GARP Multicast
Protocol. GMRP provides a mechanism so that bridges can know
multicast frames should be forwarded on each port
2.2. Remote Transparent
There exist two basic sorts of bridges -- those that
LANs directly, called Local Bridges, and those that interconnect
via an intermediate medium such as a leased line, called
Bridges. PPP may be used to connect Remote Bridges
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
The IEEE 802.1G Remote MAC Bridging committee has proposed a model
a Remote Bridge in which a set of two or more Remote Bridges that
interconnected via remote lines are termed a Remote Bridge Group
Within a Group, a Remote Bridge Cluster is dynamically formed
execution of the spanning tree as the set of bridges that may
frames among each other
This model bestows on the remote lines the basic properties of a LAN
but does not require a one-to-one mapping of lines to virtual
segments. For instance, the model of three interconnected
Bridges, A, B and C, may be that of a virtual LAN segment between
and B and another between B and C. However, if a line exists
Remote Bridges B and C, a frame could actually be sent directly
B to C, as long as there was the external appearance that it
travelled through A
IEEE 802.1G thus allows for a great deal of implementation
for features such as route optimization and load balancing, as
as the model is maintained
For simplicity, we discuss Remote Bridging in this document in
of two Remote Bridges connected by a single line
2.3. Source
The IEEE 802.1D Committee has standardized Source Routing for any
Type that allows its use. Currently, MAC Types that support
Routing are FDDI and IEEE 802.5 Token Ring
The IEEE standard defines Source Routing only as a component of
SRT bridge. However, many bridges have been implemented which
capable of performing Source Routing alone. These are most
implemented in accordance either with the IBM Token-Ring
Architecture Reference [1] or with the Source Routing Appendix
IEEE 802.1D-1998 [8].
In the Source Routing approach, the originating system has
responsibility of indicating the path that the message should follow
It does this, if the message is directed off of the local segment,
including a variable length MAC header extension called the
Information Field (RIF). The RIF consists of one 16-bit word
flags and parameters, followed by zero or more segment-and-
identifiers. Each bridge en route determines from this source
list whether it should accept the message and how to forward it
In order to discover the path to a destination, the
system transmits an Explorer frame. An All-Routes Explorer (ARE
frame follows all possible paths to a destination. A Spanning
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Explorer (STE) frame follows only those paths defined by Bridge
that the Spanning Tree Algorithm has put in Forwarding state.
states do not apply to ARE or Specifically-Routed Frames.
destination system replies to each copy of an ARE frame with
Specifically-Routed Frame, and to an STE frame with an ARE frame.
either case, the originating station may receive multiple replies
from which it chooses the route it will use for future Specifically
Routed Frames
The algorithm for Source Routing requires the bridge to be able
identify any interface by its segment-and-bridge identifier. When
packet is received that has the RIF present, a boolean in the RIF
inspected to determine whether the segment-and-bridge identifiers
to be inspected in "forward" or "reverse" sense. In its search,
bridge looks for the segment-and-bridge identifier of the
the packet was received on, and forwards the packet toward
segment identified in the segment-and-bridge identifier that
it
GVRP and GMRP are available and effective on Source Routing networks
2.4. Remote Source Route
There is no Remote Source Route Bridge proposal in IEEE 802.1 at
time, although many vendors ship remote Source Routing Bridges
We allow for modelling the line either as a connection
between two halves of a "split" Bridge (the split-bridge model),
as a LAN segment between two Bridges (the independent-bridge model).
In the latter case, the line requires a LAN Segment ID
By default, PPP Source Route Bridges use the independent-
model. This requirement ensures interoperability in the absence
option negotiation. In order to use the split-bridge model, a
MUST successfully negotiate the Bridge-Identification
Option
Although no option negotiation is required for a system to use
independent-bridge model, it is strongly recommended that
using this model negotiate the Line-Identification
Option. Doing so will verify correct configuration of the
Segment Id assigned to the line
When two PPP systems use the split-bridge model, the system
transmits an Explorer frame onto the PPP link MUST update the RIF
behalf of the two systems. The purpose of this constraint is
ensure interoperability and to preserve the simplicity of
bridging algorithm. For example, if the receiving system did
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
know whether the transmitting system had updated the RIF, it
have to scan the RIF and decide whether to update it. The choice
the transmitting system for the role of updating the RIF allows
system receiving the frame from the PPP link to forward the
without processing the RIF
Given that source routing is configured on a line or set of lines
the specifics of the link state with respect to STE frames
defined by the Spanning Tree Protocol in use. Choice of the split
bridge or independent-bridge model does not affect spanning
operation. In both cases, the spanning tree protocol is executed
the two systems independently
2.5. SR-TB Translational
IEEE 802 is not currently addressing bridges that translate
Transparent Bridging and Source Routing. For the purposes of
standard, such a device is either a Transparent or a Source
bridge, and will act on the line in one of these two ways, just as
does on the LAN
3. Traffic
Several services are provided for the benefit of different
types and user configurations. These include LAN Frame
Preservation, LAN Frame Checksum Generation, Tinygram Compression
and the identification of closed sets of LANs
3.1. LAN Frame Checksum
IEEE 802.1 stipulates that the Extended LAN must enjoy the
probability of undetected error that an individual LAN enjoys
Although there has been considerable debate concerning the algorithm
no other algorithm has been proposed than having the LAN
Checksum received by the ultimate receiver be the same
calculated by the original transmitter. Achieving this requires,
course, that the line protocols preserve the LAN Frame Checksum
end to end. The protocol is optimized towards this approach
3.2. Traffic having no LAN Frame
The fact that the protocol is optimized towards LAN Frame
preservation raises twin questions: "What is the approach to be
by systems which, for whatever reason, cannot easily support
Checksum preservation?" and "What is the approach to be used when
system originates a message, which therefore has no Frame
precalculated?".
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Surely, one approach would be to require stations to calculate
Frame Checksum in software if hardware support were unavailable;
would meet with profound dismay, and would raise serious questions
interpretation in a Bridge/Router
However, stations which implement LAN Frame Checksum
must already solve this problem, as they do originate traffic
Therefore, the solution adopted is that messages which have no
Checksum are tagged and carried across the line
When a system which does not implement LAN Frame
preservation receives a frame having an embedded FCS, it converts
for its own use by removing the trailing four octets. When
system forwards a frame which contains no embedded FCS to a LAN,
forwards it in a way which causes the FCS to be calculated
3.3. Tinygram
An issue in remote Ethernet bridging is that the protocols that
most attractive to bridge are prone to problems on low speed (64
and below) lines. This can be partially alleviated by observing
the vendors defining these protocols often fill the PDU with
of ZERO. Thus, an Ethernet or IEEE 802.3 PDU received from a
that is (1) smaller than the minimum PDU size, and (2) has a
Frame Checksum present, must be padded by inserting zeroes
the last four octets and the rest of the PDU before transmitting
on a LAN. These protocols are frequently used for
sessions, and therefore are frequently this small
To prevent ambiguity, PDUs requiring padding are explicitly tagged
Compression is at the option of the transmitting station, and
probably performed only on low speed lines, perhaps
configuration control
The pseudo-code in Appendix B describes the algorithms
3.4. Virtual
IEEE 802.1Q defines Virtual LANs and their exchangeable VLAN
frame format. Virtual LANs allow user multiple community groups
co-exist within one bridge. A bridging community is identified by
VLAN ID. If a system that supports Virtual LANs receives a frame
the LAN, that frame will be only emitted onto a LAN which belongs
the same community. In order to handle multiple communities on
single line, IEEE 802.1Q defines a VLAN Tagged Frame
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
For example, suppose you have the following configuration
E1 +--+ +--+ E
------------| | | |------------
| | W1 | |
|B1|------------|B2|
E2 | | | | E
------------| | | |------------
+--+ +--+
E1, E2, E3, and E4 are Ethernet LANs (or Token Ring, FDDI, etc.). W
is a WAN (PPP over T1). B1 and B2 are MAC level bridges
You want End Stations on E1 and E3 to communicate, and you want
Stations on E2 and E4 to communicate, but you do not want
Stations on E1 and E3 to communicate with End Stations on E2 and E4.
This is true for Unicast, Multicast, and Broadcast traffic. If
broadcast datagram originates on E1, you want it only to
propagated to E3, and not on E2 or E4.
Another way of looking at it is that E1 and E3 form a Virtual LAN
and E2 and E4 form a Virtual LAN, as if the following
were actually being used
E1 +--+ W2 +--+ E
------------|B3|------------|B4|------------
+--+ +--+
E2 +--+ W3 +--+ E
------------|B5|------------|B6|------------
+--+ +--+
4. A PPP Network Control Protocol for
The Bridging Control Protocol (BCP) is responsible for configuring
enabling and disabling the bridge protocol modules on both ends
the point-to-point link. BCP uses the same packet exchange
as the Link Control Protocol. BCP packets may not be exchanged
PPP has reached the Network-Layer Protocol phase. BCP
received before this phase is reached SHOULD be silently discarded
The Bridging Control Protocol is exactly the same as the Link
Protocol [6] with the following exceptions
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Frame
The packet may utilize any modifications to the basic frame
which have been negotiated during the Link Establishment phase
Implementations SHOULD NOT negotiate Address-and-Control-Field
Compression or Protocol-Field-Compression on other than low
links
Data Link Layer Protocol
Exactly one BCP packet is encapsulated in the PPP
field, where the PPP Protocol field indicates type hex 8031 (BCP).
Code
Only Codes 1 through 7 (Configure-Request, Configure-Ack
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
and Code-Reject) are used. Other Codes SHOULD be treated
unrecognized and SHOULD result in Code-Rejects
BCP packets may not be exchanged until PPP has reached
Network-Layer Protocol phase. An implementation SHOULD
prepared to wait for Authentication and Link Quality
to finish before timing out waiting for a Configure-Ack or
response. It is suggested that an implementation give up
after user intervention or a configurable amount of time
Configuration Option
BCP has a distinct set of Configuration Options, which are
in this document
4.1. Sending Bridge
Before any Bridged LAN Traffic or BPDUs may be communicated, PPP
reach the Network-Layer Protocol phase, and the Bridging
Protocol MUST reach the Opened state
Exactly one Bridged LAN Traffic or BPDU is encapsulated in the
Information field, where the PPP Protocol field indicates type
0031 (Bridged PDU).
Higashiyama & Baker Standards Track [Page 10]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
4.1.1. Maximum Receive Unit
The maximum length of a Bridged datagram transmitted over a PPP
is the same as the maximum length of the Information field of a
encapsulated packet. Since there is no standard method
fragmenting and reassembling Bridged PDUs, PPP links
Bridging MUST negotiate an MRU large enough to support the MAC
that are later negotiated for Bridging support. Because they
the MAC headers, even bridged Ethernet frames are larger than
default PPP MRU of 1500 octets
4.1.2. Loopback and Link Quality
It is strongly recommended that PPP Bridge Protocol
utilize Magic Number Loopback Detection and Link-Quality-Monitoring
The 802.1 Spanning Tree protocol, which is integral to
Transparent Bridging and Source Routing (as standardized),
unidirectional during normal operation. Configuration BPDUs
from the Root system in the general direction of the leaves,
any reverse traffic except in response to network events
4.1.3. Message
The multiple link case requires consideration of
sequentiality. The transmitting system may determine either that
protocol being bridged requires transmissions to arrive in the
of their original transmission, and enqueue all transmissions on
given conversation onto the same link to force order preservation,
that the protocol does NOT require transmissions to arrive in
order of their original transmission, and use that knowledge
optimize the utilization of several links, enqueuing traffic
multiple links to minimize delay
In the absence of such a determination, the transmitting system
act as though all protocols require order preservation.
protocols designed primarily for use on a single LAN require
preservation
PPP Multilink [7] and its multi-class extension [11] may be used
allow the use of multiple PPP links between a pair of systems
loss of message sequentiality. It treats the group of links as
single link with speed equal to the sum of the speeds of the links
the group
Higashiyama & Baker Standards Track [Page 11]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
4.1.4. Separation of Spanning Tree
It is conceivable that a network manager might wish to inhibit
exchange of BPDUs on a link in order to logically divide two
into separate Spanning Trees with different Roots (and
different Spanning Tree implementations or algorithms). In order
do that, he should configure both ends to not exchange BPDUs on
link. An implementation that does not support any spanning
protocol MUST silently discard any received IEEE 802.1D BPDU packets
If a bridge is connected to an old BCP bridge [10], the other
cannot operate according to this specification. Options are
to decide that
(a) If the bridge wants to terminate the connection, it sends
Terminate-Request and terminate the connection
(b) If the bridge wants to run the connection but not receive
BPDUs, its only option is to run without spanning tree on
link at all, which is dangerous. It should Configure-Reject
option and advise the network administration that it has done so
(c) If the bridge chooses to be entirely backward compatible,
sends Configure-Ack and operates in the manner described
Appendix A
In the event that both the new Management-Inline Option and
Spanning-Tree-Protocol-Configuration Option are configure-rejected
indicating that the peer implements no spanning tree protocol at
and doesn't understand the options, it is an
implementation. For safety reasons the system should cease
to configure bridging, and log the fact. If the peer was configure
rejecting the options in order to disable spanning tree entirely,
understood the option but could not within its configuration comply
It should have sent the Spanning-Tree-Protocol-Configuration
with the value NULL
Implementations SHOULD implement a backward compatibility mode
4.2. Bridged LAN Traffic (IEEE 802 Untagged Frame
For Bridging LAN traffic, the format of the frame on the line
shown below. This format is used if the traffic does not include
ID and priority
The fields are transmitted from left to right
Higashiyama & Baker Standards Track [Page 12]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
802.3 Frame format (IEEE 802 Un-tagged Frame
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| HDLC FLAG |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address and Control | 0x00 | 0x31 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|0|Z|0| Pads | MAC Type | Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source MAC Address | Length/Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LLC data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LAN FCS (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| potential line protocol pad |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame FCS | HDLC FLAG |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
802.4/802.5/FDDI Frame format (IEEE 802 Un-tagged Frame
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| HDLC FLAG |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address and Control | 0x00 | 0x31 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|0|Z|0| Pads | MAC Type | Pad Byte | Frame Control |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address | Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LLC data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LAN FCS (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional Data Link Layer padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame FCS | HDLC FLAG |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address and
As defined by the framing in use
PPP
0x0031 for PPP
bit F: Set if the LAN FCS Field is
bit Z: Set if IEEE 802.3 Pad must be zero filled to minimum
bit 0: reserved, must be
Any PPP frame may have padding inserted in the "Optional Data
Layer Padding" field. This number tells the receiving system
many pad octets to strip off
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
MAC
Up-to-date values of the MAC Type field are specified in the
recent "Assigned Numbers" RFC [4]. Current values are assigned
follows
0:
1: IEEE 802.3/Ethernet with canonical
2: IEEE 802.4 with canonical
3: IEEE 802.5 with non-canonical
4: FDDI with non-canonical
5-10:
11: IEEE 802.5 with canonical
12: FDDI with canonical
"Canonical" is the address format defined as standard
representation by the IEEE. In this format, the bit within
byte that is to be transmitted first on a LAN is represented
the least significant bit. In contrast, in non-canonical form
the bit within each byte that is to be transmitted first
represented as the most-significant bit. Many LAN
implementations use non-canonical form. In both formats,
are represented in the order of transmission
If an implementation supports a MAC Type that is the higher
numbered format of that MAC Type, then it MUST also support
lower-numbered format of that MAC Type. For example, if
implementation supports FDDI with canonical address format,
it MUST also support FDDI with non-canonical address format.
purpose of this requirement is to provide backward
with earlier versions of this specification
A system MUST NOT transmit a MAC Type numbered higher than 4
unless it has received from its peer a MAC-Support
Option indicating that the peer is willing to receive frames
that MAC Type
Frame
On 802.4, 802.5, and FDDI LANs, there are a few octets
the Destination MAC Address, one of which is protected by the FCS
The MAC Type of the frame determines the contents of the
Control field. A pad octet is present to provide 32-bit
alignment
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Destination MAC
As defined by the IEEE. The MAC Type field defines the
ordering
Source MAC
As defined by the IEEE. The MAC Type field defines the
ordering
LLC
This is the remainder of the MAC frame which is (or would be
it present) protected by the LAN FCS
For example, the 802.5 Access Control field, and Status
are not meaningful to transmit to another ring, and are omitted
LAN
If present, this is the LAN FCS which was calculated by (or
appears to have been calculated by) the originating station.
the LAN FCS flag is not set, then this field is not present,
the PDU is four octets shorter
Optional Data Link Layer
Any PPP frame may have padding inserted between the
field and the Frame FCS. The Pads field contains the length
this padding, which may not exceed 15 octets
The PPP LCP Extensions [5] specify a self-describing pad
Implementations are encouraged to set the Pads field to zero,
use the self-describing pad instead
Frame
Mentioned primarily for clarity. The FCS used on the PPP link
separate from and unrelated to the LAN FCS
4.3. Bridged LAN Traffic in IEEE 802 Tagged
To connect two or more Virtual LAN segments, the frame MUST
its VLAN ID and priority. An IEEE 802 Tagged Frame may be used if
IEEE-802-Tagged-Frame Option is accepted by the peer. The format
the frame on the line is shown below
The fields are transmitted from left to right
Higashiyama & Baker Standards Track [Page 16]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
802.3 Frame format (IEEE 802 Tagged Frame
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| HDLC FLAG |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address and Control | 0x00 | 0x31 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|0|Z|0| Pads | MAC Type | Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source MAC Address | 0x81 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Pri |C| VLAN ID | Length/Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LLC data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LAN FCS (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| potential line protocol pad |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame FCS | HDLC FLAG |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Higashiyama & Baker Standards Track [Page 17]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
802.4/802.5/FDDI Frame format (IEEE 802 Tagged Frame
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| HDLC FLAG |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address and Control | 0x00 | 0x31 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|0|Z|0| Pads | MAC Type | Pad Byte | Frame Control |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address | Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SNAP-encoded TPID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SNAP-encoded TPID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Pri |C| VLAN ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LLC data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LAN FCS (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional Data Link Layer padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame FCS | HDLC FLAG |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address and
As defined by the framing in use
PPP
0x0031 for PPP
bit F: Set if the LAN FCS Field is
bit Z: Set if IEEE 802.3 Pad must be zero filled to minimum
bit 0: reserved, must be
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Any PPP frame may have padding inserted in the "Optional Data
Layer Padding" field. This number tells the receiving system
many pad octets to strip off
MAC
Up-to-date values of the MAC Type field are specified in the
recent "Assigned Numbers" RFC [4]. Current values are assigned
follows
0:
1: IEEE 802.3/Ethernet with canonical
2: IEEE 802.4 with canonical
3: IEEE 802.5 with non-canonical
4: FDDI with non-canonical
5-10:
11: IEEE 802.5 with canonical
12: FDDI with canonical
"Canonical" is the address format defined as standard
representation by the IEEE. In this format, the bit within
byte that is to be transmitted first on a LAN is represented
the least significant bit. In contrast, in non-canonical form
the bit within each byte that is to be transmitted first
represented as the most-significant bit. Many LAN
implementations use non-canonical form. In both formats,
are represented in the order of transmission
If an implementation supports a MAC Type that is the higher
numbered format of that MAC Type, then it MUST also support
lower-numbered format of that MAC Type. For example, if
implementation supports FDDI with canonical address format,
it MUST also support FDDI with non-canonical address format.
purpose of this requirement is to provide backward
with earlier versions of this specification
A system MUST NOT transmit a MAC Type numbered higher than 4
unless it has received from its peer a MAC-Support
Option indicating that the peer is willing to receive frames
that MAC Type
Frame
On 802.4, 802.5, and FDDI LANs, there are a few octets
the Destination MAC Address, one of which is protected by the FCS
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
The MAC Type of the frame determines the contents of the
Control field. A pad octet is present to provide 32-bit
alignment
Destination MAC
As defined by the IEEE. The MAC Type field defines the
ordering
Source MAC
As defined by the IEEE. The MAC Type field defines the
ordering
3 bit priority value as defined by IEEE 802.1D
Canonical flag as defined by IEEE 802.1Q. It must be set if
data is present in the LLC data
VLAN
12 bit VLAN identifier number as defined by IEEE 802.1Q
LLC
This is the remainder of the MAC frame which is (or would be
it present) protected by the LAN FCS
For example, the 802.5 Access Control field, and Status
are not meaningful to transmit to another ring, and are omitted
LAN
If present, this is the LAN FCS which was calculated by (or
appears to have been calculated by) the originating station.
the LAN FCS flag is not set, then this field is not present,
the PDU is four octets shorter
Optional Data Link Layer
Any PPP frame may have padding inserted between the
field and the Frame FCS. The Pads field contains the length
this padding, which may not exceed 15 octets
The PPP LCP Extensions [5] specify a self-describing pad
Implementations are encouraged to set the Pads field to zero,
use the self-describing pad instead
Higashiyama & Baker Standards Track [Page 20]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Frame
Mentioned primarily for clarity. The FCS used on the PPP link
separate from and unrelated to the LAN FCS
4.4. Bridge protocols and GARP
To avoid network loops and improve redundancy, Bridges exchange
Spanning Tree Protocol data unit known as BPDU. Bridges also
a Generic Attributes Registration Protocol data unit to carry
GARP VLAN Registration Protocol (GVRP) data and GARP
Registration Protocol (GMRP). GVRP allow the Bridges to create
groups dynamically. GMRP allows bridges to filter Multicast data
the receiver is absent from the network. These Bridge
include Spanning Tree Protocol and GARP protocols data units
carried with a special destination address assigned by the IEEE
These bridge protocols data units and GARP protocol data units
be carried in the frame format shown in section 4.2 or 4.3.
Bridge that receives these data units identifies these
based on the destination address in the frame format, just like
operation of receiving frames from a LAN segment
Bridge protocols and GARP protocols data units MUST be recognized
checking the destination addresses, which are assigned by IEEE
01-80-c2-00-00-00 Bridge Group Address (used by STP
01-80-c2-00-00-01 IEEE Std. 802.3x Full Duplex PAUSE
01-80-c2-00-00-10 Bridge Management Group
01-80-c2-00-00-20 GARP Multicast Registration Protocol (GMRP
01-80-c2-00-00-21 GARP VLAN Registration Protocol (GVRP
But there is one exception to this rule: if the bridge is
to an old BCP bridge [10] and can support backward compatibility,
MUST send the BPDU in the old format described in Appendix A
5. BCP Configuration
BCP Configuration Options allow modifications to the
characteristics of the network-layer protocol to be negotiated. If
Configuration Option is not included in a Configure-Request packet
the default value for that Configuration Option is assumed
BCP uses the same Configuration Option format defined for LCP [6],
with a separate set of Options
Higashiyama & Baker Standards Track [Page 21]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Up-to-date values of the BCP Option Type field are specified in
most recent "Assigned Numbers" RFC [4]. Current values are
as follows
1 Bridge-
2 Line-
3 MAC-
4 Tinygram-
5 LAN-Identification (obsoleted
6 MAC-
7 Spanning-Tree-Protocol (old formatted
8 IEEE 802 Tagged
9 Management
5.1. Bridge-
The Bridge-Identification Configuration Option is designed for
when the line is an interface between half bridges
virtual or physical LAN segments. Since these remote bridges
modeled as a single bridge with a strange internal interface,
remote bridge needs to know the LAN segment and bridge numbers
the adjacent remote bridge. This option MUST NOT be included
the same Configure-Request as the Line-Identification option
The Source Routing Route Descriptor and its use are specified
the IEEE 802.1D Appendix on Source Routing. It identifies
segment to which the interface is attached by its
segment number, and itself by bridge number on the segment
The two half bridges MUST agree on the bridge number. If a
number is not agreed upon, the Bridging Control Protocol MUST
enter the Opened state
Since mismatched bridge numbers are indicative of a
error, a correct configuration requires that either the
declare the misconfiguration or choose one of the options.
allow two systems to proceed to the Opened state despite
mismatch, a system MAY change its bridge number to the higher
the two numbers. A higher-numbered system MUST NOT change
bridge number to a lower number. It should, however, inform
network administration of the misconfiguration in any case
By default, a system that does not negotiate this option
assumed to be configured not to use the model of the two
as two halves of a single source-route bridge. It is
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RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
assumed to be configured to use the model of the two systems
two independent bridges
If System A announces LAN Segment AAA, Bridge #1, and System
announces LAN Segment BBB, Bridge #1, then the resulting
Routing configuration (read in the appropriate direction) is
AAA,1,BBB
A summary of the Bridge-Identification Option format is shown below
The fields are transmitted from left to right
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | LAN Segment Number |Bridge#|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1
4
LAN Segment
A 12-bit number identifying the LAN segment, as defined in
IEEE 802.1D Source Routing Specification
Bridge
A 4-bit number identifying the bridge on the LAN segment,
defined in the IEEE 802.1D Source Routing Specification
5.2. Line-
The Line-Identification Configuration Option is designed for
when the line is assigned a LAN segment number as though it were
two system LAN segment in accordance with the Source
algorithm
Higashiyama & Baker Standards Track [Page 23]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
The Source Routing Route Descriptor and its use are specified
the IEEE 802.1D Appendix on Source Routing. It identifies
segment to which the interface is attached by its
segment number, and itself by bridge number on the segment
The two bridges MUST agree on the LAN segment number. If a
segment number is not agreed upon, the Bridging Control
MUST NOT enter the Opened state
Since mismatched LAN segment numbers are indicative of
configuration error, a correct configuration requires that
the bridge declare the misconfiguration or choose one of
options. To allow two systems to proceed to the Opened
despite a mismatch, a system MAY change its LAN segment number
the higher of the two numbers. A higher-numbered system MUST
change its LAN segment number to a lower number. It should
however, inform the network administration of the
in any case
By default, a system that does not negotiate this option
assumed to have its LAN segment number correctly configured by
user
A summary of the Line-Identification Option format is shown below
The fields are transmitted from left to right
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | LAN Segment Number |Bridge#|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2
4
LAN Segment
A 12-bit number identifying the LAN segment, as defined in
IEEE 802.1D Source Routing Specification
Higashiyama & Baker Standards Track [Page 24]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Bridge
A 4-bit number identifying the bridge on the LAN segment,
defined in the IEEE 802.1D Source Routing Specification
5.3. MAC-
The MAC-Support Configuration Option is provided to
implementations to indicate the sort of traffic they are
to receive. Negotiation of this option is strongly recommended
By default, when an implementation does not announce the MAC
that it supports, all MAC Types are sent by the peer which
capable of being transported given other configuration parameters
The receiver will discard those MAC Types that it does
support
A device supporting a 1600 octet MRU might not be willing
support 802.5, 802.4 or FDDI, which each support frames
than 1600 octets
By announcing the MAC Types it will support, an implementation
advising its peer that all unspecified MAC Types will
discarded. The peer MAY then reduce bandwidth usage by
sending the unsupported MAC Types
Announcement of support for multiple MAC Types is accomplished
placing multiple options in the Configure-Request
The nature of this option is advisory only. This option MUST
be included in a Configure-Nak
A summary of the MAC-Support Option format is shown below.
fields are transmitted from left to right
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | MAC Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3
Higashiyama & Baker Standards Track [Page 25]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
3
MAC
One of the values of the PDU MAC Type field (previously
in the "Bridged LAN Traffic" section) that this system is
to receive and service
5.4. Tinygram-
This Configuration Option permits the implementation to
support for Tinygram compression
Not all systems are prepared to make modifications to messages
transit. On high speed lines, it is probably not worth
effort
This option MUST NOT be included in a Configure-Nak if it has
received in a Configure-Request. This option MAY be included in
Configure-Nak in order to prompt the peer to send the option
its next Configure-Request
By default, no compression is allowed. A system which does
negotiate, or negotiates this option to be disabled, should
receive a compressed packet
A summary of the Tinygram-Compression Option format is shown below
The fields are transmitted from left to right
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Enable/Disable
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4
3
Higashiyama & Baker Standards Track [Page 26]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Enable/
If the value is 1, Tinygram-Compression is enabled. If the
is 2, Tinygram-Compression is disabled, and no decompression
occur
The implementations need not agree on the setting of
parameter. One may be willing to decompress and the other not
5.5. MAC-
The MAC-Address Configuration Option enables the implementation
announce its MAC address or have one assigned. The MAC address
represented in IEEE 802.1 Canonical format, which is to say
the multicast bit is the least significant bit of the first
of the address
If the system wishes to announce its MAC address, it sends
option with its MAC address specified. When specifying a non-
MAC address in a Configure-Request, any inclusion of this
in a Configure-Nak MUST be ignored
If the implementation wishes to have a MAC address assigned,
sends the option with a MAC address of 00-00-00-00-00-00.
that have no mechanism for address assignment will Configure
Reject the option
A Configure-Nak MUST specify a valid IEEE 802.1 format
address; the multicast bit MUST be zero. It is
recommended (although not mandatory) that the "locally
address" bit (the second least significant bit in the first octet
be set, indicating a locally assigned address
A summary of the MAC-Address Option format is shown below.
fields are transmitted from left to right
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |MAC byte 1 |L|M| MAC byte 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC byte 3 | MAC byte 4 | MAC byte 5 | MAC byte 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Higashiyama & Baker Standards Track [Page 27]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
6
8
MAC
Six octets of MAC address in 802.1 Canonical order. For clarity
the position of the Local Assignment (L) and Multicast (M)
are shown in the diagram
5.6. Spanning-Tree-Protocol (old format
The Spanning-Tree-Protocol Configuration enables a Bridge
remain compatible with older implementations of BCP [10].
configuration option is, however, incompatible with
Management-Inline option, which enables a bridge to implement
many protocols that IEEE now expects a bridge to be able to use
If the peer rejects the Management-Inline configuration option,
sending configure-reject, it must be an implementation of [10],
which is described in Appendix A. The system may
terminate the negotiation or offer to negotiate in that manner
In this case, if both bridges support a spanning tree protocol
they MUST agree on the protocol to be supported. The old
described in Appendix A MUST be used rather than the format
in section 4.2 or 4.3. When the two disagree, the lower-
of the two spanning tree protocols should be used. To resolve
conflict, the system with the lower-numbered protocol
Configure-Nak the option, suggesting its own protocol for use.
a spanning tree protocol is not agreed upon, except for the
in which one system does not support any spanning tree protocol
the Bridging Control Protocol MUST NOT enter the Opened state
Most systems will only participate in a single spanning
protocol. If a system wishes to participate simultaneously
more than one spanning tree protocol, it MAY include all of
appropriate protocol types in a single Spanning-Tree-
Configuration Option. The protocol types MUST be specified
increasing numerical order. For the purpose of comparison
negotiation, the protocol numbers MUST be considered to be
single number. For instance, if System A includes protocols 01
Higashiyama & Baker Standards Track [Page 28]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
and 03 and System B indicates protocol 03, System B
Configure-Nak and indicate a protocol type of 03 since 0103
greater than 03.
By default, an implementation MUST either support the IEEE 802.1
spanning tree or support no spanning tree protocol.
implementation that does not support any spanning tree
MUST silently discard any received IEEE 802.1D BPDU packets,
MUST either silently discard or respond to other received
packets with an LCP Protocol-Reject packet in this case
A summary of the Spanning-Tree-Protocol Option format is shown below
The fields are transmitted from left to right
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Type | Length | Protocol 1 | Protocol 2 | ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
7
2 octets plus 1 additional octet for each protocol that will
actively supported. Most systems will only support a
spanning tree protocol, resulting in a length of 3.
Protocol
Each Protocol field is one octet and indicates a desired
tree protocol. Up-to-date values of the Spanning-Tree-
field are specified as PPP DLL numbers in the most
"Assigned Numbers" RFC [4]. Current values are assigned
follows
Value
0 Null (no Spanning Tree protocol supported
1 IEEE 802.1D spanning
2 IEEE 802.1G extended spanning tree
3 IBM Source Route Spanning tree
4 DEC LANbridge 100 Spanning tree
Higashiyama & Baker Standards Track [Page 29]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
5.7. IEEE-802-Tagged-
This configuration option permits the implementation to
support for IEEE 802 Tagged Frame. Negotiation of this option
strongly recommended
A device supporting IEEE 802 Tagged Frame must be willing
support IEEE 802 Tagged Frame shown in section 4.3.
By default, IEEE 802 Tagged Frame is not supported. A system
does not negotiate, or negotiates this option to be disabled
should never receive a IEEE 802 Tagged Frame
A summary of the IEEE 802 Tagged Frame Option format is shown below
The fields are transmitted from left to right
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Enable/Disable
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8
3
Enable/
If the value is 1, IEEE-802-Tagged-Frame is enabled. If the
is 2, IEEE-802-Tagged-Frame is disabled, and MUST not send
IEEE-802-Tagged-Frame packet
5.8. Management-
The Management-Inline Configuration Option indicates that
system is willing to receive any IEEE-defined inter-
protocols, such as bridge protocol data units and GARP
data units, in the frame format shown in section 4.2 or 4.3.
Higashiyama & Baker Standards Track [Page 30]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
Old BCP [10] implementations will use the negotiation
described in section 5.6. Implementations of this procedure
use this option to indicate compliance with the new BCP and
optionally negotiate the section 5.6 procedure, either on the
configure-request or in response to a configure-reject, as well
It is recommended that the configure-request only show this
when it is relevant, and that it reply with the Spanning-Tree
Protocol (old formatted) option if a configure-reject is received
as in the normal case one can expect it to be the
negotiation
If a system receives a configure-request offering
alternatives, it should accept this procedure and reject
Spanning-Tree-Protocol (old format) option
One can expect old BCP [10] implementations to not understand
option and issue a configure-reject
By default, Management-Inline is not allowed. A system which
not negotiate, or negotiates this option to be disabled,
never receive a Bridge Protocol data unit or GARP protocol
unit inline
A summary of the Management-Inline Option format is shown below
The fields are transmitted from left to right
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9
2
6. Changes From RFC 1638
This section enumerates changes made to old BCP [10] to produce
document
(1) Remove all LAN Identification descriptions and replace with
802.1Q VLAN descriptions
Higashiyama & Baker Standards Track [Page 31]
RFC 2878 PPP Bridging Control Protocol (BCP) July 2000
(2) Remove LAN Identification field from frame format and I
from flag field
(3) Merge the Spanning Tree BPDU frame format with Bridged traffic
7. Security
This network control protocol compares the configurations of
devices and seeks to negotiate an acceptable subset of
intersection, to enable correct interoperation even in the
of minor configuration or implementation differences. In the
that a major misconfiguration is detected, the negotiation will
complete successfully, resulting in the link coming down or
coming up. It is possible that if a bridged link comes up with
rogue peer, network information may be learned from
multicast traffic, or denial of service attacks may be created
closing loops that should be detected and isolated or by
rogue load
Such attacks are not isolated to this NCP; any PPP NCP is subject
attack when connecting to a foreign or compromised device. However
no situations arise which are not common to all NCPs; any NCP
comes up with a rogue peer is subject to snooping and other attacks
Therefore, it is recommended that links on which this may
should be configured to use PPP authentication during the LCP start
up phase
8. Intellectual Property
The IETF takes no position regarding the validity or scope of
intellectual 