Internet DRAFT - draft-ietf-bess-vpls-multihoming
draft-ietf-bess-vpls-multihoming
Network Working Group B. Kothari
Internet-Draft Augtera Networks
Updates: 4761 (if approved) K. Kompella
Intended status: Standards Track Juniper Networks
Expires: January 26, 2020 W. Henderickx
Nokia
F. Balus
Cisco
J. Uttaro
AT&T
July 25, 2019
BGP based Multi-homing in Virtual Private LAN Service
draft-ietf-bess-vpls-multihoming-05.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. This document updates RFC 4761 by defining
new flags in the Control Flags field of the Layer2 Info Extended
Community.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 26, 2020.
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. General Terminology . . . . . . . . . . . . . . . . . . . 3
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. VPLS Multi-homing Considerations . . . . . . . . . . . . 5
3. Multi-homing Operation . . . . . . . . . . . . . . . . . . . 6
3.1. Customer Edge (CE) NLRI . . . . . . . . . . . . . . . . . 6
3.2. Deployment Considerations . . . . . . . . . . . . . . . . 7
3.3. Designated Forwarder Election . . . . . . . . . . . . . . 8
3.3.1. Attributes . . . . . . . . . . . . . . . . . . . . . 8
3.3.2. Variables Used . . . . . . . . . . . . . . . . . . . 9
3.3.3. Election Procedures . . . . . . . . . . . . . . . . . 11
3.4. DF Election on PEs . . . . . . . . . . . . . . . . . . . 13
3.5. Pseudowire and Site-ID Binding Properties . . . . . . . . 13
4. Multi-AS VPLS . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Route Origin Extended Community . . . . . . . . . . . . . 13
4.2. VPLS Preference . . . . . . . . . . . . . . . . . . . . . 14
4.3. Use of BGP attributes in Inter-AS Methods . . . . . . . . 15
4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS
Information between ASBRs . . . . . . . . . . . . . . 15
4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of
VPLS Information between ASes . . . . . . . . . 16
5. MAC Flush Operations . . . . . . . . . . . . . . . . . . . . 17
5.1. MAC Flush Indicators . . . . . . . . . . . . . . . . . . 17
5.2. Minimizing the effects of fast link transitions . . . . . 18
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 18
6.1. BGP based VPLS . . . . . . . . . . . . . . . . . . . . . 18
6.2. LDP VPLS with BGP Auto-discovery . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
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9. Contributing Authors . . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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.
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. The terms "VPLS instance" and "VPLS domain"
are used interchangeably in this document.
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A VPLS site is a grouping of ports on a PE that belong to the same
VPLS domain. The terms "VPLS instance" and "VPLS domain" are used
interchangeably in this document.
If the CE devices that connect to a VPLS site's ports have
connectivity to any other PE device then the VPLS site is called a
multi-homed VPLS site. Otherwise, it is called a single-homed VPLS
site. The ports are partitioned between VPLS sites such that each
port is in no more than one VPLS site. The terms "VPLS site" and "CE
site" are used interchangeably in this document.
A BGP VPLS NLRI for the base VPLS instance that has non-zero VE block
offset, VE block size and label base is called as VE NLRI in this
document. Each VPLS instance is uniquely identified by a VE-ID. VE-
ID is carried in the BGP VPLS NLRI as specified in section 3.2.2 in
[RFC4761].
A VPLS NLRI with value zero for the VE block offset, VE block size
and label base is called as CE NLRI in this document.
Section Section 3.1 defines CE NLRI and provides more detail.
A Multi-homed (MH) site is uniquely identified by a CE-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). A single-homed site can also be assigned a
CE-ID, but it is not mandatory to configure a CE-ID for single-homed
sites. Section Section 3.1 provides detail on CE-ID.
1.2. Conventions
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.
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2.1. Scenarios
In Figure 1, 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
In Figure 2, 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
into the SP cloud. That is to say, load balancing techniques will
not work. All other PEs MUST choose the same designated forwarder
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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. Customer Edge (CE) NLRI
Section 3.2.2 in [RFC4761] specifies a NLRI to be used for BGP based
VPLS (BGP VPLS NLRI). The format of the BGP VPLS NLRI is shown
below.
+------------------------------------+
| Length (2 octets) |
+------------------------------------+
| Route Distinguisher (8 octets) |
+------------------------------------+
| VE ID (2 octets) |
+------------------------------------+
| VE Block Offset (2 octets) |
+------------------------------------+
| VE Block Size (2 octets) |
+------------------------------------+
| Label Base (3 octets) |
+------------------------------------+
Figure 3: BGP VPLS NLRI
For multi-homing operation, a customer-edge NLRI (CE NLRI) is
proposed that uses BGP VPLS NLRI with the following fields set to
zero: VE Block Offset, VE Block Size and Label Base. In addition,
the VE-ID field of the NLRI is set to CE-ID. Thus, the CE NLRI
contains 2 octets indicating the length, 8 octets for Route
Distinguisher, 2 octets for CE-ID and 7 octets with value zero.
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. VPLS operations,
including multi-homing, in such a case are outside the scope of this
document. However, for interoperability with existing deployments
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that use non-zero VE block offset, VE block size and label base for
multi-homing operation, Section 6.1 provides more detail.
Wherever VPLS NLRI is used in this document, context must be used to
infer if it is applicable for CE NLRI, VE NLRI or for both.
3.2. Deployment Considerations
It is mandatory that each instance within a VPLS domain MUST be
provisioned with a unique Route Distinguisher value. Unique Route
Distinguisher allows VPLS advertisements from different VPLS PEs to
be distinct even if the advertisements have the same VE-ID, which can
occur in case of multi-homing. This allows standard BGP path
selection rules to be applied to VPLS advertisements.
Each VPLS PE must advertise a unique VE-ID with non-zero VE Block
Offset, VE Block Size and Label Base values in the BGP NLRI. VE-ID
is associated with the base VPLS instance and the NLRI associated
with it must be used for creating PWs among VPLS PEs. Any single-
homed customer sites connected to the VPLS instance do not require
any special addressing. However, an administrator (SP operator) can
choose to have a CE-ID for a single-homed site as well. Any multi-
homed customer sites connected to the VPLS instance require special
addressing, which is achieved by use of CE-ID. A set of customer
sites are distinguished as multi-homed if they all have the same CE-
ID. The following examples illustrate the use of VE-ID and CE-ID.
Figure 1 shows a customer site, CE1, multi-homed to two VPLS PEs, PE1
and PE2. In order for all VPLS PEs to set up PWs to each other, each
VPLS PE must be configured with a unique VE-ID for its base VPLS
instance. In addition, 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
VPLS site ID (CE-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 CE-ID. Thus, same CE-ID MUST be
assigned on all VPLS PEs that are multi-homed to the same customer
site.
Figure 2 shows two customer sites, CE1 and CE4, connected to PE1 with
CE1 multi-homed to PE1 and PE2. Similar to Figure 1 provisioning
model, each VPLS PE must be configured with a unique VE-ID for it
base VPLS instance. CE1 which is multi-homed to PE1 and PE2 requires
configuration of CE-ID and both PE1 and PE2 MUST be provisioned with
the same CE-ID for CE1. CE2 and CE3 are single-homed sites and do
not require special addressing. However, an operator must configure
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a CE-ID for CE4 on PE1. By doing so, remote PEs can determine that
PE1 has two VPLS sites, CE1 and CE4. If both CE1 and CE4
connectivity to PE1 is down, remote PEs can choose based on D bit in
VE NLRI not to send multicast traffic to PE1 as there are no VPLS
sites reachable via PE1. If CE4 was not assigned a unique CE-ID,
remote PEs have no way to know if there are other VPLS sites attached
and hence, would always send multicast traffic to PE1. While CE2 and
CE3 can also be configured with unique CE-IDs, there is no advantage
in doing so as both PE3 and PE4 have exactly one VPLS site.
Note that a CE-ID=0 is invalid and a PE should discard such an
advertisement.
Use of multiple VE-IDs per VPLS instance for either multi-homing
operation or for any other purpose is outside the scope of this
document. However, for interoperability with existing deployments
that use multiple VE-IDs, Section 6.1 provides more detail.
3.3. Designated Forwarder Election
BGP-based multi-homing for VPLS relies on standard BGP path selection
and VPLS DF election. The net result of doing both BGP path
selection 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. These election
algorithms are applicable to all VPLS NLRIs, and not just to CE
NLRIs. 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, SITE-ID, VBO, DOM, ACS, PREF
and PE-ID. The notation ADV -> <RD, SITE-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). These attributes are required for inter-AS operation;
for generality, the procedures below show how they are to be used.
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The procedures also outline how to handle the case that either or
both are not present.
For BGP-based Multi-homing, ADV MUST contain an L2-info extended
community as specified in [RFC4761]. Within this community are
various control flags. Two new control flags are proposed in this
document. Figure 4 shows the position of the new 'D' and 'F' flags.
Control Flags Bit Vector
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|D|Z|F|Z|Z|Z|C|S| (Z = MUST Be Zero)
+-+-+-+-+-+-+-+-+
Figure 4
1. 'D' (Down): Indicates connectivity status. In case of CE NLRI,
the connectivity status is between a CE site and a VPLS PE. In
case of VE NLRI, the connectivity status is for the VPLS
instance. In case of CE NLRI, the bit MUST be set to one if all
the attachment circuits connecting a CE site to a VPLS PE are
down. In case of VE NLRI, the bit must be set to one if the VPLS
instance is operationally down. Note that a VPLS instance that
has no connectivity to any of its sites must be considered as
operationally down.
2. 'F' (Flush): Indicates when to flush MAC state. A designated
forwarder must set the F bit and a non-designated forwarder must
clear the F bit when sending BGP CE NLRIs for multi-homed sites.
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. Refer to Section 5 for more details on the use case.
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. 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.
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3.3.2.2. SITE-ID
SITE-ID is simply set to the VE-ID field in the NLRI part of the ADV.
Note that no distinction is made whether VE-ID is for a multi-homed
site or not.
3.3.2.3. VBO
VBO is simply set to the VE Block Offset field in the NLRI part of
ADV.
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.
ACS is set to the value of the 'D' bit in ADV that belongs to CE
NLRI. If ADV belongs to base VPLS instance (VE NLRI) with non-zero
label block values, no change must be made to ACS.
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 |
+---------+---------------+----------+------------------------------+
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.
3.3.3. Election Procedures
The election procedures described in this section apply equally to
BGP VPLS and LDP VPLS. A distinction MUST NOT be made on whether the
NLRI is a multi-homing NLRI or not. Subset of these procedures
documented in standard BGP best path selection deals with general IP
Prefix BGP route selection processing as defined in [RFC4271]. A
separate part of the algorithm defined under VPLS DF election is
specific to designated forwarded election procedures performed on
VPLS advertisements. A concept of bucketization is introduced to
define route selection rules for VPLS advertisements. 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 for standard BGP path selection
An advertisement
ADV -> <RD, SITE-ID, VBO, ACS, PREF, PE-ID>
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is put into the bucket for <RD, SITE-ID, VBO>. In other words, the
information in BGP path selection consists of <RD, SITE-ID, VBO> and
only advertisements with exact same <RD, SITE-ID, VBO> are candidates
for BGP path selection procedure as defined in [RFC4271].
3.3.3.2. Bucketization for VPLS DF Election
An advertisement
ADV -> <RD, SITE-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, SITE-ID>. In other words, all
advertisements for a particular VPLS domain that have the same SITE-
ID are candidates for VPLS DF election.
3.3.3.3. Tie-breaking Rules
This section describes the tie-breaking rules for VPLS 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, SITE-ID1, VBO1, DOM1, ACS1, PREF1, PE-ID1>
ADV2 -> <RD2, SITE-ID2, VBO2, DOM2, ACS2, PREF2, PE-ID2>
Note that SITE-ID1 = SITE-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
If there is no winner and ADV1 and ADV2 are from the same PE, a VPLS
PE MUST retain both ADV1 and ADV2.
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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 lose the DF
election for a SITE-ID MUST put the ACs associated with the SITE-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 choose 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 and 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
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.
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.
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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 5.
+------------------------------------+
| Extended community type (2 octets) |
+------------------------------------+
| Encaps Type (1 octet) |
+------------------------------------+
| Control Flags (1 octet) |
+------------------------------------+
| Layer-2 MTU (2 octet) |
+------------------------------------+
| VPLS Preference (2 octets) |
+------------------------------------+
Figure 5: 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.
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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 CE-ID 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 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 6 for inter-AS VPLS with multi-homed customer sites.
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 6: 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), CE-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
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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.
In Figure 6, 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
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.
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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.
5.1. MAC Flush Indicators
If 'D' bit in the control flags is set in a received VE NLRI, the
receiving PE SHOULD flush all the MAC addresses learned from the PE
advertising the failure.
Anytime a designated forwarder change occurs, a remote PE SHOULD
flush all the MAC addresses it learned from the PE that lost the DF
election (old designated forwarder). 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. However, procedures for flushing a limited set
of MAC addresses are beyond the scope of this document. Use of
either 'D' or 'F' bit in control flags only allows to flush all MAC
addresses associated with a PE.
Designated forwarder change can occur in absence of failures, such as
when an attachment circuit comes up. 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 CE advertisement for CE1 to all it's peers. If PE3
performs the DF election before PE2, there is a chance that PE3 might
learn MAC addresses from PE2 after it was done electing PE1. This
can happen since PE2 has not yet processed the BGP CE advertisement
from PE1 and as a result continues to send traffic to PE3. This can
cause traffic from PE3 to CE1 to black-hole until those MAC addresses
are deleted due to age out timers. Therefore, to avoid such race-
conditions, a designated forwarder must set the F bit and a non-
designated forwarder must clear the F bit when sending BGP CE
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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.
5.2. 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 CE 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 compression
for F bit transitions. A timer value more than the time it takes BGP
to converge in the network is recommended.
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 CE-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 CE-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.
For compatibility with PEs that use multiple VE-IDs with non-zero
label block values for multi-homing operation, it is a requirement
that a PE receiving such advertisements must use the labels in the
NLRIs associated with lowest VE-ID for PW creation. It is possible
that maintaining PW association with lowest VE-ID can result in PW
flap, and thus, traffic loss. However, it is necessary to maintain
the association of PW with the lowest VE-ID as it provides
deterministic DF election among all the VPLS PEs.
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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 CE 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 MAC Flush procedures as described in
Section 5
7. Security Considerations
No new security issues are introduced beyond those that are described
in [RFC4761] and [RFC4762].
8. IANA Considerations
IANA already has a registry for "Layer2 Info Extended Community
Control Flags Bit Vector" <https://www.iana.org/assignments/bgp-
extended-communities>
This document requires two new bit flags to be assigned as follows:
Value Name Reference
----- -------------------------------- --------------
D Down connectivity status This document
F MAC flush indicator This document
9. Contributing Authors
The authors would also like to thank Senad Palislamovic and Wen Lin
for their contribution to the development of this document.
Senad Palislamovic
Nokia
Email: senad.palislamovic@nokia.com
Wen Lin
Juniper Networks
Email: wlin@juniper.net
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10. Acknowledgments
The authors would like to thank Yakov Rekhter, Nischal Sheth, Mitali
Singh, Ian Cowburn and Jonathan Hardwick for their insightful
comments and probing questions.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4761] Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
<https://www.rfc-editor.org/info/rfc4761>.
[RFC6074] Rosen, E., Davie, B., Radoaca, V., and W. Luo,
"Provisioning, Auto-Discovery, and Signaling in Layer 2
Virtual Private Networks (L2VPNs)", RFC 6074,
DOI 10.17487/RFC6074, January 2011,
<https://www.rfc-editor.org/info/rfc6074>.
11.2. Informative References
[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.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <https://www.rfc-editor.org/info/rfc4360>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
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[RFC4762] Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol (LDP)
Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
<https://www.rfc-editor.org/info/rfc4762>.
Authors' Addresses
Bhupesh Kothari
Augtera Networks
Email: bhupesh@anvaya.net
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: kireeti.kompella@gmail.com
Wim Henderickx
Nokia
Email: wim.henderickx@nokia.com
Florin Balus
Cisco
Email: fbalus@gmail.com
James Uttaro
AT&T
200 S. Laurel Avenue
Middletown, NJ 07748
US
Email: uttaro@att.com
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