📄 rfc1490.txt
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protocol address when the hardware address is already known. In
Frame Relay's case, the known hardware address is the DLCI. Using
Inverse ARP for Frame Relay follows the same pattern as ARP and RARP
use. That is the source hardware address is inserted at the
receiving station.
In our example, station A may use Inverse ARP to discover the
protocol address of the station associated with its DLCI 50. The
Inverse ARP request would be as follows:
InARP Request from A (DLCI 50)
ar$op 8 (InARP request)
ar$sha unknown
ar$spa pA
ar$tha 0x0C21 (DLCI 50)
ar$tpa unknown
When Station B receives this packet, it will modify the source
hardware address with the Q.922 address from the Frame Relay header.
This way, the InARP request from A will become:
ar$op 8 (InARP request)
ar$sha 0x1061
ar$spa pA
ar$tha 0x0C21
ar$tpa unknown.
Station B will format an Inverse ARP response and send it to station
A as it would for any ARP message.
8. IP over Frame Relay
Internet Protocol [9] (IP) datagrams sent over a Frame Relay network
conform to the encapsulation described previously. Within this
context, IP could be encapsulated in two different ways.
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RFC 1490 Multiprotocol over Frame Relay July 1993
1. NLPID value indicating IP
+-----------------------+-----------------------+
| Q.922 Address |
+-----------------------+-----------------------+
| Control (UI) 0x03 | NLPID = 0xCC |
+-----------------------+-----------------------+
| IP Packet . |
| . |
| . |
+-----------------------+-----------------------+
2. NLPID value indicating SNAP
+-----------------------+-----------------------+
| Q.922 Address |
+-----------------------+-----------------------+
| Control (UI) 0x03 | pad 0x00 |
+-----------------------+-----------------------+
| NLPID = 0x80 | | SNAP Header
+-----------------------+ OUI = 0x00-00-00 + Indicating
| | IP
+-----------------------+-----------------------+
| PID = 0x0800 |
+-----------------------+-----------------------+
| IP packet |
| . |
| . |
| . |
+-----------------------+-----------------------+
Although both of these encapsulations are supported under the given
definitions, it is advantageous to select only one method as the
appropriate mechanism for encapsulating IP data. Therefore, IP data
shall be encapsulated using the NLPID value of 0xCC indicating IP as
shown in option 1 above. This (option 1) is more efficient in
transmission (48 fewer bits), and is consistent with the
encapsulation of IP in X.25.
9. Other Protocols over Frame Relay
As with IP encapsulation, there are alternate ways to transmit
various protocols within the scope of this definition. To eliminate
the conflicts, the SNAP encapsulation is only used if no NLPID value
is defined for the given protocol.
As an example of how this works, ISO CLNP has a NLPID defined (0x81).
Bradley, Brown & Malis [Page 25]
RFC 1490 Multiprotocol over Frame Relay July 1993
Therefore, the NLPID field will indicate ISO CLNP and the data packet
will follow immediately. The frame would be as follows:
+---------------------------------------------+
| Q.922 Address |
+----------------------+----------------------+
| Control (0x03) | NLPID - 0x81 (CLNP) |
+----------------------+----------------------+
| remainder of CLNP packet |
| . |
| . |
+---------------------------------------------+
In this example, the NLPID is used to identify the data packet as
CLNP. It is also considered part of the CLNP packet and as such, the
NLPID should not be removed before being sent to the upper layers for
processing. The NLPID is not duplicated.
Other protocols, such as IPX, do not have a NLPID value defined. As
mentioned above, IPX would be encapsulated using the SNAP header. In
this case, the frame would be as follows:
+---------------------------------------------+
| Q.922 Address |
+----------------------+----------------------+
| Control 0x03 | pad 0x00 |
+----------------------+----------------------+
| NLPID - 0x80 (SNAP) | OUI - 0x00 00 00 |
+----------------------+ |
| |
+---------------------------------------------+
| PID = 0x8137 |
+---------------------------------------------+
| IPX packet |
| . |
| . |
+---------------------------------------------+
10. Bridging Model for Frame Relay
The model for bridging in a Frame Relay network is identical to the
model for remote bridging as described in IEEE P802.1g "Remote MAC
Bridging" [13] and supports the concept of "Virtual Ports". Remote
bridges with LAN ports receive and transmit MAC frames to and from
the LANS to which they are attached. They may also receive and
transmit MAC frames through virtual ports to and from other remote
bridges. A virtual port may represent an abstraction of a remote
bridge's point of access to one, two or more other remote bridges.
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RFC 1490 Multiprotocol over Frame Relay July 1993
Remote Bridges are statically configured as members of a remote
bridge group by management. All members of a remote bridge group are
connected by one or more virtual ports. The set of remote MAC bridges
in a remote bridge group provides actual or *potential* MAC layer
interconnection between a set of LANs and other remote bridge groups
to which the remote bridges attach.
In a Frame Relay network there must be a full mesh of Frame Relay VCs
between bridges of a remote bridge group. If the frame relay network
is not a full mesh, then the bridge network must be divided into
multiple remote bridge groups.
The frame relay VCs that interconnect the bridges of a remote bridge
group may be combined or used individually to form one or more
virtual bridge ports. This gives flexibility to treat the Frame
Relay interface either as a single virtual bridge port, with all VCs
in a group, or as a collection of bridge ports (individual or grouped
VCs).
When a single virtual bridge port provides the interconnectivity for
all bridges of a given remote bridge group (i.e. all VCs are combined
into a single virtual port), the standard Spanning Tree Algorithm may
be used to determine the state of the virtual port. When more than
one virtual port is configured within a given remote bridge group
then an "extended" Spanning Tree Algorithm is required. Such an
extended algorithm is defined in IEEE 802.1g [13]. The operation of
this algorithm is such that a virtual port is only put into backup if
there is a loop in the network external to the remote bridge group.
The simplest bridge configuration for a Frame Relay network is the
LAN view where all VCs are combined into a single virtual port.
Frames, such as BPDUs, which would be broadcast on a LAN, must be
flooded to each VC (or multicast if the service is developed for
Frame Relay services). Flooding is performed by sending the packet to
each relevant DLC associated with the Frame Relay interface. The VCs
in this environment are generally invisible to the bridge. That is,
the bridge sends a flooded frame to the frame relay interface and
does not "see" that the frame is being forwarded to each VC
individually. If all participating bridges are fully connected (full
mesh) the standard Spanning Tree Algorithm will suffice in this
configuration.
Typically LAN bridges learn which interface a particular end station
may be reached on by associating a MAC address with a bridge port.
In a Frame Relay network configured for the LAN-like single bridge
port (or any set of VCs grouped together to form a single bridge
port), however, the bridge must not only associated a MAC address
with a bridge port, but it must also associate it with a connection
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RFC 1490 Multiprotocol over Frame Relay July 1993
identifier. For Frame Relay networks, this connection identifier is
a DLCI. It is unreasonable and perhaps impossible to require bridges
to statically configure an association of every possible destination
MAC address with a DLC. Therefore, Frame Relay LAN-modeled bridges
must provide a mechanism to allow the Frame Relay bridge port to
dynamically learn the associations. To accomplish this dynamic
learning, a bridged packet shall conform to the encapsulation
described within section 7. In this way, the receiving Frame Relay
interface will know to look into the bridged packet to gather the
appropriate information.
A second Frame Relay bridging approach, the point-to-point view,
treats each Frame Relay VC as a separate bridge port. Flooding and
forwarding packets are significantly less complicated using the
point-to-point approach because each bridge port has only one
destination. There is no need to perform artificial flooding or to
associate DLCIs with destination MAC addresses. Depending upon the
interconnection of the VCs, an extended Spanning Tree algorithm may
be required to permit all virtual ports to remain active as long as
there are no true loops in the topology external to the remote bridge
group.
It is also possible to combine the LAN view and the point-to-point
view on a single Frame Relay interface. To do this, certain VCs are
combined to form a single virtual bridge port while other VCs are
independent bridge ports.
The following drawing illustrates the different possible bridging
configurations. The dashed lines between boxes represent virtual
circuits.
+-------+
-------------------| B |
/ -------| |
/ / +-------+
/ |
+-------+/ \ +-------+
| A | -------| C |
| |-----------------------| |
+-------+\ +-------+
\
\ +-------+
\ | D |
-------------------| |
+-------+
Since there is less than a full mesh of VCs between the bridges in
this example, the network must be divided into more than one remote
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RFC 1490 Multiprotocol over Frame Relay July 1993
bridge group. A reasonable configuration is to have bridges A, B,
and C in one group, and have bridges A and D in a second.
Configuration of the first bridge group combines the VCs
interconnection the three bridges (A, B, and C) into a single virtual
port. This is an example of the LAN view configuration. The second
group would also be a single virtual port which simply connects
bridges A and D. In this configuration the standard Spanning Tree
Algorithm is sufficient to detect loops.
An alternative configuration has three individual virtual ports in
the first group corresponding to the VCs interconnecting bridges A, B
and C. Since the application of the standard Spanning Tree Algorithm
to this configuration would detect a loop in the topology, an
extended Spanning Tree Algorithm would have to be used in order for
all virtual ports to be kept active. Note that the second group
would still consist of a single virtual port and the standard
Spanning Tree Algorithm could be used in this group.
Using the same drawing, one could construct a remote bridge scenario
with three bridge groups. This would be an example of the point-to-
point case. Here, the VC connecting A and B, the VC connecting A and
C, and the VC connecting A and D are all bridge groups with a single
virtual port.
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