Updates on EVPN BUM Procedures
draft-zzhang-bess-evpn-bum-procedure-updates-01
The information below is for an old version of the document.
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| Authors | Zhaohui (Jeffrey) Zhang , Wen Lin , Jorge Rabadan , Keyur Patel | ||
| Last updated | 2015-12-18 | ||
| Replaced by | draft-ietf-bess-evpn-bum-procedure-updates | ||
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draft-zzhang-bess-evpn-bum-procedure-updates-01
BESS Z. Zhang
Internet-Draft W. Lin
Updates: 7432 (if approved) Juniper Networks, Inc.
Intended status: Standards Track J. Rabadan
Expires: June 20, 2016 Alcatel-Lucent
K. Patel
Cisco Systems
December 18, 2015
Updates on EVPN BUM Procedures
draft-zzhang-bess-evpn-bum-procedure-updates-01
Abstract
This document specifies procedure updates for broadcast, unknown
unicast, and multicast (BUM) traffic in Ethernet VPNs (EVPN),
including selective multicast, and provider tunnel segmentation.
Requirements Language
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.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 20, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Reasons for Tunnel Segmentation . . . . . . . . . . . . . 4
3. Additional Route Types of EVPN NLRI . . . . . . . . . . . . . 5
3.1. Per-Region I-PMSI A-D route . . . . . . . . . . . . . . . 6
3.2. S-PMSI A-D route . . . . . . . . . . . . . . . . . . . . 6
3.3. Leaf-AD route . . . . . . . . . . . . . . . . . . . . . . 6
4. Selective Multicast . . . . . . . . . . . . . . . . . . . . . 7
5. Inter-AS Segmentation . . . . . . . . . . . . . . . . . . . . 7
5.1. Changes to Section 7.2.2 of RFC 7117 . . . . . . . . . . 7
5.2. I-PMSI Leaf Tracking . . . . . . . . . . . . . . . . . . 8
5.3. Backward Compatibility . . . . . . . . . . . . . . . . . 9
6. Inter-Region Segmentation . . . . . . . . . . . . . . . . . . 10
6.1. Area vs. Region . . . . . . . . . . . . . . . . . . . . . 10
6.2. Per-region Aggregation . . . . . . . . . . . . . . . . . 12
6.3. Use of S-NH-EC . . . . . . . . . . . . . . . . . . . . . 13
6.4. Ingress PE's I-PMSI Leaf Tracking . . . . . . . . . . . . 13
7. Intra-region Segmentation and Assisted Ingress Replication . 13
7.1. Reducing Leaf A-D Routes . . . . . . . . . . . . . . . . 14
7.2. Mix of inter-region and intra-region segmentation . . . . 15
8. Multi-homing Support . . . . . . . . . . . . . . . . . . . . 15
9. EVPN DCI . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Non-GW Option . . . . . . . . . . . . . . . . . . . . . . 16
9.2. GW option . . . . . . . . . . . . . . . . . . . . . . . . 17
10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Terminology
To be added
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2. Introduction
RFC 7432 specifies procedures to handle broadcast, unknown unicast,
and multicast (BUM) traffic in Section 11, 12 and 16, using Inclusive
Multicast Ethernet Tag Route. A lot of details are referred to RFC
7117 (VPLS Multicast). In particular, selective multicast is briefly
mentioned for Ingress Replication but referred to RFC 7117.
RFC 7117 specifies procedures for using both inclusive tunnels and
selective tunnels, similar to MVPN procedures specified in RFC 6513
and RFC 6514. A new SAFI "MCAST-VPLS" is introduced, with two types
of NLRIs that match MVPN's S-PMSI A-D routes and Leaf A-D routes.
The same procedures can be applied to EVPN selective multicast for
both Ingress Replication and other tunnel types, but new route types
need to be defined under the same EVPN SAFI.
MVPN uses terms I-PMSI and S-PMSI A-D Routes. For consistency and
convenience, this document will use the same I/S-PMSI terms for VPLS
and EVPN. In particular, EVPN's Inclusive Multicast Ethernet Tag
Route and VPLS's VPLS A-D route carrying PTA (PMSI Tunnel Attribute)
for BUM traffic purpose will all be referred to as I-PMSI A-D routes.
Depending on the context, they may be used interchangeably.
MVPN provider tunnels and EVPN/VPLS BUM provider tunnels, which are
referred to as MVPN/EVPN/VPLS provider tunnels in this document for
simplicity, can be segmented for technical or administrative reasons,
which are summarized in Section 2.1 of this document. RFC 6513/6514
cover MVPN inter-as segmentation, RFC 7117 covers VPLS multicast
inter-as segmentation, and RFC 7524 (Seamless MPLS Multicast) covers
inter-area segmentation for both MVPN and VPLS.
There is a difference between MVPN and VPLS multicast inter-as
segmentation. For simplicity, EVPN uses the same procedures as in
MVPN. All ASBRs can re-advertise their choice of the best route.
Each can become the root of its intra-AS segment and inject traffic
it receives from its upstream, while each downstream PE/ASBR will
only pick one of the upstream ASBRs as its upstream. This is also
the behavior even for VPLS in case of inter-area segmentation.
For inter-area segmentation, RFC 7524 requires the use of Inter-area
P2MP Segmented Next-Hop Extended Community (S-NH-EC), and the setting
of "Leaf Information Required" (LIR) flag in PTA in certain
situations. Either of these could be optional in case of EVPN.
Removing these requirements would make the segmentation procedures
transparent to ingress and egress PEs.
RFC 7524 assumes that segmentation happens at area borders. However,
it could be at "regional" borders, where a region could be a sub-
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area, or even an entire AS plus its external links (Section 6). That
would allow for more flexible deployment scenarios (e.g. for single-
area provider networks).
This document specifies/clarifies/redefines certain/additional EVPN
BUM procedures, with a salient goal that they're better aligned among
MVPN, EVPN and VPLS. For brevity, only changes/additions to relevant
RFC 7117 and RFC 7524 procedures are specified, instead of repeating
the entire procedures. Note that these are to be applied to EVPN
only, even though sometimes they may sound to be updates to RFC
7117/7524.
2.1. Reasons for Tunnel Segmentation
Tunnel segmentation may be required and/or desired because of
administrative and/or technical reasons.
For example, an MVPN/VPLS/EVPN network may span multiple providers
and Inter-AS Option-B has to be used, in which the end-to-end
provider tunnels have to be segmented at and stitched by the ASBRs.
Different providers may use different tunnel technologies (e.g.,
provider A uses Ingress Replication, provider B uses RSVP-TE P2MP
while provider C uses mLDP). Even if they use the same tunnel
technology like RSVP-TE P2MP, it may be impractical to set up the
tunnels across provider boundaries.
The same situations may apply between the ASes and/or areas of a
single provider. For example, the backbone area may use RSVP-TE P2MP
tunnels while non-backbone areas may use mLDP tunnels.
Segmentation can also be used to divide an AS/area to smaller
regions, so that control plane state and/or forwarding plane state/
burden can be limited to that of individual regions. For example,
instead of Ingress Replicating to 100 PEs in the entire AS, with
inter-area segmentation [RFC 7524] a PE only needs to replicate to
local PEs and ABRs. The ABRs will further replicate to their
downstream PEs and ABRs. This not only reduces the forwarding plane
burden, but also reduces the leaf tracking burden in the control
plane. This inter-region segmentation can be further extended to
intra-region as an alternative way to achieve Assisted Replication as
proposed in [draft-rabadan-bess-evpn-optimized-ir], and it works for
MPLS encapsulation.
Smaller regions also have the benefit that, in case of tunnel
aggregation, it is easier to find congruence among the segments of
different constituent (service) tunnels and the resulting aggregation
(base) tunnel in a region. This leads to better bandwidth
efficiency, because the more congruent they are, the fewer leaves of
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the base tunnel need to discard traffic when a service tunnel's
segment does not need to receive the traffic (yet it is receiving the
traffic due to aggregation).
Another advantage of the smaller region is smaller BIER sub-domains.
In this new multicast architecture BIER, packets carry a BitString,
in which the bits correspond to edge routers that needs to receive
traffic. Smaller sub-domains means smaller BitStrings can be used
without having to send multiple copies of the same packet.
Finally, EVPN tunnel segmentation can be used for EVPN DCIs, as
discussed in Section 9. It follows the same concepts discussed
above.
3. Additional Route Types of EVPN NLRI
RFC 7432 defines the format of EVPN NLRI as the following:
+-----------------------------------+
| Route Type (1 octet) |
+-----------------------------------+
| Length (1 octet) |
+-----------------------------------+
| Route Type specific (variable) |
+-----------------------------------+
So far five types have been defined:
+ 1 - Ethernet Auto-Discovery (A-D) route
+ 2 - MAC/IP Advertisement route
+ 3 - Inclusive Multicast Ethernet Tag route
+ 4 - Ethernet Segment route
+ 5 - IP Prefix Route
This document defines three additional route types:
+ 6 - Per-Region I-PMSI A-D route
+ 7 - S-PMSI A-D route
+ 8 - Leaf A-D route
The "Route Type specific" field of the type 6 and type 7 EVPN NLRIs
starts with a type 1 RD, whose Administrative sub-field MUST match
that of the RD in all the EVPN routes from the same advertising
router for a given EVI, except the Leaf A-D route (Section 3.3).
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3.1. Per-Region I-PMSI A-D route
The Per-region I-PMSI A-D route has the following format. Its usage
is discussed in Section 6.2.
+-----------------------------------+
| RD (8 octets) |
+-----------------------------------+
| Ethernet Tag ID (4 octets) |
+-----------------------------------+
| Extended Community (8 octets) |
+-----------------------------------+
After Ethernet Tag ID, an Extended Community (EC) is used to identify
the region. Various types and sub-types of ECs provide maximum
flexibility. Note that this is not an EC Attribute, but an 8-octet
field embedded in the NLRI itself, following EC encoding scheme.
3.2. S-PMSI A-D route
The S-PMSI A-D route has the following format:
+-----------------------------------+
| RD (8 octets) |
+-----------------------------------+
| Ethernet Tag ID (4 octets) |
+-----------------------------------+
| Multicast Source Length (1 octet) |
+-----------------------------------+
| Multicast Source (Variable) |
+-----------------------------------+
| Multicast Group Length (1 octet) |
+-----------------------------------+
| Multicast Group (Variable) |
+-----------------------------------+
| Originating Router's IP Addr |
+-----------------------------------+
Other than the addition of Ethernet Tag ID, it is identical to the
S-PMSI A-D route as defined in RFC 7117. The procedures in RFC 7117
also apply (including wildcard functionality), except that the
granularity level is per Ethernet Tag.
3.3. Leaf-AD route
The Route Type specific field of a Leaf A-D route consists of the
following:
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+-----------------------------------+
| Route Key (variable) |
+-----------------------------------+
| Originating Router's IP Addr |
+-----------------------------------+
A Leaf A-D route is originated in response to a PMSI route, which
could be an Inclusive Multicast Tag route, a per-region I-PMSI A-D
route, an S-PMSI A-D route, or some other types of routes that may be
defined in the future that triggers Leaf A-D routes. The Route Key
is the "Route Type Specific" field of the route for which this Leaf
A-D route is generated.
The general procedures of Leaf A-D route are first specified in RFC
6514 for MVPN. The principles apply to VPLS and EVPN as well. RFC
7117 has details for VPLS Multicast, and this document points out
some specifics for EVPN, e.g. in Section 5.
4. Selective Multicast
RFC 7117 specifies Selective Multicast for VPLS. Other than that
different route types and formats are specified with EVPN SAFI for
S-PMSI A-D and Leaf A-D routes (Section 3), all procedures in RFC
7117 with respect to Selective Multicast apply to EVPN as well,
including wildcard procedures.
5. Inter-AS Segmentation
5.1. Changes to Section 7.2.2 of RFC 7117
The first paragraph of Section 7.2.2.2 of RFC 7117 says:
"... The best route procedures ensure that if multiple
ASBRs, in an AS, receive the same Inter-AS A-D route from their EBGP
neighbors, only one of these ASBRs propagates this route in Internal
BGP (IBGP). This ASBR becomes the root of the intra-AS segment of
the inter-AS tree and ensures that this is the only ASBR that accepts
traffic into this AS from the inter-AS tree."
The above VPLS behavior requires complicated VPLS specific procedures
for the ASBRs to reach agreement. For EVPN, a different approach is
used and the above quoted text is not applicable to EVPN.
The Leaf A-D based procedure is used for each ASBR who re-advertises
into the AS to discover the leaves on the segment rooted at itself.
This is the same as the procedures for S-PMSI in RFC 7117 itself.
The following text at the end of the second bullet:
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"................................................... If, in order
to instantiate the segment, the ASBR needs to know the leaves of
the tree, then the ASBR obtains this information from the A-D
routes received from other PEs/ASBRs in the ASBR's own AS."
is changed to the following:
"................................................... If, in order
to instantiate the segment, the ASBR needs to know the leaves of
the tree, then the ASBR MUST set the LIR flag to 1 in the PTA to
trigger Leaf A-D routes from egress PEs and downstream ASBRs.
It MUST be (auto-)configured with an import RT, which controls
acceptance of leaf A-D routes by the ASBR."
Accordingly, the following paragraph in Section 7.2.2.4:
"If the received Inter-AS A-D route carries the PMSI Tunnel attribute
with the Tunnel Identifier set to RSVP-TE P2MP LSP, then the ASBR
that originated the route MUST establish an RSVP-TE P2MP LSP with the
local PE/ASBR as a leaf. This LSP MAY have been established before
the local PE/ASBR receives the route, or it MAY be established after
the local PE receives the route."
is changed to the following:
"If the received Inter-AS A-D route has the LIR flag set in its PTA,
then a receiving PE must originate a corresponding Leaf A-D route,
and a receiving ASBR must originate a corresponding Leaf A-D route
if and only if it received and imported one or more corresponding Leaf
A-D routes from its downstream IBGP or EBGP peers, or it has non-null
downstream forwarding state for the PIM/mLDP tunnel that instantiates
its downstream intra-AS segment. The ASBR that (re-)advertised the
Inter-AS A-D route then establishes a tunnel to the leaves discovered
by the Leaf A-D routes."
5.2. I-PMSI Leaf Tracking
An ingress PE does not set the LIR flag in its I-PMSI's PTA, even
with Ingress Replication or RSVP-TE P2MP tunnels. It does not rely
on the Leaf A-D routes to discover leaves in its AS, and Section 11.2
of RFC 7432 explicitly states that the LIR flag must be set to zero.
An implementation of RFC 7432 might have used the Originating
Router's IP Address field of the Inclusive Multicast Ethernet Tag
routes to determine the leaves, or might have used the Next Hop field
instead. Within the same AS, both will lead to the same result.
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With segmentation, an ingress PE MUST determine the leaves in its AS
from the BGP next hops in all its received I-PMSI A-D routes, so it
does not have to set the LIR bit set to request Leaf A-D routes. PEs
within the same AS will all have different next hops in their I-PMSI
A-D routes (hence will all be considered as leaves), and PEs from
other ASes will have the next hop in their I-PMSI A-D routes set to
addresses of ASBRs in this local AS, hence only those ASBRs will be
considered as leaves (as proxies for those PEs in other ASes). Note
that in case of Ingress Replication, when an ASBR re-advertises IBGP
I-PMSI A-D routes, it MUST advertise the same label for all those for
the same Ethernet Tag ID and the same EVI. When an ingress PE builds
its flooding list, multiple routes may have the same (nexthop, label)
tuple and they will only be added as a single branch in the flooding
list.
5.3. Backward Compatibility
The above procedures assume that all PEs are upgraded to support the
segmentation procedures:
o An ingress PE uses the Next Hop instead of Originating Router's IP
Address to determine leaves for the I-PMSI tunnel.
o An egress PE sends Leaf A-D routes in response to I-PMSI routes,
if the PTA has the LIR flag set (by the re-advertising ASBRs).
o In case of Ingress Replication, when an ingress PE builds its
flooding list, multiple I-PMSI routes may have the same (nexthop,
label) tuple and only a single branch for those will be added in
the flooding list.
If a deployment has legacy PEs that does not support the above, then
a legacy ingress PE would include all PEs (including those in remote
ASes) as leaves of the inclusive tunnel and try to send traffic to
them directly (no segmentation), which is either undesired or not
possible; a legacy egress PE would not send Leaf A-D routes so the
ASBRs would not know to send external traffic to them.
To address this backward compatibility problem, the following
procedure can be used (see Section 6.2 for per-PE/AS/region I-PMSI
A-D routes):
o An upgraded PE indicates in its per-PE I-PMSI A-D route that it
supports the new procedures. Details will be provided in a future
revision.
o All per-PE I-PMSI A-D routes are restricted to the local AS and
not propagated to external peers.
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o The ASBRs in an AS originate per-region I-PMSI A-D routes and
advertise to their external peers to advertise tunnels used to
carry traffic from the local AS to other ASes. Depending on the
types of tunnels being used, the LIR flag in the PTA may be set,
in which case the downstream ASBRs and upgraded PEs will send Leaf
A-D routes to pull traffic from their upstream ASBRs. In a
particular downstream AS, one of the ASBRs is elected, based on
the per-region I-PMSI A-D routes for a particular source AS, to
send traffic from that source AS to legacy PEs in the downsrream
AS. The traffic arrives at the elected ASBR on the tunnel
announced in the best per-region I-PMSI A-D route for the source
AS, that the ASBR has selected of all those that it received over
EBGP or IBGP sessions. Details of the election procedure will be
provided in a future revision.
o In an ingress AS, if and only if an ASBR has active downstream
receivers (PEs and ASBRs), which are learned either explicitly via
Leaf AD routes or implicitly via PIM join or mLDP label mapping,
the ASBR originates a per-PE I-PMSI A-D route (i.e., regular
Inclusive Multicast Ethernet Tag route) into the local AS, and
stitches incoming per-PE I-PMSI tunnels into its per-region I-PMSI
tunnel. With this, it gets traffic from local PEs and send to
other ASes via the tunnel announced in its per-region I-PMSI A-D
route.
Note that, even if there is no backward compatibility issue, the
above procedures has the benefit of keeping all per-PE I-PMSI A-D
routes in their local ASes, greatly reducing the flooding of the
routes and their corresponding Leaf A-D routes (when needed), and the
number of inter-as tunnels.
6. Inter-Region Segmentation
6.1. Area vs. Region
RFC 7524 is for MVPN/VPLS inter-area segmentation and does not
explicitly cover EVPN. However, if "area" is replaced by "region"
and "ABR" is replaced by "RBR" (Regional Border Router) then
everything still works, and can be applied to EVPN as well.
A region can be a sub-area, or can be an entire AS including its
external links. Instead of automatic region definition based on IGP
areas, a region would be defined as a BGP peer group. In fact, even
with IGP area based region definition, a BGP peer group listing the
PEs and ABRs in an area is still needed.
Consider the following example diagram:
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--------- ------ ---------
/ \ / \ / \
/ \ / \ / \
| PE1 o ASBR1 -- ASBR2 ASBR3 -- ASBR4 o PE2 |
\ / \ / \ /
\ / \ / \ /
--------- ------ ---------
AS 100 AS 200 AS 300
|-----------|--------|---------|--------|------------|
segment1 segment2 segment3 segment4 segment5
The inter-as segmentation procedures specified so far (RFC 6513/6514,
7117, and Section 5 of this document) requires all ASBRs to be
involved, and Ingress Replication is used between two ASBRs in
different ASes.
In the above diagram, it's possible that ASBR1/4 does not support
segmentation, and the provider tunnels in AS 100/300 can actually
extend across the external link. In the case, the inter-region
segmentation procedures can be used instead - a region is the entire
(AS100 + ASBR1-ASBR2 link) or (AS300 + ASBR3-ASBR4 link). ASBR2/3
would be the RBRs, and ASBR1/4 will just be a transit core router
with respect to provider tunnels.
As illustrated in the diagram below, ASBR2/3 will establish a
multihop EBGP session with either a RR or directly with PEs in the
neighboring AS. I/S-PMSI A-D routes from ingress PEs will not be
processed by ASBR1/4. When ASBR2 re-advertises the routes into AS
200, it changes the next hop to its own address and changes PTA to
specify the tunnel type/identification in its own AS. When ASBR3 re-
advertises I/S-PMSI A-D routes into the neighboring AS 300, it
changes the next hop to its own address and changes PTA to specify
the tunnel type/identification in the neighboring region 3. Now the
segment is rooted at ASBR3 and extends across the external link to
PEs.
--------- ------ ---------
/ RR....\.mh-ebpg / \ mh-ebgp/....RR \
/ : \ `. / \ .' / : \
| PE1 o ASBR1 -- ASBR2 ASBR3 -- ASBR4 o PE2 |
\ / \ / \ /
\ / \ / \ /
--------- ------ ---------
AS 100 AS 200 AS 300
|-------------------|----------|---------------------|
segment 1 segment 2 segment 3
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6.2. Per-region Aggregation
Notice that every I/S-PMSI route from each PE will be propagated
throughout all the ASes or regions. They may also trigger
corresponding Leaf A-D routes depending on the types of tunnels used
in each region. This may become too many - routes and corresponding
tunnels. To address this concern, the I-PMSI routes from all PEs in
a AS/region can be aggregated into a single I-PMSI route originated
from the RBRs, and traffic from all those individual I-PMSI tunnels
will be switched into the single I-PMSI tunnel. This is like the
MVPN Inter-AS I-PMSI route originated by ASBRs.
The MVPN Inter-AS I-PMSI A-D route can be better called as per-AS
I-PMSI A-D route, to be compared against the (per-PE) Intra-AS I-PMSI
A-D routes originated by each PE. In this document we will call it
as per-region I-PMSI A-D route, in case we want to apply the
aggregation at regional level. The per-PE I-PMSI routes will not be
propagated to other regions. If multiple RBRs are connected to a
region, then each will advertise such a route, with the same route
key (Section 3.1). Similar to the per-PE I-PMSI A-D routes, RBRs/PEs
in a downstream region will each select a best one from all those re-
advertised by the upstream RBRs, hence will only receive traffic
injected by one of them.
MVPN does not aggregate S-PMSI routes from all PEs in an AS like it
does for I-PMSIs routes, because the number of PEs that will
advertise S-PMSI routes for the same (s,g) or (*,g) is small. This
is also the case for EVPN, i.e., there is no per-region S-PMSI
routes.
Notice that per-region I-PMSI routes can also be used to address
backwards compatibility issue, as discussed in Section 5.3.
The per-region I-PMSI route uses an embedded EC in NLRI to identify a
region. As long as it uniquely identify the region and the RBRs for
the same region uses the same EC it is permitted. In the case where
an AS number or area ID is needed, the following can be used:
o For a two-octet AS number, a Transitive Two-Octet AS-Specific EC
of sub-type 0x09 (Source AS), with the Global Administrator sub-
field set to the AS number and the Local Administrator sub-field
set to 0.
o For a four-octet AS number, a Transitive Four-Octet AS-Specific EC
of sub-type 0x09 (Source AS), with the Global Administrator sub-
field set to the AS number and the Local Administrator sub-field
set to 0.
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o For an area ID, a Transitive IPv4-Address-Specific EC of any sub-
type.
Uses of other particular ECs may be specified in other documents.
6.3. Use of S-NH-EC
RFC 7524 specifies the use of S-NH-EC because it does not allow ABRs
to change the BGP next hop when they re-advertise I/S-PMSI AD routes
to downstream areas. That is only to be consistent with the MVPN
Inter-AS I-PMSI A-D routes, whose next hop must not be changed when
they're re-advertised by the segmenting ABRs for reasons specific to
MVPN. For EVPN, it is perfectly fine to change the next hop when
RBRs re-advertise the I/S-PMSI A-D routes, instead of relying on S-
NH-EC. As a result, this document specifies that RBRs change the BGP
next hop when they re-advertise I/S-PMSI A-D routes and do not use S-
NH-EC. if a downstream PE/RBR needs to originate Leaf A-D routes, it
simply uses the BGP next hop in the corresponding I/S-PMSI A-D routes
to construct Route Targets.
The advantage of this is that neither ingress nor egress PEs need to
understand/use S-NH-EC, and consistent procedure (based on BGP next
hop) is used for both inter-as and inter-region segmentation.
6.4. Ingress PE's I-PMSI Leaf Tracking
RFC 7524 specifies that when an ingress PE/ASBR (re-)advertises an
VPLS I-PMSI A-D route, it sets the LIR flag to 1 in the route's PTA.
Similar to the inter-as case, this is actually not really needed for
EVPN. To be consistent with the inter-as case, the ingress PE does
not set the LIR flag in its originated I-PMSI A-D routes, and
determines the leaves based on the BGP next hops in its received
I-PMSI A-D routes, as specified in Section 5.2.
The same backward compatibility issue exists, and the same solution
as in the inter-as case applies, as specified in Section 5.3.
7. Intra-region Segmentation and Assisted Ingress Replication
[draft-rabadan-bess-evpn-optimized-ir] describes "Assisted Ingress
Replication", which reduces the burden of NVEs by having them
replicate to only one of a few designated replicators, which will
then replicate to other relevant NVEs. The tunnel segmentation
procedures can be extended to achieve the same, even with the support
for MPLS encapsulation.
With inter-region segmentation, an RBR, which is a Route Reflector,
changes the BGP Next Hop to one of its own addresses when it re-
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advertises an I/S-PMSI route to other regions, and sets the LIR bit
in the PTA Flag field when necessary, but it does not do so when re-
advertising to NVEs in its own region. If it does that even when re-
advertising to local NVEs, then it becomes a replicator as in [draft-
rabadan-bess-evpn-optimized-ir]: NVEs will respond with Leaf AD
routes to individual I-PMSI routes from NVEs, but targeted to the re-
advertising RBR of the selected best one (out of all those same
routes re-advertised by different RBRs). so that the sending NVEs
will only replicate to the RBRs, which will in turn replicate to
NVEs.
In case of MPLS encapsulation, for split-horizon purpose, NVEs MUST
set the LIR bit in their I-PMSI A-D routes to trigger corresponding
Leaf A-D routes from RBRs, with different labels advertised in the
Leaf A-D routes for different NVEs, so that RBRs know the source NVEs
of incoming packets, and will not relay the traffic back to the
source NVE.
A RNVE (Regular, or legacy NVE that does not support the procedures
discussed in this section) replicate traffic directly to all NVEs/
RNVEs. RNVEs can be identified by the lack of indication as
discussed in Section 5.3 in their I-PMSI A-D routes. In case of MPLS
encapsulation, NVEs and RNVEs advertise a label in their I-PMSI A-D
routes, and RBRs MUST not change that when re-advertise the routes.
Note that, the label is advertised even though an NVE sets the LIR
bit.
A RNVE is not able to send back Leaf A-D routes, so RBRs won't relay
received traffic to them. An ingress NVE (legacy or not) always send
to RNVEs directly. For comparison, in inter-as scenario
(Section 5.3) an ASBR is elected to relay traffic but in this intra-
region case, it is reasonable for the ingress NVE to send to RNVEs
directly - it is feasible and simpler.
7.1. Reducing Leaf A-D Routes
To address the possible concern with too many Leaf A-D routes (every
NVE responds with one to its selected RBR for each I-PMSI A-D route),
a RBR can clear the LIR bit when it re-advertises the I-PMSI routes
so that no Leaf A-D routes will be triggered for the per-PE I-PMSI
routes. It also originates a per-region I-PMSI A-D route
(Section 6.2), but instead of into other regions, it is back into the
same region. The route has the LIR bit set so that NVEs will respond
with a Leaf A-D route, allowing a RBR to determine the set of NVEs
that it is responsible for relaying incoming traffic to.
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The per-region I-PMSI A-D routes from the RBRs and corresponding Leaf
A-D routes from NVEs are comparable to the Replicator-AR and Leaf-AR
routes with the Optimized IR method (Selective Mode).
This is also comparable to the per-region aggregation discussed
earlier, only that the per-region I-PMSI A-D route is advertised back
to the same region instead of to other regions. Similarly, the RBRs
could terminate the per-PE I-PMSI A-D routes if there are no RNVEs.
7.2. Mix of inter-region and intra-region segmentation
Some more details may need to be spelled out when intra-region
segmentation is used for IR optimization while in the mean time
inter-region segmentation is used, with RNVEs present in different
regions.
8. Multi-homing Support
If multi-homing does not span across different ASes or regions,
existing procedures work with segmenation. If an ES is multi-homed
to PEs in different ASes or regions, additional procedures are needed
to work with segmentation. The procedures are well understood but
omitted here until the requirement becomes clear.
9. EVPN DCI
In addition to inter-as/region segmentation uses cases, EVPN Overlay
DC Interconnect is another important use case for EVPN tunnel
segmentation.
Section 5.1.1.1 and 5.1.1.2 of [draft-ietf-bess-evpn-overlay] discuss
two options of interconnecting EVPN Overlay DCs. With the GW option,
DC EVPNs and Interconnect EVPN (DCI) are independent and terminate at
the GWs. With the non-GW option, DC EVPNs and Interconnect EVPN form
an integral EVPN, just like EVPN inter-as option-B. The GW option is
discussed in details in section 3.4 of [draft-ietf-bess-dci-evpn-
overlay].
The non-GW option can only be used when PEs can use VNI/VSID that has
local significance (like mpls labels), and the GW option must be used
otherwise. With the GW option, mac lookup must be performed when
traffic comes from where non-local VNI/VSID are used. Otherwise,
label/VNI/VSID switching can be used (typical inter-as option-B
behavior).
Note that with either option, BUM traffic forwarding can be based on
tunnel stitching instead of mac lookup (except if IR is used together
with non-local VNI/VSID), because BUM traffic goes to all PEs on
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corresponding provider tunnels instead of to targeted PEs. The
following sections discusses some specific details for each option.
9.1. Non-GW Option
The non-GW option can be easily compared to EVPN/mpls inter-region
scenario where a region spans an entire AS - assuming that each DC is
in its own AS that is different from the DCI's and other DCs'..
Consider the following diagram:
+--------------+
+---------+ | | +---------+
+----+ | +----+ +----+ | +----+
|NVE1|--| |RBR1| |RBR3| |--|NVE3|
+----+ | | | | | | +----+
| +----+ +----+ |
| DC1 | WAN | DC2 |
| +----+ +----+ |
| |RBR3| |RBR4| |
+----+ | | | | | | +----+
|NVE2|--| +----+ +----+ |--|NVE4|
+----+ +---------+ | | +---------+ +----+
+--------------+
|---EVPN-Overlay----|---EVPN-MPLS---|----EVPN-Overlay---|
Data Center Interconnect without Gateway
The RBRs are WAN Edge routers. They re-advertise I/S-PMSI routes
from one side to the other, following the previous described
segmentation procedures. For example, the Inclusive Multicast Route
from NVE1 is re-advertised into the WAN side by both RBR 1 and RBR2,
with the LIR flag bit set in the PTA, and then re-advertised into DC2
by RBR3/4. NVE3/4 could both choose either the one re-advertised by
RBR3 or by RBR4, or could each choose a different one (e.g., NVE3
chooses the one re-advertised by RBR3 while NVE4 chooses the one re-
advertised by RBR4). Each either joins the advertised PIM tunnel or
send a corresponding Leaf A-D route to the re-advertiser of the
chosen best route. RBR3 and/or RBR4 repeat the process, followed by
RBR1 and/or RBR2 doing the same. At the end, a segmented tunnel is
established to reach all NVE3/4. When BUM traffic arrives on RBR1/2
from NVE1 via the tunnel segment in DC1, the multicast VXLAN
encapsulation is removed and the traffic is directly switched into
the segment in the WAN w/o going through mac lookup.
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The per-region aggregation method (Section 6.2) can be used to limit
the I-PMSI A-D routes to each DC.
9.2. GW option
Consider the following diagram adapted from section 3.4 of [draft-
ietf-bess-dci-evpn-overlay]:
+--------------+
+---------+ | | +---------+
+----+ | +---+ +---+ | +----+
|NVE1|--| | | | | |--|NVE3|
+----+ | |GW1| |GW3| | +----+
| +---+ +---+ |
| DC1 | WAN | DC2 |
| +---+ +---+ |
| | | | | |
+----+ | |GW2| |GW4| | +----+
|NVE2|--| +---+ +---+ |--|NVE4|
+----+ +---------+ | | +---------+ +----+
+--------------+
|---EVPN-Overlay----|---EVPN-MPLS---|----EVPN-Overlay---|
The GWs consumes EVPN routes from the DC side and re-originate new
ones into the WAN side, and vice versa. All GWs will advertise their
own I-PMSI A-D route to the DC and WAN side, but only the DF on an
internal ESI (I-ESI) for the local DC will forward BUM traffic from
one EVPN domain to the other. For example, BUM traffic from NVE1
will reach both GW1 and GW2, but only the DF, say GW1, will forward
to the WAN side. The traffic will then reach both GW3 and GW4, but
again only the DF (for the I-ESI for DC2, say GW4) will forward
traffic into DC2.
In [draft-ietf-bess-dci-evpn-overlay], the traffic forwarding by GWs
is based on mac lookup - because of global significance of VNIs in
DCs, the VXLAN encapsulation cannot indicate to which remote NVE a
known unicast packet should be forwarded to. However for BUM
traffic, this is not a problem - a BUM packet only need to be put
onto the appropriate tunnel. As a result, the DF GW on the I-ESI for
a local DC can stitch all incoming BUM tunnels from local NVEs to its
tunnel on the WAN side, and stitch all incoming BUM tunnels from
remote GWs in the DCI into its tunnel on the DC side. This way, BUM
traffic will be switched via label/VNI/VSID or multicast vxlan tunnel
destination, bypassing mac lookup. Note that, this works only if
Ingress Replication is not used for BUM traffic in an EVPN Overlay
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DC, because in that case the only way to distinguish BUM traffic from
known uncast traffic is by checking mac address of the packets.
Because the I-PMSI routes/tunnels are terminated in each DC/DCI, the
I-PMSI routes originated by GWs are somewhat similar to the per-
region I-PMSI routes discussed in the previous section. However, the
per-region I-PMSI routes from RBRs in the same DC have the same route
key and NVEs will only receive traffic from one of the RBRs based on
best route selection, while the per-GW I-PMSI routes are distinct and
all NVEs receive traffic from the same one of the GWs because only
the DF on the I-ESI can forward traffic.
10. Security Considerations
This document does not seem to introduce new security risks, though
this may be revised after further review and scrutiny.
11. Acknowledgements
The authors thank Eric Rosen, John Drake, and Ron Bonica for their
comments and suggestions.
12. References
12.1. Normative References
[I-D.ietf-bess-ir]
Rosen, E., Subramanian, K., and J. Zhang, "Ingress
Replication Tunnels in Multicast VPN", draft-ietf-bess-
ir-00 (work in progress), January 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7117] Aggarwal, R., Ed., Kamite, Y., Fang, L., Rekhter, Y., and
C. Kodeboniya, "Multicast in Virtual Private LAN Service
(VPLS)", RFC 7117, DOI 10.17487/RFC7117, February 2014,
<http://www.rfc-editor.org/info/rfc7117>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <http://www.rfc-editor.org/info/rfc7432>.
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[RFC7524] Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
Point-to-Multipoint (P2MP) Segmented Label Switched Paths
(LSPs)", RFC 7524, DOI 10.17487/RFC7524, May 2015,
<http://www.rfc-editor.org/info/rfc7524>.
12.2. Informative References
[I-D.ietf-bess-dci-evpn-overlay]
Rabadan, J., Sathappan, S., Henderickx, W., Palislamovic,
S., Balus, F., Sajassi, A., and D. Cai, "Interconnect
Solution for EVPN Overlay networks", draft-ietf-bess-dci-
evpn-overlay-00 (work in progress), January 2015.
[I-D.ietf-bess-evpn-overlay]
Sajassi, A., Drake, J., Bitar, N., Isaac, A., Uttaro, J.,
and W. Henderickx, "A Network Virtualization Overlay
Solution using EVPN", draft-ietf-bess-evpn-overlay-01
(work in progress), February 2015.
[I-D.rabadan-bess-evpn-optimized-ir]
Rabadan, J., Sathappan, S., Henderickx, W., Sajassi, A.,
and A. Isaac, "Optimized Ingress Replication solution for
EVPN", draft-rabadan-bess-evpn-optimized-ir-00 (work in
progress), October 2014.
[I-D.wijnands-bier-architecture]
Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
S. Aldrin, "Multicast using Bit Index Explicit
Replication", draft-wijnands-bier-architecture-05 (work in
progress), March 2015.
[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
2012, <http://www.rfc-editor.org/info/rfc6513>.
[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
<http://www.rfc-editor.org/info/rfc6514>.
Authors' Addresses
Zhaohui Zhang
Juniper Networks, Inc.
EMail: zzhang@juniper.net
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Wen Lin
Juniper Networks, Inc.
EMail: wlin@juniper.net
Jorge Rabadan
Alcatel-Lucent
EMail: jorge.rabadan@alcatel-lucent.com
Keyur Patel
Cisco Systems
EMail: keyupate@cisco.com
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