Internet Engineering Task Force B. Kothari
Internet Draft Cohere Networks
Updates: 4761 (if approved)
Intended status: Standards Track K. Kompella
Expires: April 2013 Contrail Systems
W. Henderickx
F. Balus
S. Palislamovic
Alcatel-Lucent
J. Uttaro
AT&T
W. Lin
Juniper Networks
October 21, 2012
BGP based Multi-homing in Virtual Private LAN Service
draft-ietf-l2vpn-vpls-multihoming-04.txt
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.
Status of this Memo
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Table of Contents
1. Introduction...................................................4
1.1. General Terminology.......................................4
1.2. Conventions used in this document.........................5
2. Background.....................................................6
2.1. Scenarios.................................................6
2.2. VPLS Multi-homing Considerations..........................7
3. Multi-homing Operations........................................8
3.1. Provisioning Model........................................8
3.2. Multi-homing NLRI.........................................8
3.3. Designated Forwarder Election.............................9
3.3.1. Attributes...........................................9
3.3.2. Variables Used.......................................9
3.3.2.1. RD.............................................10
3.3.2.2. MH-ID..........................................10
3.3.2.3. VBO............................................10
3.3.2.4. DOM............................................10
3.3.2.5. ACS............................................10
3.3.2.6. PREF...........................................11
3.3.2.7. PE-ID..........................................11
3.4. VPLS DF Election on PEs..................................14
3.5. Pseudowire binding and traffic forwarding rules..........14
3.5.1. Site-ID Binding Properties..........................14
3.5.2. Standby Pseudowire Properties.......................15
4. Multi-AS VPLS.................................................16
4.1. Route Origin Extended Community..........................16
4.2. Preference...............................................16
4.3. Use of BGP-MH attributes in Inter-AS Methods.............17
4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS
Information between ASBRs ......18
4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of
VPLS Information between ASes ..............19
5. MAC Flush Operations..........................................20
5.1. MAC List FLush...........................................20
5.2. Implicit MAC Flush.......................................21
5.3. Minimizing the effects of fast link transitions..........22
6. Backwards Compatibility.......................................23
6.1. BGP based VPLS...........................................23
6.2. LDP VPLS with BGP Auto-discovery.........................23
7. Security Considerations.......................................24
8. IANA Considerations...........................................25
9. Acknowledgments...............................................26
10. References...................................................27
10.1. Normative References....................................27
10.2. Informative References..................................27
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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. [RFC6074] 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.
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
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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 RFC-2119 [RFC2119].
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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
CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for
redundant connectivity.
...............
. . ___ 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.
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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 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.
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3. Multi-homing Operations
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 with the following fields: VE-ID, VE block offset, VE block
size and label base. While this NLRI MAY be used for multi-homing,
a modified version of it, as detailed in this paragraph, 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
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. As stated above, to ensure backward capabilities, it is valid
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to use BGP VPLS NLRI for multi-homing operations. As such, it is
valid to have non-zero VE block offset, VE block size and label base
in the VPLS NLRI for multi-homed site.
3.3. Designated Forwarder Election
BGP-based multi-homing for VPLS relies on Standard BGP best path
selection and VPLS DF election. The net result of doing both
elections 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. Given that
semantics of BGP VPLS NLRI does not necessarily follow a standard IP
Prefix form, a construct of advertisement (ADV) with the variables
of interest is defined. The variables of interest are: RD, MH-ID,
VBO, DOM, ACS, PREF and PE-ID, so the notation of:
ADV = <RD, MH-ID, VBO, DOM, ACS, PREF, PE-ID>
The following sections describe two attributes needed for standard
BGP best path selection and VPLS DF election, the variables derived
from fields in VPLS advertisement ADV, and finally elaborate on the
selection processes.
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). These attributes are required for inter-AS operation; for
generality, the procedures below show how they are to be used. The
procedures also outline how to handle the case that either or both
are not present.
3.3.2. Variables Used
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3.3.2.1. RD
RD is simply set to the Route Distinguisher field in the NLRI part
of ADV. Actual process of assigning Route Distinguisher values must
guarantee its uniqueness per PE node. Therefore, two multi-homed
PEs offering the same VPLS service to a common set of CEs MUST
allocate different RD values for this site respectively.
3.3.2.2. MH-ID
MH-ID is simply set to the VE-ID field in the NLRI part of ADV. The
same MH-ID MUST be assigned to all PEs that are connected to the
same multi-homed site.
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.
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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.
+---------+---------------+----------+------------------------------+
| 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 |
+---------+---------------+----------+------------------------------+
Table 1
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.
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3.3.3. Election Procedures
The election procedures described in this section apply equally to
BGP VPLS and LDP VPLS. Subset of these procedures documented in
standard BGP best path selection deals with general IP Prefix BGP
route selection processing as defined in RFC 4271 and RFC 4364. A
separate part of the algorithm defined under VPLS DF election is
very specific to designated forwarded election procedures performed
on per VPLS instance bases. Given that the notion of VPLS
advertisement is not commonly used destination IP Prefix, a concept
of bucketization is introduced. By bucketizing advertisements and
running them through two different sets of procedures based on
variables of interest, we are effectively adopting common sets of
route selection rules to the VPLS environment. A distinction MUST
NOT be made on whether the NLRI is a multi-homing NLRI or not. 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.
3.3.3.1. Bucketization and BGP Best Path Selection
From the advertisement
ADV -> <RD, MH-ID, VBO, ACS, PREF, PE-ID>
we select variables of interest that satisfy a notion of the same
route as it is applicable to BGP election. As such, advertisements
with the exact same RD, MH-ID and VBO are candidates for BGP
Selection and put into BGP election bucket.
ADV -> <RD, MH-ID, VBO>
A standard set of BGP path selection rules, as defined in RFC 4271
and 4264 is applied as tie-breaking mechanism. These tie-breaking
rules are applied to candidate advertisements by a BGP speaker
responsible for processing and redistribution of BGP VPLS and MH
NLRI. If there is no winner and both ADVs are from the same PE, BGP
path selection should simply consider this an update.
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3.3.3.2. Bucketization and 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
and common DOM are candidates for VPLS DF election. Tie breaking
rules for VPLS DF election are different from standard BGP best path
selection. As outlined in 3.3, the main reason for that is the fact
that only single VPLS PE can be a designated forwarder for a given
site. 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>
Note that MH-ID1 = MH-ID2 and DOM1 = DOM2, since ADV1 and ADV2 came
from the same bucket. 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
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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. VPLS 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 lose 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 PE2 is not
the designated forwarder for any customer site and it has no other
single homed sites connected to it.
3.5. Pseudowire binding and traffic forwarding rules
3.5.1. Site-ID Binding Properties
For the use case where a single PE provides connectivity to a set of
CEs from which some on multi-homed and others are not, only single
pseudowire MAY be established. For example, if PE1 provides VPLS
service to CE1 and CE4 which are both part of the same VPLS domain,
but different sites, and CE1 is multi-homed, but CE4 is not (as
described in figure 2), PE3 would establish only single pseudowire
toward PE1. A design needs to ensure that regardless of PE1's
forwarding state in respect to DF or non-DF for multi-homed CE1,
PE3s access to CE4 is established. Since label allocation and
pseudowire established is tied to site-ID, we need to ensure that
proper pseudowire bindings are established.
For set of given advertisements with the common DOM but with
different Site-ID values, a VPLS PE speaker SHOULD instantiate and
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bind the pseudowire based on advertisement with the lowest Site-ID
value. Otherwise, binding would be completely random and during DF
changes for multi-homed site, non-multi-homed CE might suffer
traffic loss.
3.5.2. Standby Pseudowire Properties
As the notion of the convergence is addressed for transport plane by
use of RSVP FRR and LDP LFA, it is evident that similar solution is
required at the service level plane as well. This is most evident
in large-scale deployments as it takes quite long time to converge.
Ingress PE usually has to handle multiple relatively larger tasks
and re-signal all pseudowires affected by egress PE or AC failure.
Therefore, an implementation MAY choose to optimize the convergence
by pre-signaling the second, standby, pseudowire toward non-DF end
point for every active VPLS in 1:1 fashion. This greatly improves
the convergence times. However, details of such implementation are
still under research.
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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. 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 3: Layer2 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 VPLS preference is used, then BGP
Local Preference MUST be set to the value of VPLS preference. Note
that a Local Preference value of zero for a MH-ID is not valid nless
'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 [RFC6074] 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 4 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 4: 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
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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.
In Figure 4, PE1 will send the VPLS advertisements with Route Origin
Extended Community containing its loopback address. PE2 will do the
same. Even though PE3 receives the VPLS advertisements for VE-ID 1
and 2 from the same BGP nexthop, ASBR2, the source PE address
contained in the Route Origin Extended Community is different for
the CE1 and CE2 advertisements, and thus, PE3 creates two PWs, one
for CE1 (for VE-ID 1) and another one for CE2 (for VE-ID 2).
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
an 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.
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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 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 MAY send a list of MAC addresses that belong to CE1 to all its
BGP peers.
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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
Implicit MAC Flush refers to the use of BGP MH advertisements by the
PEs to flush the MAC addresses learned from the previous designated
forwarder.
In case of a failure, 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. In such cases, the remote
PEs can flush all the MACs that were learned from the PE which
reported the failure.
However, in cases when a designated forwarder change occurs in
absence of failures, such as when an attachment circuit comes up,
the BGP MH advertisement from the PE reporting the change is not
sufficient for MAC flush procedures. Consider the case in Figure 2
where PE1-CE1 link is non-operational and PE2 is the designated
forwarder for CE1. Also assume that Local Preference of PE1 is
higher than PE2. When PE1-CE1 link becomes operational, PE1 will
send a BGP MH advertisement to all it's peers. If PE3 elects PE1 as
the new designated forwarder for CE1 and as a result flushes all the
MACs learned from PE1 before PE2 elects itself as the non-designated
forwarder, there is a chance that PE3 might learn MAC addresses from
PE2 and as a result may black-hole traffic until those MAC addresses
are deleted due to age out timers.
A new flag 'F' is introduced in the Control Flags Bit Vector as a
deterministic way to indicate when to flush.
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Control Flags Bit Vector
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|D|A|F|Z|Z|Z|C|S| (Z = MUST Be Zero)
+-+-+-+-+-+-+-+-+
Figure 5
A designated forwarder must set the F bit and a non-designated
forwarder must clear the F bit when sending BGP MH advertisements.
A state transition from one to zero for the F bit can be used by a
remote PE to flush all the MACs learned from the PE that is
transitioning from designated forwarder to non-designated forwarder.
5.3. Minimizing the effects of fast link transitions
Certain failure scenarios may result in fast transitions of the link
towards the multi-homing CE which in turn will generate fast status
transitions of one or multiple multi-homed sites reflected through
multiple BGP MH advertisements and LDP MAC Flush messages.
It is recommended that a timer to damp the link flaps be used for
the port towards the multi-homed CE to minimize the number of MAC
Flush events in the remote PEs and the occurrences of BGP state
compressions for F bit transitions. A timer value more than the
time it takes BGP to converge in the network is recommended.
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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.
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. MH PEs may use existing LDP MAC Flush to flush the
remote LDP VPLS PEs or may use the implicit MAC Flush procedure.
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7. Security Considerations
No new security issues are introduced beyond those that are
described in [RFC4761] and [RFC4762].
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8. IANA Considerations
At this time, this memo includes no request to IANA.
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9. Acknowledgments
The authors would like to thank Ian Cowburn, Yakov Rekhter, Nischal
Sheth, and Mitali Singh for their insightful comments and probing
questions.
This document was prepared using 2-Word-v2.0.template.dot.
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10. References
10.1. 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.
[RFC6074] Rosen, E., "Provisioning, Autodiscovery, and Signaling
in L2VPNs", RFC 6074, January 2011.
10.2. Informative References
[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.
[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.
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[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.
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Authors' Addresses
Bhupesh Kothari
Cohere Networks
Email: bhupesh@cohere.net
Kireeti Kompella
Contrail Systems
Email: kireeti.kompella@gmail.com
Wim Henderickx
Alcatel-Lucent
Email: wim.henderickx@alcatel-lucent.be
Florin Balus
Alcatel-Lucent
Email: florin.balus@alcatel-lucent.com
Senad Palislamovic
Alcatel-Lucent
Email: senad.palislamovic@alcatel-lucent.com
James Uttaro
AT&T
200 S. Laurel Avenue
Middletown, NJ 07748, US
Email: uttaro@att.com
Wen Lin
Juniper Networks
Email: wlin@juniper.net
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