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Versions: (draft-perlman-trill-rbridge-af) 00
01 02 03 04 05 RFC 6439
TRILL Working Group Radia Perlman
INTERNET-DRAFT Intel Labs
Intended status: Proposed Standard Donald Eastlake 3rd
Updates: RFCtrill Huawei
Ayan Banerjee
Cisco
Hu Fangwei
ZTE
Expires: October 16, 2011 April 17, 2011
RBridges: Appointed Forwarders
<draft-ietf-trill-rbridge-af-02.txt>
Abstract
The IETF TRILL protocol provides optimal pair-wise data forwarding
without configuration, 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 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 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. The purpose of this document is to improve the
documentation of the Appointed Forwarder mechanism.
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
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
R. Perlman, et al [Page 1]
INTERNET-DRAFT RBridges: Appointed Forwarders
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Acknowledgements
The authors of [RFCtrill] and [RFCadj], those listed in the
Acknowledgements section of [RFCtrill] and [RFCadj], 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............................8
2.2.3 Appointed Forwarders Limit...........................9
2.3 Local Configuration Action Appointment Effects.........9
3. The Inhibition Mechanism...............................10
4. Inhibited Appointed Forwarder Behavior.................13
5. Multiple Ports on the Same Link........................14
6. Security Considerations................................15
7. IANA Considerations....................................15
8. References.............................................16
8.1 Normative References..................................16
8.2 Informative References................................16
Appendix: Change Record...................................18
From -00 to -01...........................................18
From -01 to -02...........................................18
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1. Introduction
The IETF TRILL protocol [RFCtrill] provides optimal pair-wise data
frame forwarding without configuration, 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] [RFC1195] link state routing and encapsulating traffic using a
header that includes a hop count. The design supports VLANs 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 [RFCadj] 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 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. 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 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 [RFCadj].
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 [RFCtrill] and
[RFCadj]. Familiarity with [RFCtrill] and [RFCadj] is assumed in this
document. In case of conflict between this document and [RFCtrill],
this document prevails.
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1.1 Terminology and Acronyms
This document uses the acronyms defined in [RFCtrill].
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. While
TRILL tries to avoid such a situation, 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 [RFCadj], if there are multiple ports with the
same MAC address on a link, all but one will be suspended. While the
cases of multiple ports on a link for one RBridge and multiple ports
with the same MAC address are fully accommodated, 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
[RFCadj] 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).
When an RBridge believes that it has lost DRB status on a link or it
observes a change in the RBridge that is DRB for the link without
itself becoming DRB, it loses all Appointed Forwarder status.
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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 [RFCtisis] 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.
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 [RFCadj], 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
RBridges 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 and none of
these sub-TLVs appoints the receiving RBridge, then it loses all
Appointed Forwarder status and is no longer Appointed Forwarder for
any VLAN. If one or more Appointed Forwarder sub-TLVs are present
that appoint the receiving RBridge in a Hello from the winning port
they are handled as described below.
An appointment in an Appointed Forwarder sub-TLV is for a specific
RBridge and a contiguous interval of VLAN IDs; however, it actually
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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. The RBridge becomes
Appointed Forwarder for all such VLANs in all such Appointed
Forwarder sub-TLVs in the Hello PDU and, if it was Appointed
Forwarder for any additional VLANs, it loses Appointed Forwarder
status for such additional VLANs.
The network manager normally controls what VLANs are enabled on
RBridge ports and which ports are configured as trunk ports. Thus the
network manager can appoint an RBridge forwarder for an arbitrary set
of 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, to facilitate
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 1 or VLAN 0xFFE.
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
4,095.
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 to 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.
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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
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 cancels any Appointed Forwarder
status that depends on enabled VLANs on that port.
Causing a port to no longer be configured as a trunk port or enabling
VLAN-x on a port cannot, 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.
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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 on the link by the
RBridge and this SHOULD be done. 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 and
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. 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 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, because each port will initially believe it is DRB,
is set in accordance with item 2 below.
2. When an RBridge believes that it has become DRB on a link, 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 believes 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 loses the DRB election to a
different port of the same RBridge on that link (perhaps due to
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INTERNET-DRAFT RBridges: Appointed Forwarders
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 on VLAN-x from some
other RBridge RB2 and RB2 asserts in the Hello that it is
Appointed Forwarder, then RB1 sets the VLAN-x inhibition timer
for that link to the maximum of its 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 all of RB1's
ports on that link, it sets the 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 port as long as there
is some chance it might be later configured to not be a trunk
port.
6. When an RBridge detects a spanning tree root bridge change on a
port, it sets the 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-2005]) is running
throughout the attached bridged LAN, it should be safe to
configure this inhibition time 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 the
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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 unless (1)
its DRB inhibition timer for that link, (2) its root inhibition timer
for that link, and (3) its VLAN inhibition timer for that link for
VLAN-x, are all three 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. However, it does not output it to or queue it for that link
although, if appropriate, 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.
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 it is
Appointed Forwarder for VLAN-x.
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
[RFCadj]. 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. 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
port, 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 [RFCtrill].
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
[IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to
Intermediate System Intra-Domain Routing 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.
[RFCtisis] - Eastlake, D., A. Banerjee, D. Dutt, R. Perlman, A.
Ghanwani, "TRILL Use of IS-IS", draft-ietf-isis-trill-05, in
RFC Editor's queue.
[RFCtrill] - Perlman, R., D. Eastlake, D. Dutt, S. Gai, and A.
Ghanwani, "RBridges: Base Protocol Specification", draft-ietf-
trill-rbridge-protocol-16.txt, in RFC Editor's queue.
[RFCadj] - Eastlake, D., R. Perlman, A. Ghanwani, D. Dutt, V. Manral,
"RBridges: Adjacency", draft-ietf-trill-adj, work in progress.
8.2 Informative References
None.
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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 3rd
Huawei Technologies
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
Ayan Banerjee
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134 USA
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: 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.
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