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IETF RFC 6439
Last modified on Tuesday, November 22nd, 2011
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Internet Engineering Task Force (IETF) R. Perlman
Request for Comments: 6439 Intel Labs
Updates: 6325 D. Eastlake 3rd
Category: Standards Track Y. Li
ISSN: 2070-1721 Huawei Technologies
A. Banerjee
Cisco Systems
F. Hu
ZTE Corporation
November 2011
Routing Bridges (RBridges): Appointed Forwarders
 Abstract
The IETF TRILL (TRansparent Interconnection of Lots of Links)
protocol provides least cost pair-wise data forwarding without
configuration in multi-hop networks with arbitrary topology, safe
forwarding even during periods of temporary loops, and support for
multipathing of both unicast and multicast traffic. TRILL
accomplishes this by using IS-IS (Intermediate System to Intermediate
System) link state routing and by encapsulating traffic using a
header that includes a hop count. Devices that implement TRILL are
called "RBridges" (Routing Bridges).
TRILL supports multi-access LAN (Local Area Network) links that can
have multiple end stations and RBridges attached. Where multiple
RBridges are attached to a link, native traffic to and from end
stations on that link is handled by a subset of those RBridges called
"Appointed Forwarders", with the intent that native traffic in each
VLAN (Virtual LAN) be handled by at most one RBridge. The purpose of
this document is to improve the documentation of the Appointed
Forwarder mechanism; thus, it updates RFC 6325.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6439.
Perlman, et al. Standards Track PAGE 1 
RFC 6439 RBridges: Appointed Forwarders November 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
1.1. Terminology and Acronyms ...................................3
2. Appointed Forwarders and Their Appointment ......................4
2.1. Appointment Effects of DRB Elections .......................5
2.2. Appointment and Removal by the DRB .........................5
2.2.1. Processing Forwarder Appointments ...................6
2.2.2. Frequency of Appointments ...........................7
2.2.3. Appointed Forwarders Limit ..........................8
2.3. Local Configuration Action Appointment Effects .............8
2.4. VLAN Mapping within a Link .................................9
3. The Inhibition Mechanism ........................................9
4. Inhibited Appointed Forwarder Behavior .........................11
5. Multiple Ports on the Same Link ................................12
6. Security Considerations ........................................12
7. Acknowledgements ...............................................13
8. References .....................................................13
8.1. Normative References ......................................13
8.2. Informative References ....................................13
Appendix. VLAN Inhibition Example .................................14
1. Introduction
The IETF TRILL (TRansparent Interconnection of Lots of Links)
protocol [RFC 6325] provides optimal pair-wise data frame forwarding
without configuration in multi-hop networks with arbitrary topology,
safe forwarding even during periods of temporary loops, and support
for multipathing of both unicast and multicast traffic. TRILL
accomplishes this by using IS-IS (Intermediate System to Intermediate
System) [IS-IS] [RFC 1195] link state routing and encapsulating
traffic using a header that includes a hop count. The design
Perlman, et al. Standards Track PAGE 2
RFC 6439 RBridges: Appointed Forwarders November 2011
supports VLANs (Virtual Local Area Networks) and optimization of the
distribution of multi-destination frames based on VLANs and IP-
derived multicast groups. Devices that implement TRILL are called
"RBridges" (Routing Bridges).
Section 2 of [RFC 6327] explains the environment for which the TRILL
protocol is designed and the differences between that environment and
the typical Layer 3 routing environment.
TRILL supports multi-access LAN (Local Area Network) links that can
have multiple end stations and RBridges attached. Where multiple
RBridges are attached to a link, native traffic to and from end
stations on that link is handled by a subset of those RBridges called
"Appointed Forwarders", with the intent that native traffic in each
VLAN be handled by at most one RBridge. An RBridge can be Appointed
Forwarder for many VLANs.
The purpose of this document is to improve the documentation of the
Appointed Forwarder mechanism; thus, it updates RFC 6325. It
includes reference implementation details. Alternative
implementations that interoperate on the wire are permitted.
The Appointed Forwarder mechanism is irrelevant to any link on which
end station service is not offered. This includes links configured
as point-to-point IS-IS links and any link with all RBridge ports on
that link configured as trunk ports. (In TRILL, configuration of a
port as a "trunk port" just means that no end station service will be
provided. It does not imply that all VLANs are enabled on that
port.)
The Appointed Forwarder mechanism has no effect on the formation of
adjacencies, the election of the Designated RBridge (DRB) for a link,
MTU matching, or pseudonode formation. Those topics are covered in
[RFC 6327]. Furthermore, Appointed Forwarder status has no effect on
the forwarding of TRILL Data frames. It only affects the handling of
native frames.
For other aspects of the TRILL base protocol, see [RFC 6325] and
[RFC 6327]. Familiarity with [RFC 6325] and [RFC 6327] is assumed in
this document. In case of conflict between this document and
[RFC 6325], this document prevails.
1.1. Terminology and Acronyms
This document uses the acronyms defined in [RFC 6325].
A "trunk port" is a port configured with the "end station service
disable" bit on, as described in Section 4.9.1 of [RFC 6325].
Perlman, et al. Standards Track PAGE 3
RFC 6439 RBridges: Appointed Forwarders November 2011
In this document, the term "link" means "bridged LAN", that is to say
some combination of physical links with zero or more bridges, hubs,
repeaters, or the like.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC 2119].
2. Appointed Forwarders and Their Appointment
The Appointed Forwarder on a link for VLAN-x is the RBridge that
ingresses native frames from the link and egresses native frames to
the link in VLAN-x. By default, the DRB (Designated RBridge) on a
link is in charge of native traffic for all VLANs on the link. The
DRB may, if it wishes, act as Appointed Forwarder for any VLAN and it
may appoint other RBridges that have ports on the link as Appointed
Forwarder for one or more VLANs.
It is important that there not be two Appointed Forwarders on a link
that are ingressing and egressing native frames for the same VLAN at
the same time. Should this occur, it could form a loop where frames
are not protected by a TRILL Hop Count for part of the loop. (Such a
condition can even occur through two Appointed Forwarders for two
different VLANs, VLAN-x and VLAN-y, if ports or bridges inside the
link are configured to map frames between VLAN-x and VLAN-y as
discussed in Section 2.4.) While TRILL tries to avoid such
situations, for loop safety there is also an "inhibition" mechanism
(see Section 3) that can cause an RBridge that is an Appointed
Forwarder to not ingress or egress native frames.
As discussed in Section 5, an RBridge may have multiple ports on a
link. As discussed in [RFC 6327], if there are multiple ports with
the same Media Access Control (MAC) address on a link, all but one
will be suspended. The case of multiple ports on a link for one
RBridge and the case of multiple ports with the same MAC address on a
link and combinations of these cases are fully accommodated; however,
multiple ports on a link for one RBridge is expected to be a rare
condition and duplicate MAC addresses are not recommended by either
TRILL or IEEE 802.1 standards.
Appointed Forwarder status has no effect on the forwarding of TRILL
Data frames. It only affects the handling of native frames.
Perlman, et al. Standards Track PAGE 4
RFC 6439 RBridges: Appointed Forwarders November 2011
There are three mechanisms by which an RBridge can be appointed or
un-appointed as Appointed Forwarder: as a result of the DRB elections
[RFC 6327] as discussed in Section 2.1, as a result of action by the
DRB as discussed in Section 2.2, as a result of a local configuration
action as discussed in Section 2.3.
2.1. Appointment Effects of DRB Elections
When an RBridge believes that it has become the DRB on a link, by
default, it can act as Appointed Forwarder for any VLANs on that link
that it chooses as long as its port is not configured as a trunk port
and has that VLAN enabled (or at least one of its ports meets these
criteria, if it has more than one port on the link).
An RBridge loses all Appointed Forwarder status when:
1. it decides that it has lost the status of being the DRB for a
link; or
2. it observes a change in the RBridge that is the DRB for the link
without itself becoming the DRB.
In the rare corner case where an RBridge has more than one port on a
link, one of which was previously the DRB election winner but has
just lost the DRB election to a different port of the same RBridge
(possibly due to management configuration of port priorities), there
is no change in which RBridge is the DRB. Therefore, neither of the
above points applies and there is no change in Appointed Forwarder
status.
2.2. Appointment and Removal by the DRB
The DRB may appoint other RBridges on the link through inclusion of
one or more Appointed Forwarders sub-TLVs [RFC 6326] in a TRILL Hello
it sends on the Designated VLAN out the port that won the DRB
election. When the DRB sends any appointments in a TRILL Hello, it
must send all appointments for that link in that Hello. Any previous
appointment not included is implicitly revoked.
Although the DRB does not need to announce the VLANs for which it has
chosen to act as Appointed Forwarder by sending appoints for itself,
if the DRB wishes to revoke all appointments for RBridges other than
itself on the link, it is recommended that it send a TRILL Hello with
an appointment for itself for some VLAN.
The DRB MUST NOT send any appointments on a link unless its DRB
inhibition timer (see Section 3) for that link is expired.
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RFC 6439 RBridges: Appointed Forwarders November 2011
How the DRB decides what other RBridges on the link, if any, to
appoint forwarder for which VLANs is beyond the scope of this
document.
2.2.1. Processing Forwarder Appointments
When a non-DRB RBridge that can offer end station service on a link
receives a TRILL Hello that is not discarded for one of the reasons
given in [RFC 6327], it checks the source MAC address and the Port ID
and System ID in the Hello to determine if it is from the winning DRB
port. If it is not from that port, any Appointed Forwarder sub-TLVs
in the Hello are ignored, and there is no change in the receiving
RBridge's Appointed Forwarder status. Also, if no Appointed
Forwarder sub-TLVs are present in the TRILL Hello, there is no change
in the receiver's Appointed Forwarder status.
However, if the TRILL Hello is from the winning DRB port and the
Hello includes one or more Appointed Forwarder sub-TLVs, then the
receiving RBridge becomes appointed for the VLANs that are both
listed for it in the Hello and are enabled on the receiving port.
(If the appointment includes VLAN IDs 0x000 or 0xFFF, they are
ignored, but any other VLAN IDs are still effective.) If the
receiver was Appointed Forwarder for any other VLANs, its Appointed
Forwarder status for such other VLANs is revoked. For example, if
none of these sub-TLVs in a Hello appoints the receiving RBridge,
then it loses all Appointed Forwarder status and is no longer
Appointed Forwarder for any VLAN on the port where the Hello was
received.
The handling of one or more Appointed Forwarder sub-TLVs in a Hello
from the winning port that appoints the receiving RBridge is as
follows. An appointment in an Appointed Forwarder sub-TLV is for a
specific RBridge and a contiguous interval of VLAN IDs; however, as
stated above, it actually appoints that RBridge forwarder only for
the VLAN(s) in that range that are enabled on one or more ports that
RBridge has on the link (ignoring any ports configured as trunk ports
or as IS-IS point-to-point ports). If the RBridge was Appointed
Forwarder for any additional VLANs beyond the VLANs for which it was
being appointed, it loses Appointed Forwarder status for such
additional VLANs.
There is no reason for an RBridge to remember that it received a
valid appointment message for a VLAN that was ineffective because the
VLAN was not enabled on the port where the message was received or
because the port was a trunk or point-to-point port. It does not
become Appointed Forwarder for such a VLAN just because that VLAN is
later enabled or the port later reconfigured.
Perlman, et al. Standards Track PAGE 6
RFC 6439 RBridges: Appointed Forwarders November 2011
It should be straightforward for the DRB to send, within one Hello,
the appointments for several dozen VLAN IDs or several dozen blocks
of contiguous VLAN IDs. Should the VLANs the DRB wishes to appoint
be inconveniently distributed, for example, the proverbial case where
the DRB RB1 wishes to appoint RB2 forwarder for all even-numbered
VLANs and appoint RB3 forwarder for all odd-numbered VLANs, the
following method may be used. The network manager normally controls
what VLANs are enabled on RBridge port. Thus, the network manager
can appoint an RBridge forwarder for an arbitrary set of scattered
VLANs by enabling only those VLANs on the relevant port (or ports)
and then having the DRB send an appointment that appears to appoint
the target RBridge forwarder for all VLANs. However, for proper
operation and inter-RBridge communication, the Designated VLAN for a
link SHOULD be enabled on all RBridge ports on that link, and it may
not be desired to appoint the RBridge forwarder for the Designated
VLAN. Thus, in the general case, it would require two appointments,
although it would still only require one appointment if the
Designated VLAN were an extreme low or high value such as VLAN 0xFFE
or the default VLAN 1.
For example, assume the DRB wants RB2 to be Appointed Forwarder for
all even-numbered VLANs and the Designated VLAN for the link is VLAN
101. The network manager could cause all even-numbered VLANs plus
VLAN 101 to be enabled on the relevant port of RB2 and then, with the
desired effect, cause the DRB to send appointments to RB2 appointing
it forwarder for all VLANs from 1 through 100 and from 102 through
4,094.
Should the network manager have misconfigured the enabled VLANs and
Appointed Forwarders, resulting in two RBridges believing they are
Appointed Forwarders for the same VLAN, then item 4 in Section 3 will
cause one or more of the RBridges to be inhibited for that VLAN.
2.2.2. Frequency of Appointments
It is not necessary for the DRB to include the forwarder appointments
in every TRILL Hello that it sends on the Designated VLAN for a link.
For loop safety, every RBridge is required to indicate, in every
TRILL Hello it sends in VLAN-x on a link, whether it is an Appointed
Forwarder for VLAN-x for that link (see item 4 in Section 3). It is
also RECOMMENDED that the DRB have all VLANs for which end station
service will be offered on the link as well as the Designated VLAN,
enabled. Thus, the DRB will generally be informed by other RBridges
on the link of the VLANs for which they believe they are Appointed
Forwarder. If this matches the appointments the DRB wishes to make,
it is not required to re-send its forwarder appointments; however,
for robustness, especially in cases such as VLAN misconfigurations in
Perlman, et al. Standards Track PAGE 7
RFC 6439 RBridges: Appointed Forwarders November 2011
a bridged LAN link, it is RECOMMENDED that the DRB send its forwarder
appointments on the Designated VLAN at least once per its Holding
Time on the port that won the DRB election.
2.2.3. Appointed Forwarders Limit
The mechanism of DRB forwarder appointment and the limited length of
TRILL Hellos impose a limit on the number of RBridges on a link that
can be Appointed Forwarders. To obtain a conservative estimate,
assume that no more than 1000 bytes are available in a TRILL Hello
for such appointments. Assume it is desired to appoint various
RBridges on a link forwarder for arbitrary non-intersecting sets of
VLANs. Using the technique discussed above would generally require
two appointments, or 12 bytes, per RBridge. With allowance for
sub-TLV and TLV overhead, appointments for 83 RBridges would fit in
under 1000 bytes. Including the DRB, this implies a link with 84 or
more RBridges attached. Links with more than a handful of RBridges
attached are expected to be rare.
Note: If the Designated VLAN were an extreme low or high value, such
as VLAN 1, which is the default and may be a common value in
practice, only 6 bytes per RBridge would be required. This would
permit twice as many different Appointed Forwarder RBridges than
indicated by the general analysis above or, alternatively, would take
only half as much space to appoint the same number of Appointed
Forwarders.
Unnecessary changes in Appointed Forwarders SHOULD NOT be made as
they may result in transient lack of end station service. Large
numbers of Appointed Forwarders on a link (in excess of 65) are NOT
RECOMMENDED due to the complexity of their establishment and
maintenance.
2.3. Local Configuration Action Appointment Effects
Disabling VLAN-x at an RBridge port cancels any Appointed Forwarder
status that RBridge has for VLAN-x unless VLAN-x is enabled on some
other port that the RBridge has connected to the same link.
Configuring a port as a trunk port or point-to-point port revokes any
Appointed Forwarder status that depends on enabled VLANs at that
port.
Causing a port to no longer be configured as a trunk or point-to-
point port or enabling VLAN-x on a port does not, in itself, cause
the RBridge to become an Appointed Forwarder for the link that port
is on. However, such actions can allow the port's RBridge to become
Appointed Forwarder by choice if it is the DRB or by appointment, if
it is not the DRB on the link.
Perlman, et al. Standards Track PAGE 8
RFC 6439 RBridges: Appointed Forwarders November 2011
2.4. VLAN Mapping within a Link
TRILL Hellos include a field that is set to the VLAN in which they
are sent. If they arrive on a different VLAN, then VLAN mapping is
occurring within the link. (Such VLAN mapping within a link between
RBridges should not be confused with VLAN mapping inside an RBridge
[VLANMAP]). VLAN mapping between VLAN-x and VLAN-y can lead to a
loop if the Appointed Forwarders for the VLANs are different. If
such mapping within a link was allowed and occurred on two or more
links so that there was a cycle of VLAN mappings, a broadcast frame,
for example, would loop forever.
To prevent this potential problem, if the DRB on a link detects VLAN
mapping by receiving a Hello in VLAN-x that was sent on VLAN-y, it
MUST make or revoke appointments so as to assure that the same
RBridge (possibly the DRB) is the Appointed Forwarder on the link for
both VLAN-x and VLAN-y.
3. The Inhibition Mechanism
An RBridge has, for every link on which it can offer end station
service (that is every link for which it can act as an Appointed
Forwarder), the following timers denominated in seconds:
- a DRB inhibition timer,
- a root change inhibition timer, and
- up to 4,094 VLAN inhibition timers, one for each legal VLAN ID.
The DRB and root change inhibition timers MUST be implemented.
The loss of native traffic due to inhibition will be minimized by
logically implementing a VLAN inhibition timer per each VLAN for
which end station service will ever be offered by the RBridge on the
link; this SHOULD be done. (See the Appendix for an example
motivating VLAN inhibition timers.) However, if implementation
limitations make a full set of such timers impractical, the VLAN
inhibition timers for more than one VLAN can, with care, be merged
into one timer. In particular, an RBridge MUST NOT merge the VLAN
inhibition timers together for two VLANs if it is the Appointer
Forwarder for one and not for the other, as this can lead to
unnecessary indefinitely prolonged inhibition. In the limit, there
will be safe operations, albeit with more native frame loss than
would otherwise be required, even if only two VLAN inhibition timers
are provided: one for VLANs for which the RBridge is the Appointed
Forwarder and one for all other VLANs. At least two VLAN inhibition
Perlman, et al. Standards Track PAGE 9
RFC 6439 RBridges: Appointed Forwarders November 2011
timers MUST be implemented. Where a VLAN inhibition timer represents
more than one VLAN, an update or test that would have been done to
the timer for any of the VLANs is performed on the merged timer.
These timers are set as follows:
1. On booting or management reset, each port will have its own set
of timers, even if two or more such ports are on the same link,
because the RBridge will not have had a chance to learn that yet.
All inhibition timers are set to expired except the DRB
inhibition timer that is set in accordance with item 2 below.
The DRB inhibition timer is handled differently because each port
will initially believe it is the DRB.
2. When an RBridge decides that it has become the DRB on a link,
including when it is first booted or reset by management, it sets
the DRB inhibition timer to the Holding Time of its port on that
link that won the DRB election.
3. When an RBridge decides that it has lost DRB status on a link, it
sets the DRB inhibition timer to expired.
Note: In the rare corner case where one port of an RBridge was
the DRB election winner, but later lost the DRB election to a
different port of the same RBridge on that link (perhaps due to
management configuration of port priority), neither 2 nor 3 above
applies, and the DRB timer is not changed.
4. When an RBridge RB1 receives a TRILL Hello asserting that the
sender is the Appointed Forwarder that either (1) arrives on
VLAN-x or (2) was sent on VLAN-x as indicated inside the Hello,
then RB1 sets its VLAN-x inhibition timer for the link to the
maximum of that timer's existing value and the Holding Time in
the received Hello. An RBridge MUST maintain VLAN inhibition
timers for a link to which it connects if it can offer end
station service on that link even if it is not currently
Appointed Forwarder for any VLAN on that link.
5. When an RBridge RB1 enables VLAN-x on a port connecting to a link
and VLAN-x was previously not enabled on any of RB1's ports on
that link, it sets its VLAN inhibition timer for VLAN-x for that
link to its Holding Time for that port. This is done even if the
port is configured as a trunk or point-to-point port as long as
there is some chance it might later be configured not to be a
trunk or point-to-point port.
Perlman, et al. Standards Track PAGE 10
RFC 6439 RBridges: Appointed Forwarders November 2011
6. When an RBridge detects a change in the common spanning tree root
bridge on a port, it sets its root change inhibition timer for
the link to an amount of time that defaults to 30 seconds and is
configurable to any value from 30 down to zero seconds. This
condition will not occur unless the RBridge is receiving Bridge
PDU (BPDUs) on the port from an attached bridged LAN. It is safe
to configure this inhibition time to the settling time of an
attached bridged LAN. For example, if it is known that Rapid
Spanning Tree Protocol (RSTP [802.1Q]) is running throughout the
attached bridged LAN, it should be safe to configure this
inhibition time to 7 seconds or, if the attached bridges have
been configured to have a minimum Bridge Hello Timer, safe to
configure it to 4 seconds. Note that, while an RBridge could
determine what version of spanning tree is running on the
physical link between it and any directly connected bridge by
examination of the BPDUs it receives, it could not tell if
inter-bridge links beyond those directly connected bridges were
running classic Spanning Tree Protocol (STP), which might require
the root change inhibition timer to be set to 30 seconds for
safety.
7. When an RBridge decides that one of its ports (or a set of its
ports) P1 is on the same link as another of its ports (or set of
its ports) P2, then the inhibition timers are merged to a single
set of inhibition timers by using the maximum value of the
corresponding timers.
8. When an RBridge decides that a set of its ports that it had been
treating as being on the same link are no longer on the same
link, those ports will necessarily be on two or more links (one
link per port in the limit). This is handled by cloning a copy
of the timers for each of the two or more links to which the
RBridge has decided these ports connect.
4. Inhibited Appointed Forwarder Behavior
An Appointed Forwarder for a link is inhibited for VLAN-x if:
1. its DRB inhibition timer for that link is not expired, or
2. its root change inhibition timer for that link is not expired, or
3. its VLAN inhibition timer for that link for VLAN-x is not
expired.
If a VLAN-x Appointed Forwarder for a link is inhibited and receives
a TRILL Data frame whose encapsulated frame is in VLAN-x and would
normally be egressed to that link, it decapsulates the native frame
Perlman, et al. Standards Track PAGE 11
RFC 6439 RBridges: Appointed Forwarders November 2011
as usual. However, it does not output it to or queue it for that
link, although, if appropriate (for example, the frame is multi-
destination), it may output it to or queue it for other links.
If a VLAN-x Appointed Forwarder for a link is inhibited and receives
a native frame in VLAN-x that would normally be ingressed from that
link, the native frame is ignored except for address learning.
An RBridge with one or more unexpired inhibition timers, possibly
including an unexpired inhibition timer for VLAN-x, is still required
to indicate in TRILL Hellos it sends on VLAN-x whether or not it is
Appointed Forwarder for VLAN-x for the port on which it sends the
Hello.
Inhibition has no effect on the receipt or forwarding of TRILL Data
frames.
5. Multiple Ports on the Same Link
An RBridge may have multiple ports on the same link. Some of these
ports may be suspended due to MAC address duplication as described in
[RFC 6327]. Suspended ports never ingress or egress native frames.
If an RBridge has one or more non-suspended ports on a link and those
ports offer end station service, that is, those ports are not
configured as point-to-point or trunk ports, then that RBridge is
eligible to be an Appointed Forwarder for that link. It can become
Appointed Forwarder either by its choice, because it is the DRB, or
by appointment by the DRB as described in Sections 2.1 and 2.2.
If an RBridge that is the Appointed Forwarder for VLAN-x on a link
has multiple non-suspended ports on that link, it may load share the
task of ingressing and egressing VLAN-x native frames across those
ports however it chooses, as long as there is no case in which a
frame it egresses onto the link from one port can be ingressed on
another of its ports, creating a loop. If the RBridge is the
Appointed Forwarder for multiple VLANs, a straightforward thing to do
would be to partition those VLANs among the ports it has on the link.
6. Security Considerations
This memo provides improved documentation of the TRILL Appointed
Forwarder mechanism. It does not change the security considerations
of the TRILL base protocol. See Section 6 of [RFC 6325].
Perlman, et al. Standards Track PAGE 12
RFC 6439 RBridges: Appointed Forwarders November 2011
7. Acknowledgements
The authors of [RFC 6325] and [RFC 6327], those listed in the
Acknowledgements section of [RFC 6325] and [RFC 6327], and Ron Bonica,
Stewart Bryant, Linda Dunbar, Les Ginsberg, Erik Nordmark, Dan
Romascanu, and Mike Shand are hereby thanked for their contributions.
8. References
Normative and Informative references for this document are listed
below.
8.1. Normative References
[802.1Q] IEEE 802.1, "IEEE Standard for Local and metropolitan
area networks - Virtual Bridged Local Area Networks",
IEEE Std 802.1Q-2011, May 2011.
[IS-IS] ISO/IEC 10589:2002, Second Edition, "Intermediate System
to Intermediate System Intra-Domain Routeing Exchange
Protocol for use in Conjunction with the Protocol for
Providing the Connectionless-mode Network Service (ISO
8473)", 2002.
[RFC 1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC 6326] Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and A.
Ghanwani, "Transparent Interconnection of Lots of Links
(TRILL) Use of IS-IS", RFC 6326, July 2011.
[RFC 6327] Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D.,
and V. Manral, "Routing Bridges (RBridges): Adjacency",
RFC 6327, July 2011.
8.2. Informative References
[VLANMAP] Perlman, R., Dutt, D., Banerjee, A., Rijhsinghani, A.,
and D. Eastlake, "RBridges: Campus VLAN and Priority
Regions", Work in Progress, October 2011.
Perlman, et al. Standards Track PAGE 13
RFC 6439 RBridges: Appointed Forwarders November 2011
Appendix. VLAN Inhibition Example
The per-VLAN inhibition timers (or the equivalent) are needed to be
loop safe in the case of misconfigured bridges on a link.
For a simple example, assume that RB1 and RB2 are the only RBridges
on the link, that RB1 is higher priority to be the DRB, and that they
both want VLAN 1 (the default) to be the Designated VLAN. However,
there is a bridge between them configured so that RB1 can see all the
frames sent by RB2 but none of the frames from RB1 can get through to
RB2.
Both will think they are the DRB. RB1 because it is higher priority
even though it sees the Hellos from RB2, and RB2 because it doesn't
see the Hellos from RB1 and therefore thinks it is highest priority.
Say RB1 chooses to act as Appointed Forwarder for VLANs 2 and 3 while
RB2 chooses to act as Appointed Forwarder for VLANs 3 and 4. There
is no problem with VLANs 2 and 4 but if you do not do something about
it, you could have a loop involving VLAN 3. RB1 will see the Hellos
RB2 issues on VLAN 3 declaring itself Appointed Forwarder, so RB1
will be inhibited on VLAN 3. RB2 does not see the Hellos issued by
RB1 on VLAN 3, so RB2 will become uninhibited and will handle VLAN 3
native traffic.
However, this situation may change. RB2 might crash, the bridge
might crash, or RB2 might be reconfigured so it no longer tried to
act as Appointed Forwarder for VLAN 3, or other issues may occur.
So, RB1 has to maintain a VLAN 3 inhibition timer, and if it sees no
Hellos from any other RBridge on the link claiming to be Appointed
Forwarder for VLAN 3 in a long enough time, then RB1 becomes
uninhibited for that VLAN on the port in question and can handle end
station traffic in VLAN 3.
Perlman, et al. Standards Track PAGE 14
RFC 6439 RBridges: Appointed Forwarders November 2011
Authors' Addresses
Radia Perlman
Intel Labs
2200 Mission College Blvd.
Santa Clara, CA 95054 USA
Phone: +1-408-765-8080
EMail: Radia@alum.mit.edu
Donald Eastlake 3rd
Huawei Technologies
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
EMail: d3e3e3@gmail.com
Yizhou Li
Huawei Technologies
101 Software Avenue,
Nanjing 210012, China
Phone: +86-25-56622310
EMail: liyizhou@huawei.com
Ayan Banerjee
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134 USA
Phone: +1-408-333-7149
EMail: ayabaner@cisco.com
Fangwei Hu
ZTE Corporation
889 Bibo Road
Shanghai 201203
China
Phone: +86-21-68896273
EMail: hu.fangwei@zte.com.cn
Perlman, et al. Standards Track PAGE 15
RFC TOTAL SIZE: 36978 bytes
PUBLICATION DATE: Tuesday, November 22nd, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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