AC-Aware Bundling Service Interface in EVPNCisco Systemssajassi@cisco.comCisco SystemsCanadalburdet@cisco.comCisco Systemsmankamis@cisco.comNokiajorge.rabadan@nokia.comIndividualje_drake@yahoo.com
Routing
BESS Working Group
An EVPN (Ethernet VPNs) provides an extensible and flexible multihoming VPN solution
over an MPLS/IP network for intra-subnet connectivity among
Tenant Systems and End Devices that can be physical or virtual.
EVPN multihoming with Integrated Routing and Bridging (IRB) is one of the common deployment scenarios.
Some deployments requires the capability to have multiple subnets
designated with multiple VLAN IDs in the single broadcast domain.
EVPN technology defines three different types of service interface
which serve different requirements but none of them address the requirement
of supporting multiple subnets within a single broadcast domain.
In this document, we define a new service interface type to support
multiple subnets in the single broadcast domain. Service interface
proposed in this document will be applicable to multihoming cases only.
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
and RFC 8174.
EVPN based All-Active multihoming is becoming the basic building
block for providing redundancy in next generation data center
deployments as well as service provider access/aggregation network.
For EVPN IRB mode, there are deployments which expect to be able to support
multiple subnets within a single broadcast domain. Each subnet would
be differentiated by VLAN. Thus, a single IRB interface can still
serve multiple subnets.
Motivation behind such deployments are
Manageability: The support to have multiple
subnets using a single broadcast domain requires
only one broadcast domain and one IRB for "N" subnets
compare to the "N" broadcast domain and "N" IRB interface to
manage.
Simplicity: It avoids extra configuration by configuring
VLAN Range with single BD and IRB as compared to individual VLAN, BD, and IRB
interface per subnet.
defines three types of service interface. None
of them provide flexibility to achieve multiple subnets within a single broadcast domain.
The different types of service interface from are:
VLAN-Based Service Interface: With this service interface,
an EVPN instance consists of only a single broadcast
domain (e.g., a single VLAN). Therefore, there is a
one-to-one mapping between a VID on this interface and a MAC-VRF.
VLAN Bundle Service Interface: With this service interface,
an EVPN instance corresponds to multiple
broadcast domains (e.g., multiple VLANs); however, only a single
bridge table is maintained per MAC-VRF, which means multiple VLANs
share the same bridge table. The MPLS-encapsulated frames MUST
remain tagged with the originating VID. Tag translation is
NOT permitted. The Ethernet Tag ID in all EVPN routes MUST be set to 0.
VLAN-Aware Bundle Service Interface: With this service interface,
an EVPN instance consists of multiple broadcast domains
(e.g., multiple VLANs) with each VLAN having its own bridge
table -- i.e., multiple bridge tables (one per VLAN) are maintained
by a single MAC-VRF corresponding to the EVPN instance.
From the definition, it seems like VLAN Bundle Service Interface
does provide flexibility to support multiple subnets within a single
broadcast domain. However, the requirement is to have multiple subnets from
same ES on multihoming All-Active mode; that would not work. For example,
lets take the case from where PE1 learns MAC of H1 on VLAN 1 (subnet S1).
PE1 originates EVPN MAC route, as per , where the Ethernet Tag would
be set to 0. Incoming packets from the IRB interface, at PE2, are untagged packets.
PE2 does not have any associated AC information from EVPN MAC routes advertised by PE1.
PE2 can not forward traffic that is destined to H1.
This document specifies an extension to existing service interface types
defined in and defines AC-aware
Bundling service interface. AC-aware Bundling
service interface would provide a mechanism to have multiple subnets
in the single broadcast domain. This extension is applicable only for multihomed EVPN
PEs.
EVPN topology with multihoming and non multihoming PE shows sample EVPN topology where PE1 and PE2 are
multihomed PEs. PE3 is remote PE participating in the same EVPN instance
(EVI-1). It illustrates four subnets S1, S2, S3, and S4 where numerical value
provides associated VLAN information.
In Figure 1, BD-1 has multiple subnets where each subnet is distinguished by
VLAN 1, 2,3 and 4. PE1 learns MAC address MAC-1 from AC associated with
subnet S1. PE1 uses MAC route to advertise MAC-1 presence to PE
PEs. As per MAC route advertisement from PE1
does not carry any context providing information about MAC
address association with AC. When PE2 receives the MAC route with MAC-2,
it can not determine the AC associated with this MAC.
Since PE2 could not bind MAC-1 with the correct AC when it receives
data traffic destined for MAC-1, it does not know the destination AC
since multiple bridge ports have the same ESI assignment.
defines
mechanism to synchronize multicast routes between multihome
PEs. In the above case, if the receiver behind S1 sends IGMP
membership request, CE could hash it to either of the PEs. When
a multicast route is originated, it does not contain any AC
information. Once it reaches peer PE, it does not have
any information about which subnet this IGMP membership request
belongs. Similarly to the unicast traffic problem, the incoming
multicast traffic from IRB cannot be forwarded to the proper AC.
In the case of a single subnet per broadcast domain, there is a potential
case of security issue. For example, PE1 has BD1 configured
with VLAN-1 and multihome PE PE2 has BD1 configured with VLAN-2.
Each of the IGMP membership requests on PE1 would be synchronized
to PE2 and PE2 would process multicast routes and start forwarding
multicast traffic on VLAN-2, which was not intended. Again, a similar
issue can potentially be seen with unicast traffic.
AC: Attachment circuit.
BD: broadcast domain. As per , an EVI consists of a
single or multiple BDs. In case of VLAN-bundle and VLAN-based
service models (see ), a BD is equivalent to an EVI.
In the case of the VLAN-aware bundle service model, an EVI contains
multiple BDs. Also, in this document, BD and subnet are
equivalent terms.
BD Route Target: refers to the broadcast domain assigned to Route
Target . In the case of the VLAN-aware bundle service model,
all the BD instances in the MAC-VRF share the same Route Target.
BT: Bridge Table. The instantiation of a BD in a MAC-VRF,
as per .
Ethernet A-D route: Ethernet Auto-Discovery (A-D) route,
as per .
EVI: EVPN Instance spanning the NVE/PE devices that are
participating on that EVPN, as per .
EVPN: Ethernet Virtual Private Networks, as per .
IRB: Integrated Routing and Bridging interface.
It connects an IP-VRF to a BD (or subnet).
MAC-VRF: A Virtual Routing and Forwarding Table for Media
Access Control (MAC) addresses on an NVE/PE, as per .
A MAC-VRF is also an instantiation of an EVI in an NVE/PE.
PE: Provider edge device hosting EVPN instance
RT-2: EVPN route type 2, i.e., MAC/IP advertisement route,
as defined in .
RT-5: EVPN route type 5, i.e., IP Prefix route. As defined in
Section 3 of .
VLAN: The usage of VLAN refers to the 802.1Q or 802.1AD tag.
(S, G): Multicast membership request for source S and group G.
This document also assumes familiarity with the terminology
of ,,
.
A new service interface represents an attachment-circuit where
multiple VLANs are configured. Each of these VLANs is represented
by a different AC ID (Identifier) under a single broadcast domain. Service interface MUST be applicable to multihomed PEs only. Service interface MUST have an Ethernet-Segment identifier assignment. New service interface handling procedures MUST be
backward compatible with implementation procedures defined in
.
New service interface MUST support EVPN multicast routes defined
in too.
Section 9.1 of describes different
mechanism to learn Unicast MAC address locally. At those PEs
where AC aware bundling is supported, the MAC address is
learned along with VLAN associated with AC.
MAC/IP advertisement route construction follows the mechanism defined in section 9.2.1 of
. An Attachment Circuit Extended Community () MUST be attached to EVPN Route Type 2.
Presence of Attachment Circuit Extended Community
() MUST be ignored by non
multihoming PEs. Remote PE (non-multihomed PE) MUST
process MAC route as defined in .
Multihoming PE MUST process Attachment Circuit Extended Community
() to associate the remote MAC address with the appropriate AC.
From , PE2 receives MAC route for MAC-1.
It MUST get an AC-ID from the Attachment Circuit Extended Community ()
in Route Type 2 and associate the MAC address with the specific subnet.
When a local multihomed PE in a given broadcast domain receives
IGMP membership request on local AC, it MUST synchronize multicast
state by originating multicast route defined in
. When Service
interface is AC aware it MUST attach Attachment Circuit Extended Community
() along with the multicast route. For example in when
H2 sends IGMP membership requests for (S, G), and CE hashed it to one of the PEs. Lets
say PE1 received an IGMP membership request. PE1 MUST originate multicast
route to synchronize multicast state with PE2. Multicast route MUST contain
Attachment Circuit Extended Community () along with
multicast route.
PE1 MUST originate multicast route updates for any subsequent
IGMP membership requests under
same or different subnet attaching adequate Attachment
Circuit ID Extended Community ().
If multihomed PE receives a remote multicast route on the broadcast domain for a given ES,
route MUST be programmed to the correct subnet. Subnet information MUST be extracted from
Attachment Circuit Extended Community. That value maps to the VLAN of a local AC where
the multicast route is associated with.
Packet received from CE MUST follow the same procedure as defined in Section 13.1 of
.
Unknown Unicast packets from a Remote PE MUST follow the procedure
as per Section 13.2.1 of .
Known unicast Received on a remote PE MUST follow the procedure as per
section 13.2.2. In , if PE3 receives
a known unicast packet for destination MAC MAC-1, it MUST follow the procedure
defined in Section 13.2.2 of .
If destination MAC lookup is performed on a known unicast packet, destination
MAC lookup MUST provide VLAN and local AC information. For example, if PE2 receives
a unicast packet that is destined to MAC-1 (packet might be coming from IRB or remote PE
with EVPN tunnel), destination MAC lookup on PE2 MUST provide
an outgoing port along with associated VLAN value.
Multicast traffic from CE and remote PE MUST follow the procedure defined
in .
Multicast traffic received from IRB interface or EVPN tunnel,
route lookup would be performed based on IGMP
snooping state and traffic would be forwarded to the appropriate AC.
If there is misconfiguration of VLAN or VLAN range across multihoming
PEs, the same MAC address or IGMP membership requests would be learned with different VLANs per broadcast
domain. This is detected by a received Route having an ESI which is local, but
an Attachment Circuit ID which does not match any Normalized VID in the local
bridge domain (or vice-versa)
In this case, an Error message MUST be thrown for the operator to make configuration
changes. Furthermore, the errored MAC route MUST be ignored.
This document defines a new BGP Extended Community for EVPN and updates several
existing Extended Communities.
A new BGP Extended Community called Attachment Circuit
ID Extended Community is introduced. This new extended community is a
transitive extended community with the Type field of 0x06
(EVPN) and the Sub-Type of 0x0E. It is advertised along with
EVPN MAC/IP Advertisement Route (Route Type 2) per
for AC-Aware
Bundling Service Interface. It may also be advertised along with
EVPN Multicast Route (Route Type 7 and 8) as
per .
Generically speaking, the new extended community MUST be attached to
any routes that require specific VLAN identification.
The Attachment Circuit Extended Community is encoded as
an 8-octet value as follows:
Attachment Circuit Extended Community The Attachment Circuit ID field represents a Normalized VID, the value of which is derived
using the same method as in . The Instance field is used to uniqueley identify an Attachment Circuit Extended Community
when multiple instances are attached to a same route. When only a single instance is present on
a route, this field is set to 0.
The procedures described in this document are backwards compatible with
VLAN-aware bundling mode since the Ethernet Tag ID field remains intact. This, however, may present
a drawback: AC-Aware Bundle use-cases may result in multiple ACs being represented by a
single EVPN route.For instance with multicast, the same (S,G) may be used over different subnets like S1-S4 in
interface ESI-100 of . In that case, the same Route-Type 7 MUST carry multiple Attachment Circuit Extended
Communities, one instance per attachment circuit / VLAN.
Similarly for unicast MAC addresses MAC-1, MAC-2 in subnets S1-S4 of same interface ESI-100
in , separate Route Type 2 will be advertised with Attachment Circuit Extended Community according to this
document. In both cases, a single Ethernet A-D per EVI Route Type 1 is advertised.It may also happen that the number of VLAN is fairly large, and multiple routes with
different BGP Route Distinguishers may be necessary to carry the required amount of Extended
Communities.These additional Route Distinguishers or functionality implemented in concurrent specifications updating Ethernet-AD
per EVI behaviour not being defined for AC-Aware Bundling, add complexity to the overall
solution and implementation. The following subsections address this for known documents and
provide future-looking guidance for AC-Aware Bundling support.To remedy the possible large number of VLAN or uniqueness issues correlating multiple
Attachment Circuit Extended Community instances, the Attachment Circuit ID MAY be set to 0xFFFF_FFFF,
which does not correspond to a valid Normalized VID.
That value tells peer PE that the Attachment Circuit ID is carried as part of
the Ethernet Tag ID field of the route. Since the key of such an EVPN route
is now unique, multiple Attachment Circuit Extended Communities per route is no longer required.
This however poses backward-compatibility and interoperability issues with remote PE(s)
expecting a zero Ethernet Tag ID and/or with VLAN-Aware Bundle Service Interface .
The use of the EVPN Layer 2 Attributes Extended Community ("L2-Attr") for bridge domains is
instroduced in , specifically with the intent of
signaling Primary and Backup states in the Control Flags field. The extended comunity is
added to Ethernet A-D per EVI routes.
When multiple ACs / VLAN are present, DF-Election may
result in different Primary (P) and Backup (B) states for each subinterface. When this is
the case, supporting for AC-Aware Bundling is achieved by adding multiple L2-Attr Extended Communities on the Ethernet A-D per EVI route, each with a unique Instance. To
associate this L2-Attr with a specific AC, Attachment Circuit Extended
Communities are also added each with a corresponding value in their Instance field(s)
Updated EVPN Layer 2 Attributes Extended Community defines procedures for peering PEs to signal AC-Influenced DF
election and the desire to use AC-DF with the rest of the PEs in the ES. The procedure
defined further relies on withdrawing (or not advertising) the Ethernet-AD per EVI corresponding to an AC in failed or
misconfigured state.
When multiple attachment circuit / VLAN are present, each individual AC may be miconfigured
(missing) or in failed state. The Attachment
Circuit Extended Community in Ethernet A-D per EVI routes is of generalized assistance,
allowing to compare lists of local AC / VLAN Normalized VIDs to those received in remote
routes with a same ESI (peer). A PE which receives an Ethernet A-D per EVI route
without the Attachment Circuit ID corresponding to its local Normalized VID may assume the
peer has misconfigured this subnet / VLAN or the AC has failed and perform AC-Influenced DF election for
that AC as if the Ethernet A-D had been withdrawn.
defines procedures for peering PEs to
signal reroute labels with special disposition attributes in order to support a fast reroute
machcnism for traffic loss avoidance on failure(s).
These reroute labels may be allocated at the bridge or at the AC granularity. Allocation at
the bridge granulartity poses no problem for AC-Aware Bundling and the Instance field may
remain 0 (same value as previously Reserved=0).
However, to support AC allocation granularity and AC-Aware Bundling, the ESI Label Extended Community is updated to include an Instance field for
correlation against a specific Attachment Circuit Extended Community. The Attachment Circuit
ID from the extended community with matchign Instance identifies the specific AC for which
the reroute label is intended to be installed.
Updated ESI Label Extended Community
Generically speaking, any new use-case requiring a new or updated Extended Community where
information pertaining to multiple ACs is to be included in a single EVPN
route, MUST contain or be updated to include a 2 octet Instance field. An Attachment Circuit
Extended Community MUST be included for each AC, with the same unique value in its Instance
field for demultiplexing information onto the correct AC.
The same Security Considerations described in
are valid for this document.
IANA has made the following assignment in the "EVPN Extended Community Sub-Types"
registry set up by .
Sub-Type Value
Name
Reference
0x0E
Attachment Circuit Extended Community
This document
AcknowledgementsThe authors thank and
and for their careful
reviews.ContributorsIn addition to the authors listed on the front page, the following
people have also contributed to this document:Cisco SystemsCanadapbrisset@cisco.comCisco SystemsUnited States of Americasthoria@cisco.com