L2VPN Working Group B. Kothari
Internet Draft K. Kompella
Intended status: Standards Track Juniper Networks
Expires: January 2010 W. Henderickx
F. Balus
Alcatel-Lucent
July 13, 2009
BGP based Multi-homing in Virtual Private LAN Service
draft-kothari-henderickx-l2vpn-vpls-multihoming-01.txt
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Abstract
Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private
Network (VPN) that gives its customers the appearance that their
sites are connected via a Local Area Network (LAN). It is often
required for the Service Provider (SP) to give the customer redundant
connectivity to some sites, often called "multi-homing". This memo
shows how BGP-based multi-homing can be offered in the context of LDP
and BGP VPLS solutions.
Table of Contents
1. Introduction...................................................3
1.1. General Terminology.......................................4
1.2. Conventions used in this document.........................4
2. Background.....................................................4
2.1. Scenarios.................................................4
2.2. VPLS Multi-homing Considerations..........................5
3. Multi-homing Operation.........................................6
3.1. Provisioning Model........................................6
3.2. Multi-homing NLRI.........................................6
3.3. Designated Forwarder Election.............................7
3.3.1. Attributes...........................................7
3.3.2. Variables Used.......................................7
3.3.2.1. RD..............................................7
3.3.2.2. MH-ID...........................................7
3.3.2.3. VBO.............................................8
3.3.2.4. DOM.............................................8
3.3.2.5. ACS.............................................8
3.3.2.6. PREF............................................8
3.3.2.7. PE-ID...........................................9
3.3.3. Election Procedures..................................9
3.3.3.1. Bucketization for BGP DF Election..............10
3.3.3.2. Bucketization for VPLS DF Election.............10
3.3.3.3. Tie-breaking Rules.............................10
3.4. DF Election on PEs.......................................11
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4. Multi-AS VPLS.................................................12
4.1. Route Origin Extended Community..........................12
4.2. VPLS Preference..........................................12
4.3. Use of BGP-MH attributes in Inter-AS Methods.............13
4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS
Information between ASBRs..................................14
4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of
VPLS Information between ASes..............................15
5. MAC Flush Operations..........................................15
5.1. MAC List Flush...........................................16
5.2. Implicit MAC Flush.......................................16
6. Backwards Compatibility.......................................16
6.1. BGP based VPLS...........................................17
6.2. LDP VPLS with BGP Auto-discovery.........................17
7. Security Considerations.......................................17
8. IANA Considerations...........................................17
9. References....................................................17
9.1. Normative References.....................................17
9.2. Informative References...................................18
10. Acknowledgments..............................................18
1. Introduction
Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private
Network (VPN) that gives its customers the appearance that their
sites are connected via a Local Area Network (LAN). It is often
required for a Service Provider (SP) to give the customer redundant
connectivity to one or more sites, often called "multi-homing".
[RFC4761] explains how VPLS can be offered using BGP for auto-
discovery and signaling; section 3.5 of that document describes how
multi-homing can be achieved in this context. [I-D.ietf-l2vpn-
signaling] explains how VPLS can be offered using BGP for auto-
discovery (BGP-AD) and [RFC4762] explains how VPLS can be offered
using LDP for signaling. This document provides a BGP-based multi-
homing solution applicable to both BGP and LDP VPLS technologies.
Note that BGP MH can be used for LDP VPLS without the use of the BGP-
AD solution.
Section 2 lays out some of the scenarios for multi-homing, other ways
that this can be achieved, and some of the expectations of BGP-based
multi-homing. Section 3 defines the components of BGP-based multi-
homing, and the procedures required to achieve this. Section 7 may
someday discuss security considerations.
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1.1. General Terminology
Some general terminology is defined here; most is from [RFC4761],
[RFC4762] or [RFC4364]. Terminology specific to this memo is
introduced as needed in later sections.
A "Customer Edge" (CE) device, typically located on customer
premises, connects to a "Provider Edge" (PE) device, which is owned
and operated by the SP. A "Provider" (P) device is also owned and
operated by the SP, but has no direct customer connections. A "VPLS
Edge" (VE) device is a PE that offers VPLS services.
A VPLS domain represents a bridging domain per customer. A Route
Target community as described in [RFC4360] is typically used to
identify all the PE routers participating in a particular VPLS
domain. A VPLS site is a grouping of ports on a PE that belong to the
same VPLS domain. A Multi-homed (MH) site is uniquely identified by a
MH site ID (MH-ID). Sites are referred to as local or remote
depending on whether they are configured on the PE router in context
or on one of the remote PE routers (network peers).
1.2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Background
This section describes various scenarios where multi-homing may be
required, and the implications thereof. It also describes some of the
singular properties of VPLS multi-homing, and what that means from
both an operational point of view and an implementation point of
view. There are other approaches for providing multi-homing such as
Spanning Tree Protocol, and this document specifies use of BGP for
multi-homing. Comprehensive comparison among the approaches is
outside the scope of this document.
2.1. Scenarios
The most basic scenario is shown in Figure 1.
CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for redundant
connectivity.
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...............
. . ___ CE2
___ PE1 . /
/ : PE3
__/ : Service :
CE1 __ : Provider PE4
\ : : \___ CE3
\___ PE2 .
. .
...............
Figure 1 Scenario 1
CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for redundant
connectivity. However, CE4, which is also in the same VPLS domain, is
single-homed to just PE1.
CE4 ------- ...............
\ . . ___ CE2
___ PE1 . /
/ : PE3
__/ : Service :
CE1 __ : Provider PE4
\ : : \___ CE3
\___ PE2 .
. .
...............
Figure 2 Scenario 2
2.2. VPLS Multi-homing Considerations
The first (perhaps obvious) fact about a multi-homed VPLS CE, such as
CE1 in Figure 1 is that if CE1 is an Ethernet switch or bridge, a
loop has been created in the customer VPLS. This is a dangerous
situation for an Ethernet network, and the loop must be broken. Even
if CE1 is a router, it will get duplicates every time a packet is
flooded, which is clearly undesirable.
The next is that (unlike the case of IP-based multi-homing) only one
of PE1 and PE2 can be actively sending traffic, either towards CE1 or
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into the SP cloud. That is to say, load balancing techniques will not
work. All other PEs MUST choose the same designated forwarder for a
multi-homed site. Call the PE that is chosen to send traffic to/from
CE1 the "designated forwarder".
In Figure 2, CE1 and CE4 must be dealt with independently, since CE1
is dual-homed, but CE4 is not.
3. Multi-homing Operation
This section describes procedures for electing a designated forwarder
among the set of PEs that are multi-homed to a customer site. The
procedures described in this section are applicable to BGP based
VPLS, LDP based VPLS with BGP-AD or a VPLS that contains a mix of
both BGP and LDP signaled PWs.
3.1. Provisioning Model
Figure 1 shows a customer site, CE1, multi-homed to two VPLS PEs, PE1
and PE2. In order for all VPLS PEs within the same VPLS domain to
elect one of the multi-homed PEs as the designated forwarder, an
indicator that the PEs are multi-homed to the same customer site is
required. This is achieved by assigning the same multi-homed site ID
(MH-ID) on PE1 and PE2 for CE1. When remote VPLS PEs receive NLRI
advertisement from PE1 and PE2 for CE1, the two NLRI advertisements
for CE1 are identified as candidates for designated forwarder
selection due to the same MH-ID. Thus, same MH-ID SHOULD be assigned
on all VPLS PEs that are multi-homed to the same customer site. Note
that a MH-ID=0 is invalid and a PE should discard such an
advertisement.
3.2. Multi-homing NLRI
Section 3.2.2 in [RFC4761] describes the encoding of the BGP VPLS
NLRI. This NLRI contains fields VE-ID, VE block offset, VE block size
and label base. For multi-homing operation, the same NLRI is used for
identifying the multi-homed customers sites. The VE-ID field in the
NLRI is set to MH-ID; the VE block offset, VE block size and label
base are set to zero. Thus, the NLRI contains 2 octets indicating the
length, 8 octets for Route Distinguisher, 2 octets for MH-ID and 7
octets with value zero.
Figure 2 shows two customer sites, CE1 and CE4, connected to PE1 with
CE1 multi-homed to PE1 and PE2. CE4 does not require special
addressing, being associated with the base VPLS instance identified
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by the VSI-ID for LDP VPLS and VE-ID for BGP VPLS. However, CE1 which
is multi-homed to PE1 and PE2 requires configuration of MH-ID and
both PE1 and PE2 MUST be provisioned with the same MH-ID for CE1. It
is valid to have non-zero VE block offset, VE block size and label
base in the VPLS NLRI for a multi-homed site. However, multi-homing
operations in such a case are outside the scope of this document.
3.3. Designated Forwarder Election
BGP-based multi-homing for VPLS relies on BGP DF election and VPLS DF
election. The net result of doing both BGP and VPLS DF election is
that of electing a single designated forwarder (DF) among the set of
PEs to which a customer site is multi-homed. All the PEs that are
elected as non-designated forwarders MUST keep their attachment
circuit to the multi-homed CE in blocked status (no forwarding).
These election algorithms operate on VPLS advertisements, which
include both the NLRI and attached BGP attributes. In order to
simplify the explanation of these algorithms, we will use a number of
variables derived from fields in the VPLS advertisement. These
variables are: RD, MH-ID, VBO, DOM, ACS, PREF and PE-ID. The notation
ADV -> <RD, MH-ID, VBO, DOM, ACS, PREF, PE-ID> means that from a
received VPLS advertisement ADV, the respective variables were
derived. The following sections describe two attributes needed for DF
election, then describe the variables and how they are derived from
fields in VPLS advertisement ADV, and finally describe how DF
election is done.
3.3.1. Attributes
The procedures below refer to two attributes: the Route Origin
community (see Section 4.1) and the L2-info community (see Section
4.2.1). These attributes are required for inter-AS operation; for
generality, the procedures below show how they are to be used. The
procedures also say how to handle the case that either or both are
not present.
3.3.2. Variables Used
3.3.2.1. RD
RD is simply set to the Route Distinguisher field in the NLRI part of
ADV.
3.3.2.2. MH-ID
MH-ID is simply set to the VE-ID field in the NLRI part of ADV.
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3.3.2.3. VBO
VBO is simply set to the VE Block Offset field in the NLRI part of
ADV. This field will typically be zero.
3.3.2.4. DOM
This variable, indicating the VPLS domain to which ADV belongs, is
derived by applying BGP policy to the Route Target extended
communities in ADV. The details of how this is done are outside the
scope of this document.
3.3.2.5. ACS
ACS is the status of the attachment circuits for a given site of a
VPLS. ACS = 1 if all attachment circuits for the site are down, and 0
otherwise.
For BGP-based Multi-homing, ADV MUST contain an L2-info extended
community; within this community are control flags. One of these
flags is the 'D' bit, described in [I-D.kothari-l2vpn-auto-site-id].
ACS is set to the value of the 'D' bit in ADV.
3.3.2.6. PREF
PREF is derived from the Local Preference (LP) attribute in ADV as
well as the VPLS Preference field (VP) in the L2-info extended
community. If the Local Preference attribute is missing, LP is set to
0; if the L2-info community is missing, VP is set to 0. The following
table shows how PREF is computed from LP and VP.
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+---------+---------------+----------+------------------------------+
| VP | LP Value | PREF | Comment |
| Value | | Value | |
+---------+---------------+----------+------------------------------+
| 0 | 0 | 0 | malformed advertisement, |
| | | | unless ACS=1 |
| | | | |
| 0 | 1 to (2^16-1) | LP | backwards compatibility |
| | | | |
| 0 | 2^16 to | (2^16-1) | backwards compatibility |
| | (2^32-1) | | |
| | | | |
| >0 | LP same as VP | VP | Implementation supports VP |
| | | | |
| >0 | LP != VP | 0 | malformed advertisement |
+---------+---------------+----------+------------------------------+
Figure 3 PREF table
3.3.2.7. PE-ID
If ADV contains a Route Origin (RO) community (see Section 4.1) with
type 0x01, then PE-ID is set to the Global Administrator sub-field of
the RO. Otherwise, if ADV has an ORIGINATOR_ID attribute, then PE-ID
is set to the ORIGINATOR_ID. Otherwise, PE-ID is set to the BGP
Identifier.
3.3.3. Election Procedures
The election procedures described in this section apply equally to
BGP VPLS and LDP VPLS.
Election occurs in two stages. The first stage divides all received
VPLS advertisements into buckets of relevant and comparable
advertisements. Distinction MUST NOT be made on whether the NLRI is a
multi-homing NLRI or not. In this stage, advertisements may be
discarded as not being relevant to DF election. The second stage
picks a single "winner" from each bucket by repeatedly applying a
tie-breaking algorithm on a pair of advertisements from that bucket.
The tie-breaking rules are such that the order in which
advertisements are picked from the bucket does not affect the final
result. Note that this is a conceptual description of the process; an
implementation MAY choose to realize this differently as long as the
semantics are preserved.
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Note: these procedures supersede the tie breaking rules described in
(Section 9.1.2.2) [RFC4271]
3.3.3.1. Bucketization for BGP DF Election
An advertisement
ADV -> <RD, MH-ID, VBO, DOM, ACS, PREF, PE-ID>
is discarded if DOM is not of interest to the BGP speaker. Otherwise,
ADV is put into the bucket for <DOM, RD, MH-ID, VBO>. In other words,
the information in BGP DF election consists of <DOM, RD, MH-ID, VBO>
and only advertisements with exact same <DOM, RD, MH-ID, VBO, DOM>
are candidates for DF election.
3.3.3.2. Bucketization for VPLS DF Election
An advertisement
ADV -> <RD, MH-ID, VBO, DOM, ACS, PREF, PE-ID>
is discarded if DOM is not of interest to the VPLS PE. Otherwise, ADV
is put into the bucket for <DOM, MH-ID>. In other words, all
advertisements for a particular VPLS domain that have the same MH-ID
are candidates for VPLS DF election.
3.3.3.3. Tie-breaking Rules
This section describes the tie-breaking rules for both BGP and VPLS
DF election. Tie-breaking rules for BGP DF election are applied to
candidate advertisements by any BGP speaker. Since RD must be same
for advertisements to be candidates for BGP DF election, use of
unique RDs will result in no candidate advertisements for BGP tie-
breaking rules and thus, a BGP speaker in such a case will simply not
do BGP DF election. Tie-breaking rules for VPLS DF election are
applied to candidate advertisements by all VPLS PEs and the actions
taken by VPLS PEs based on the VPLS DF election result are described
in Section 3.4.
Given two advertisements ADV1 and ADV2 from a given bucket, first
compute the variables needed for DF election:
ADV1 -> <RD1, MH-ID1, VBO1, DOM1, ACS1, PREF1, PE-ID1>
ADV2 -> <RD2, MH-ID2, VBO2, DOM2, ACS2, PREF2, PE-ID2>
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Note that MH-ID1 = MH-ID2 and DOM1 = DOM2, since ADV1 and ADV2 came
from the same bucket. If this is for BGP DF election, RD1 = RD2 and
VBO1 = VBO2 as well. Then the following tie-breaking rules MUST be
applied in the given order.
1. if (ACS1 != 1) AND (ACS2 == 1) ADV1 wins; stop
if (ACS1 == 1) AND (ACS2 != 1) ADV2 wins; stop
else continue
2. if (PREF1 > PREF2) ADV1 wins; stop;
else if (PREF1 < PREF2) ADV2 wins; stop;
else continue
3. if (PE-ID1 < PE-ID2) ADV1 wins; stop;
else if (PE-ID1 > PE-ID2) ADV2 wins; stop;
else ADV1 and ADV2 are from the same VPLS PE
For BGP DF election, if there is no winner and ADV1 and ADV2 are from
the same PE, BGP DF election should simply consider this as an
update.
For VPLS DF election, if there is no winner and ADV1 and ADV2 are
from the same PE, a VPLS PE MUST retain both ADV1 and ADV2.
3.4. DF Election on PEs
DF election algorithm MUST be run by all multi-homed VPLS PEs. In
addition, all other PEs SHOULD also run the DF election algorithm. As
a result of the DF election, multi-homed PEs that loose the DF
election for a MH-ID MUST put the ACs associated with the MH-ID in
non-forwarding state.
DF election result on the egress PEs can be used in traffic
forwarding decision. Figure 2 shows two customer sites, CE1 and CE4,
connected to PE1 with CE1 multi-homed to PE1 and PE2. If PE1 is the
designated forwarder for CE1, based on the DF election result, PE3
can chose to not send unknown unicast and multicast traffic to PE2 as
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PE2 is not the designated forwarder for any customer site and it has
no other single homed sites connected to it.
4. Multi-AS VPLS
This section describes multi-homing in an inter-AS context.
4.1. Route Origin Extended Community
Due to lack of information about the PEs that originate the VPLS
NLRIs in inter-AS operations, Route Origin Extended Community
[RFC4360] is used to carry the source PE's IP address.
To use Route Origin Extended Community for carrying the originator
VPLS PE's loopback address, the type field of the community MUST be
set to 0x01 and the Global Administrator sub-field MUST be set to the
PE's loopback IP address.
4.2. VPLS Preference
When multiple PEs are assigned the same site ID for multi-homing, it
is often desired to be able to control the selection of a particular
PE as the designated forwarder. Section 3.5 in [RFC4761] describes
the use of BGP Local Preference in path selection to choose a
particular NLRI, where Local Preference indicates the degree of
preference for a particular VE. The use of Local Preference is
inadequate when VPLS PEs are spread across multiple ASes as Local
Preference is not carried across AS boundary. A new field, VPLS
preference (VP), is introduced in this document that can be used to
accomplish this. VPLS preference indicates a degree of preference for
a particular customer site. VPLS preference is not mandatory for
intra-AS operation; the algorithm explained in Section 3.3 will work
with or without the presence of VPLS preference.
Section 3.2.4 in [RFC4761] describes the Layer2 Info Extended
Community that carries control information about the pseudowires. The
last two octets that were reserved now carries VPLS preference as
shown in Figure 3.
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+------------------------------------+
| Extended community type (2 octets) |
+------------------------------------+
| Encaps Type (1 octet) |
+------------------------------------+
| Control Flags (1 octet) |
+------------------------------------+
| Layer-2 MTU (2 octet) |
+------------------------------------+
| VPLS Preference (2 octets) |
+------------------------------------+
Figure 4 Layer 2 Info Extended Community
A VPLS preference is a 2-octets unsigned integer. A value of zero
indicates absence of a VP and is not a valid preference value. This
interpretation is required for backwards compatibility.
Implementations using Layer2 Info Extended Community as described in
(Section 3.2.4) [RFC4761] MUST set the last two octets as zero since
it was a reserved field.
For backwards compatibility, if VP is used, then BGP Local Preference
MUST be set to the value of VP. Note that a Local Preference value of
zero for a MH-Site is not valid unless 'D' bit in the control flags
is set (see [I-D.kothari-l2vpn-auto-site-id]). In addition, Local
Preference value greater than or equal to 2^16 for VPLS
advertisements is not valid.
4.3. Use of BGP-MH attributes in Inter-AS Methods
Section 3.4 in [RFC4761] and section 4 in [I-D.ietf-l2vpn-signaling]
describe three methods (a, b and c) to connect sites in a VPLS to PEs
that are across multiple AS. Since VPLS advertisements in method (a)
do not cross AS boundaries, multi-homing operations for method (a)
remain exactly the same as they are within as AS. However, for method
(b) and (c), VPLS advertisements do cross AS boundary. This section
describes the VPLS operations for method (b) and method (c). Consider
Figure 5 for inter-AS VPLS with multi-homed customer sites.
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4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS Information
between ASBRs
AS1 AS2
........ ........
CE2 _______ . . . .
___ PE1 . . PE3 --- CE3
/ : . . :
__/ : : : :
CE1 __ : ASBR1 --- ASBR2 :
\ : : : :
\___ PE2 . . PE4 ---- CE4
. . . .
........ ........
Figure 5 Inter-AS VPLS
A customer has four sites, CE1, CE2, CE3 and CE4. CE1 is multi-homed
to PE1 and PE2 in AS1. CE2 is single-homed to PE1. CE3 and CE4 are
also single homed to PE3 and PE4 respectively in AS2. Assume that in
addition to the base LDP/BGP VPLS addressing (VSI-IDs/VE-IDs), MH ID
1 is assigned for CE1. After running DF election algorithm, all four
VPLS PEs must elect the same designated forwarder for CE1 site. Since
BGP Local Preference is not carried across AS boundary, VPLS
preference as described in Section 4.2 MUST be used for carrying site
preference in inter-AS VPLS operations.
For Inter-AS method (b) ASBR1 will send a VPLS NLRI received from PE1
to ASBR2 with itself as the BGP nexthop. ASBR2 will send the received
NLRI from ASBR1 to PE3 and PE4 with itself as the BGP nexthop. Since
VPLS PEs use BGP Local Preference in DF election, for backwards
compatibility, ASBR2 MUST set the Local Preference value in the VPLS
advertisements it sends to PE3 and PE4 to the VPLS preference value
contained in the VPLS advertisement it receives from ASBR1. ASBR1
MUST do the same for the NLRIs it sends to PE1 and PE2. If ASBR1
receives a VPLS advertisement without a valid VPLS preference from a
PE within its AS, then ASBR1 MUST set the VPLS preference in the
advertisements to the Local Preference value before sending it to
ASBR2. Similarly, ASBR2 must do the same for advertisements without
VPLS Preference it receives from PEs within its AS. Thus, in method
(b), ASBRs MUST update the VPLS and Local Preference based on the
advertisements they receive either from an ASBR or a PE within their
AS.
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In Figure 5, PE1 will send the VPLS advertisements, including the
ones for MH site CE1, with Route Origin Extended Community containing
its loopback address. PE2 will do the same. Even though PE3 receives
the VPLS advertisements from the same BGP nexthop, ASBR2, the source
PE address contained in the Route Origin Extended Community is
different for the VPLS advertisements received from PE1 and PE2, and
thus, PE3 can apply correctly the DF Election algorithm as the
resulting PE-IDs are different.
4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of VPLS
Information between ASes
In this method, there is a multi-hop E-BGP peering between the PEs or
Route Reflectors in AS1 and the PEs or Route Reflectors in AS2. There
is no VPLS state in either control or data plane on the ASBRs.
The multi-homing operations on the PEs in this method are exactly the
same as they are in intra-AS scenario. However, since Local
Preference is not carried across AS boundary, the translation of LP
to VP and vice versa MUST be done by RR, if RR is used to reflect
VPLS advertisements to other ASes. This is exactly the same as what a
ASBR does in case of method (b). A RR must set the VP to the LP value
in an advertisement before sending it to other ASes and must set the
LP to the VP value in an advertisement that it receives from other
ASes before sending to the PEs within the AS.
5. MAC Flush Operations
In a service provider VPLS network, customer MAC learning is confined
to PE devices and any intermediate nodes, such as a Route Reflector,
do not have any state for MAC addresses.
Topology changes either in the service provider's network or in
customer's network can result in the movement of MAC addresses from
one PE device to another. Such events can result into traffic being
dropped due to stale state of MAC addresses on the PE devices. Age
out timers that clear the stale state will resume the traffic
forwarding, but age out timers are typically in minutes, and
convergence of the order of minutes can severely impact customer's
service. To handle such events and expedite convergence of traffic,
flushing of affected MAC addresses is highly desirable.
This section describes the scenarios where VPLS flush is desirable
and the specific VPLS Flush TLVs that provide capability to flush the
affected MAC addresses on the PE devices. All operations described in
this section are in context of a particular VPLS domain and not
across multiple VPLS domains. Mechanisms for MAC flush are described
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in [I-D.kothari-l2vpn-vpls-flush] for BGP based VPLS and in [RFC4762]
for LDP based VPLS.
5.1. MAC List Flush
If multiple customer sites are connected to the same PE, PE1 as shown
in Figure 2, and redundancy per site is desired when multi-homing
procedures described in this document are in effect, then it is
desirable to flush just the relevant MAC addresses from a particular
site when the site connectivity is lost.
To flush particular set of MAC addresses, a PE SHOULD originate a
flush message with MAC list that contains a list of MAC addresses
that needs to be flushed. In Figure 2, if connectivity between CE1
and PE1 goes down and if PE1 was the designated forwarder for CE1,
PE1 SHOULD send a list of MAC addresses that belong to CE1 to all its
BGP peers.
It is RECOMMENDED that in case of excessive link flap of customer
attachment circuit in a short duration, a PE should have a means to
throttle advertisements of flush messages so that excessive flooding
of such advertisements do not occur.
5.2. Implicit MAC Flush
When connectivity to a customer site is lost, remote PEs learn that a
particular site is no longer reachable. The local PE either withdraws
the VPLS NLRI that it previously advertised for the site or it sends
a BGP update message for the site's VPLS NLRI with the 'D' bit set.
If a remote PE detects that a multi-homed PE has transitioned from
being a DF to a non-DF, then the remote PE can choose to flush all
MAC addresses that it learned from the multi-homed PE transitioning
from DF to non-DF. Alternatively the remote PE may chose to react
when detecting the non-DF to DF transition for a multi-homed PE by
flushing in the related VPLS context all the MACs learned with the
exception of the MACs associated with the new DF PE.
6. Backwards Compatibility
No forwarding loops are formed when PEs or Route Reflectors that do
not support procedures defined in this section co exist in the
network with PEs or Route Reflectors that do support.
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6.1. BGP based VPLS
As explained in this section, multi-homed PEs to the same customer
site MUST assign the same MH-ID and related NLRI SHOULD contain the
block offset, block size and label base as zero. Remote PEs that lack
support of multi-homing operations specified in this document will
fail to create any PWs for the multi-homed MH-IDs due to the label
value of zero and thus, the multi-homing NLRI should have no impact
on the operation of Remote PEs that lack support of multi-homing
operations specified in this document.
6.2. LDP VPLS with BGP Auto-discovery
The BGP-AD NLRI has a prefix length of 12 containing only a 8 bytes
RD and a 4 bytes VSI-ID. If a LDP VPLS PEs running BGP AD lacks
support of multi-homing operations specified in this document, it
SHOULD ignore a MH NLRI with the length field of 17. As a result it
will not ask LDP to create any PWs for the multi-homed Site-ID and
thus, the multi-homing NLRI should have no impact on LDP VPLS
operation.
7. Security Considerations
No new security issues are introduced beyond those that are described
in [RFC4761] and [RFC4762].
8. IANA Considerations
At this time, this memo includes no request to IANA.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling", RFC
4761, January 2007.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
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[I-D.ietf-l2vpn-signaling] Rosen, E., "Provisioning, Autodiscovery,
and Signaling in L2VPNs", draft-ietf-l2vpn-signaling-08
(work in progress), May 2006.
[I-D.kothari-l2vpn-vpls-flush] Kothari, B. and R. Fernando, "VPLS
Flush in BGP-based Virtual Private LAN Service", draft-
kothari-l2vpn-vpls-flush-00 (work in progress), October
2008.
[I-D.kothari-l2vpn-auto-site-id] Kothari, B., Kompella, K., and T.
IV, "Automatic Generation of Site IDs for Virtual Private
LAN Service", draft-kothari-l2vpn-auto-site-id-01 (work in
progress), October 2008.
[I-D.ietf-pwe3-redundancy-bit] Muley, P., Bocci, M., and L. Martini,
"Preferential Forwarding Status bit definition", draft-
ietf-pwe3-redundancy-bit-01 (work in progress), September
2008.
9.2. Informative References
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection:
An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456,
April 2006.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
10. Acknowledgments
The authors would like to thank Yakov Rekhter, Nischal Sheth, Mitali
Singh, Deven Raut and Nehal Bhau for their insightful comments and
probing questions.
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Authors' Addresses
Bhupesh Kothari
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089 US
Email: bhupesh@juniper.net
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089 US
Email: kireeti@juniper.net
Wim Henderickx
Alcatel-Lucent
Copernicuslaan 50
2018 Antwerp, Belgium
Email: wim.henderickx@alcatel-lucent.be
Florin Balus
Alcatel-Lucent
701 E. Middlefield Road
Mountain View, CA, USA 94043
Email: florin.balus@alcatel-lucent.com
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