
TRILL Working Group                                        Radia Perlman
INTERNET-DRAFT                                                Intel Labs
Intended status: Proposed Standard                       Donald Eastlake
Updates: 6325                                                  Yizhou Li
                                                                  Huawei
                                                           Ayan Banerjee
                                                                   Cisco
                                                              Hu Fangwei
                                                                     ZTE
Expires: March 25, 2012                               September 26, 2011


                     RBridges: Appointed Forwarders
                  <draft-ietf-trill-rbridge-af-05.txt>


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.

   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 and thus it updates RFC 6325.


Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  Distribution of this document is
   unlimited. Comments should be sent to the TRILL working group mailing
   list <rbridge@postel.org>.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference


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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html



Acknowledgements

   The authors of [RFC6325] and [RFC6327], those listed in the
   Acknowledgements section of [RFC6325] and [RFC6327], and Ron Bonica,
   Stewart Bryant, Linda Dunbar, Les Ginsberg, Erik Nordmark, Dan
   Romascanu, and Mike Shand are hereby thanked for their contributions.




































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Table of Contents

      1. Introduction............................................4
      1.1 Terminology and Acronyms...............................5

      2. Appointed Forwarders and Their Appointment..............6
      2.1 Appointment Effects of DRB Elections...................6
      2.2 Appointment and Removal by the DRB.....................7
      2.2.1 Processing Forwarder Appointments....................7
      2.2.2 Frequency of Appointments............................9
      2.2.3 Appointed Forwarders Limit...........................9
      2.3 Local Configuration Action Appointment Effects........10
      2.4 VLAN Mapping Within a Link............................10

      3. The Inhibition Mechanism...............................12
      4. Inhibited Appointed Forwarder Behavior.................15
      5. Multiple Ports on the Same Link........................16

      6. Security Considerations................................17
      7. IANA Considerations....................................17

      8. References.............................................18
      8.1 Normative References..................................18
      8.2 Informative References................................18

      Authors' Addresses........................................19

      Appendix: VLAN Inhibition Example.........................20

      Appendix Z: Change Record.................................21






















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1. Introduction

   The IETF TRILL (Transparent Interconnection of Lots of Links)
   protocol [RFC6325] 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] [RFC1195] link state routing and encapsulating
   traffic using a header that includes a hop count. The design 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.

   Section 2 of [RFC6327] 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 and 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 affect on the formation of
   adjacencies, the election of the DRB for a link, MTU matching, or
   pseudonode formation. Those topics are covered in [RFC6327].
   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 [RFC6325] and
   [RFC6327]. Familiarity with [RFC6325] and [RFC6327] is assumed in
   this document. In case of conflict between this document and
   [RFC6325], this document prevails.


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1.1 Terminology and Acronyms

   This document uses the acronyms defined in [RFC6325].

   A "trunk port" is a port configured with the "end station service
   disable" bit on, as described in Section 4.9.1 of [RFC6325].

   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
   [RFC2119].





































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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 [RFC6327], if there are multiple ports with the
   same 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 affect on the forwarding of TRILL
   Data frames. It only affects the handling of native frames.

   There are three mechanisms by which an RBridge can be appointed or
   un-appointed as Appointed Forwarder: as a result of DRB elections
   [RFC6327] 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).


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   An RBridge loses all Appointed Forwarder status when
      1. it decides that it has lost the status of being DRB for a link
         or
      2. it observes a change in the RBridge that is DRB for the link
         without itself becoming 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 that
   port 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 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 [RFC6326] 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.

   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 [RFC6327], 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


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   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.

   The handling of one or more Appointed Forwarder sub-TLVs in a Hello
   from the winning port that appoint 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 re-configured.

   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 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


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   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). And it
   is 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
   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 imposes 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 forwarders 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


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   RBridges attached. Links with more than a handful of RBridges
   attached are expected to be rare.

   (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 DRB or by appointment if it is
   not DRB on the link.



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


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   MUST make or revoke appointments so as to assure that the same
   RBridge (possibly the DRB) is Appointed Forwarder on the link for
   both VLAN-x and VLAN-y.

















































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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 and this SHOULD be done. (See 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 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 Appointed Forwarder and one for all other VLANs. At least
   two VLAN inhibition timers MUST be implemented. Where a VLAN
   inhibition timer represents more than one VLAN, an update or test
   that would have be 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, as even if two or more are on the same link as 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 DRB.

      2. When an RBridge decides that it has become 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.


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        Note: In the rare corner case where one port of an RBridge was
           the DRB election winner but later loses 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 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 be later
         configured to not be a trunk or point-to-point port.

      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
         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.


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      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 the
         RBridge has decided these ports connect to.














































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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
   as usual. But 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 un-expired 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.
























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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
   [RFC6327]. 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 DRB, or by
   appointment by the DRB as described in Sections 2.1 and 2.2.

   If an RBridge which is 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 Appointed Forwarder for
   multiple VLANs, a straightforward thing to do would be to partition
   those VLANs among the ports it has on the link.































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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 [RFC6325].




7. IANA Considerations

   This document requires no IANA actions. RFC Editor: Please delete
   this section before publication.







































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8. References

   Normative and Informational 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.

   [RFC1195] - Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
         dual environments", RFC 1195, December 1990.

   [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
         Ghanwani, "Routing Bridges (RBridges): Base Protocol
         Specification", RFC 6325, July 2011.

   [RFC6326] - 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.

   [RFC6327] - 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., D. Dutt, A. Banerjee, A. Rijhsinghani, D.
         Eastlake, "RBridges: Campus VLAN and Priority Regions", draft-
         ietf-trill-rbridge-vlan-mapping, work in progress.









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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
   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






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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 DRB, and that they
   both want VLAN 1 (the default) to be the designated VLAN. But 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 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 so 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 and so RB1
   will be inhibited on VLAN 3. RB2 does not see the Hellos issued by
   RB1 on VLAN 3 and so RB2 will become uninhibited and will handle VLAN
   3 native traffic.

   But this situation may change. RB2 might crash or the bridge might
   crash or RB2 might be re-configured so it no longer tried to act as
   appointed forwarder for VLAN 3 or ... 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.




















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Appendix Z: Change Record

   This appendix summarizes changes between versions of this draft.

   RFC Editor: Please delete this Appendix before publication.



From -00 to -01

   1. Clarify that an RBridge needs to check the source MAC, Port ID,
      and System Id in received TRILL Hellos to determine whether
      forwarder appointment sub-TLVs are ignored or take effect.

   2. Note that RB1's Appointed Forwarder status for VLAN-x is cancelled
      if VLAN-x is disabled on all ports RB1 has on a link.

   3. Minor editorial changes.



From -01 to -02

   1. Include additional appropriate references to configuring ports as
      trunk ports or to no longer be trunk ports.

   2. Minor editorial changes.



From -02 to -03

   1. Add note on "trunk port" to Section 1.1.

   2. Clarify that RBridges do not maintain state for AF appointments
      that were ineffective due to being for a disabled VLAN, trunk
      port, or point-to-point port.

   3. Add material in Section 2.2.1 pointing to Item 4 in Section 3 as
      the safety measure when you do have two AFs on a link for the same
      VLAN.

   4. Add VLAN inhibition timer example Appendix.

   5. Provide that an inhibited AF can still learn end station addresses
      from native frames it receives.

   6. Minor editorial changes.




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From -03 to -04

   1. Add a definition of "link".

   2. Specify that VLAN IDs 0x000 and 0xFFF are ignored in appointments.

   3. Add Section 2.4 on VLAN Mapping Within a Link.

   4. Note that a VLAN inhibition timer is set for the VLAN identified
      inside an appropriate Hello as well as for the VLAN in which that
      Hello is delivered.

   5. Add to Section 2.2 a recommendation that, if the DRB wants to
      clear all appointments, it just send a appointment for itself.

   6. Minor editorial changes.



From -04 to -05

   1. In Section 3, Item 6, correct the default safe RSTP root bridge
      change inhibition timer value to 7 seconds and note that it
      requires minimum Bridge Hello Timer configuration of attached
      bridges to be able to safely reduce this to 4 seconds.

   2. Update references for RFCs that have been published.

   3. Add a few acknowledgements.

   4. Note in the Abstract and Introduction that this document updates
      RFC 6325.

   5. Minor editorial changes.


















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Copyright and IPR Provisions

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   Contribution that he or she makes as part of the IETF Standards
   Process to the IETF Trust pursuant to the provisions of RFC 5378. No
   language to the contrary, or terms, conditions or rights that differ
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   published or posted by such Contributor, or included with or in such
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