Internet DRAFT - draft-ietf-bess-evpn-irb-extended-mobility
draft-ietf-bess-evpn-irb-extended-mobility
BESS WorkGroup N. Malhotra, Ed.
Internet-Draft A. Sajassi
Updates: 7432 (if approved) A. Pattekar
Intended status: Standards Track Cisco Systems
Expires: 18 April 2024 J. Rabadan
Nokia
A. Lingala
AT&T
J. Drake
Juniper Networks
16 October 2023
Extended Mobility Procedures for EVPN-IRB
draft-ietf-bess-evpn-irb-extended-mobility-17
Abstract
The procedure to handle host mobility in a layer 2 Network with EVPN
control plane is defined as part of RFC7432. EVPN has since evolved
to find wider applicability across various IRB use cases that include
distributing both MAC and IP reachability via a common EVPN control
plane. MAC Mobility procedures defined in RFC7432 are extensible to
IRB use cases if a fixed 1:1 mapping between host IP and MAC is
assumed across host moves. Generic mobility support for IP and MAC
addresses that allows these bindings to change across moves IS
REQUIRED to support a broader set of EVPN IRB use cases. EVPN all-
active multi-homing further introduces scenarios that require
additional consideration from mobility perspective. This document
enumerates a set of design considerations applicable to mobility
across these EVPN IRB use cases and updates sequence number
assignment procedures defined in RFC7432 to address these IRB use
cases.
NOTE TO IESG (TO BE DELETED BEFORE PUBLISHING): This draft lists six
authors which is above the required limit of five. Given significant
and active contributions to the draft from all six authors over the
course of six years, we would like to request IESG to allow
publication with six authors. Specifically, the three Cisco authors
are the original inventors of these procedures and contributed
heavily to rev 0 draft, most of which is still intact. AT&T is also
a key contributor towards defining the use cases that this document
addresses as well as the proposed solution. Authors from Nokia and
Juniper have further contributed to revisions and discussions
steadily over last six years to enable respective implementations and
a wider adoption.
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Status of This Memo
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This Internet-Draft will expire on 18 April 2024.
Copyright Notice
Copyright (c) 2023 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Document Structure . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language and Terminology . . . . . . . . . . . . 5
3. Background and Problem Statement . . . . . . . . . . . . . . 6
3.1. Optional MAC only RT-2 . . . . . . . . . . . . . . . . . 7
3.2. Mobility Use Cases . . . . . . . . . . . . . . . . . . . 7
3.2.1. Host MAC+IP Move . . . . . . . . . . . . . . . . . . 7
3.2.2. Host IP Move to new MAC . . . . . . . . . . . . . . . 7
3.2.2.1. VM Reload . . . . . . . . . . . . . . . . . . . . 7
3.2.2.2. MAC Sharing . . . . . . . . . . . . . . . . . . . 8
3.2.2.3. Problem . . . . . . . . . . . . . . . . . . . . . 8
3.2.3. Host MAC move to new IP . . . . . . . . . . . . . . . 9
3.2.3.1. Problem . . . . . . . . . . . . . . . . . . . . . 9
3.3. EVPN All Active multi-homed ES . . . . . . . . . . . . . 10
4. Design Considerations . . . . . . . . . . . . . . . . . . . . 12
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5. Solution Components . . . . . . . . . . . . . . . . . . . . . 13
5.1. Sequence Number Inheritance . . . . . . . . . . . . . . . 13
5.2. MAC Sharing . . . . . . . . . . . . . . . . . . . . . . . 14
5.3. Multi-homing Mobility Synchronization . . . . . . . . . . 15
6. Requirements for Sequence Number Assignment . . . . . . . . . 15
6.1. Local MAC-IP learning . . . . . . . . . . . . . . . . . . 15
6.2. Local MAC learning . . . . . . . . . . . . . . . . . . . 16
6.3. Remote MAC or MAC-IP Update . . . . . . . . . . . . . . . 16
6.4. REMOTE (SYNC) MAC update . . . . . . . . . . . . . . . . 17
6.5. REMOTE (SYNC) MAC-IP update . . . . . . . . . . . . . . . 17
6.6. Interoperability . . . . . . . . . . . . . . . . . . . . 17
6.7. MAC Sharing Race Condition . . . . . . . . . . . . . . . 18
6.8. Mobility Convergence . . . . . . . . . . . . . . . . . . 18
6.8.1. Generalized Probing Logic . . . . . . . . . . . . . . 19
7. Routed Overlay . . . . . . . . . . . . . . . . . . . . . . . 19
8. Duplicate Host Detection . . . . . . . . . . . . . . . . . . 20
8.1. Scenario A . . . . . . . . . . . . . . . . . . . . . . . 21
8.2. Scenario B . . . . . . . . . . . . . . . . . . . . . . . 21
8.2.1. Duplicate IP Detection Procedure for Scenario B . . . 21
8.3. Scenario C . . . . . . . . . . . . . . . . . . . . . . . 22
8.4. Duplicate Host Recovery . . . . . . . . . . . . . . . . . 22
8.4.1. Route Un-freezing Configuration . . . . . . . . . . . 23
8.4.2. Route Clearing Configuration . . . . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.1. Normative References . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
EVPN-IRB enables advertising both MAC and IP routes via a single
MAC+IP RT-2 advertisement. The MAC address is imported into the
local bridge MAC table and enables L2 bridged traffic across the
network overlay. The IP address is imported into the local ARP table
in an asymmetric IRB design or imported into the IP routing table in
a symmetric IRB design, and enables routed traffic across the layer 2
network overlay. Please refer to [RFC9135] for more background on
EVPN IRB forwarding modes.
To support EVPN mobility procedure, a single sequence number mobility
attribute is advertised with the combined MAC+IP route. A single
sequence number advertised with the combined MAC+IP route to resolve
both MAC and IP reachability implicitly assumes a 1:1 fixed mapping
between IP and MAC. While a fixed 1:1 mapping between IP and MAC is
a common use case that is addressed via existing MAC mobility
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procedure defined in [RFC7432], additional IRB scenarios need to be
considered, that don't necessarily adhere to this assumption. Such
use cases are common in a virtualized host environment where hosts
attached to an EVPN network are virtual machines (VM) or
containerized workloads. Following IRB mobility scenarios are
considered:
* VM move results in VM IP and MAC moving together
* VM move results in VM IP moving to a new MAC association
* VM move results in VM MAC moving to a new IP association
While existing MAC mobility procedure can be used for MAC+IP move in
the first scenario, subsequent scenarios result in a new MAC- IP
association. As a result, a single sequence number assigned
independently per-{MAC, IP} is not sufficient to determine most
recent reachability for both MAC and IP, unless the sequence number
assignment algorithm allows for changing MAC-IP bindings across
moves.
This document updates sequence number assignment procedures defined
in [RFC7432] to adequately address mobility support across EVPN-IRB
overlay use cases that allow MAC-IP bindings to change across VM
moves and can support mobility for both MAC and IP components carried
in an EVPN RT-2 for these use cases.
In addition, for hosts on an ESI multi-homed to multiple PE devices,
additional procedures are specified to ensure synchronized sequence
number assignments across the multi-homing devices.
This document covers mobility for the following cases, independent of
the overlay encapsulation (e.g.: MPLS, NVO Tunnel):
* Symmetric EVPN IRB overlay
* Asymmetric EVPN IRB overlay
* Routed EVPN overlay
1.1. Document Structure
Following sections of the document are informative:
* section 3 provides the necessary background and problem statement
being addressed in this document.
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* section 4 lists the resulting design considerations for the
document.
Following sections of the document are normative:
* section 6 describes the mobility and sequence number assigment
procedures in an EVPN-IRB overlay required to address the
scenarios described in section 4.
* section 7 describes the mobility procedures for a routed overlay
network as opposed to an IRB overlay.
* section 8 describes corresponding duplicate detection procedures
for EVPN-IRB and routed overlays.
2. Requirements Language and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
* EVPN-IRB: A BGP-EVPN distributed control plane based integrated
routing and bridging fabric overlay discussed in [RFC9135]
* Underlay: IP or MPLS fabric core network that provides IP or MPLS
routed reachability between EVPN PEs.
* Overlay: VPN or service layer network consisting of EVPN PEs OR
VPN provider-edge (PE) switch-router devices that runs on top of
an underlay routed core.
* EVPN PE: A PE switch-router in a data-center fabric that runs
overlay BGP-EVPN control plane and connects to overlay CE host
devices. An EVPN PE may also be the first-hop layer-3 gateway for
CE/host devices. This document refers to EVPN PE as a logical
function in a data-center fabric. This EVPN PE function may be
physically hosted on a top-of-rack switching device (ToR) OR at
layer(s) above the ToR in the Clos fabric. An EVPN PE is
typically also an IP or MPLS tunnel end-point for overlay VPN flow
* Symmetric EVPN-IRB: An overlay fabric first-hop routing
architecture as defined in [RFC9135], wherein, overlay host-to-
host routed inter-subnet flows are routed at both ingress and
egress EVPN PEs.
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* Asymmetric EVPN-IRB: An overlay fabric first-hop routing
architecture as defined in [RFC9135], wherein, overlay host-to-
host routed inter-subnet flows are routed and bridged at ingress
PE and bridged at egress PEs.
* ARP: Address Resolution Protocol [RFC826]. ARP references in this
document are equally applicable to ND as well.
* ND: IPv6 Neighbor Discovery Protocol [RFC4861].
* Ethernet-Segment: physical Ethernet or LAG port that connects an
access device to an EVPN PE, as defined in [RFC7432].
* EVPN all-active multi-homing: PE-CE all-active multi-homing
achieved via a multi-homed layer-2 LAG interface on a CE with
member links to multiple PEs and related EVPN procedures on the
PEs.
* RT-2: EVPN route type 2 carrying both MAC and IP reachability.
* RT-5: EVPN route type 5 carrying IP prefix reachability.
* MAC-IP: IP association for a MAC, referred to in this document may
be IPv4, IPv6 or both.
* SYNC MAC route: In the context of EVPN multi-homing, this refers
to a local MAC route SYNCed from another PE sharing the same ESI.
* SYNC MAC-IP route: In the context of EVPN multi-homing, this
refers to a local MAC-IP route SYNCed from another PE sharing the
same ESI.
* SYNC MAC sequence number: In the context of EVPN multi-homing,
this refers to sequence number received with a SYNC MAC route.
* SYNC MAC-IP sequence number: In the context of EVPN multi-homing,
this refers to sequence number received with a SYNC MAC-IP route.
* VM: Virtual Machine or containerized workloads
3. Background and Problem Statement
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3.1. Optional MAC only RT-2
In an EVPN IRB scenario, where a single MAC+IP RT-2 advertisement
carries both IP and MAC routes, a MAC only RT-2 advertisement is
redundant for host MACs that are advertised via MAC+IP RT-2. As a
result, advertisement of a local MAC only RT-2 is an optional at an
EVPN PE. This is an important consideration for mobility scenarios
discussed in subsequent sections. Note that a local MAC and its
assigned sequence number is still maintained locally on a PE, and it
is just the advertisement of this route to other PEs that is
optional.
MAC only RT-2 may still be advertised for non-IP host MACs that are
not advertised via MAC+IP RT-2.
3.2. Mobility Use Cases
This section describes the IRB mobility use cases considered in this
document. Procedures to address them are covered later in section 6
and section 7.
* Host move results in Host IP and MAC moving together
* Host move results in Host IP moving to a new MAC association
* Host move results in Host MAC moving to a new IP association
3.2.1. Host MAC+IP Move
This is the baseline case, wherein a host move results in both host
MAC and IP moving together with no change in MAC-IP binding across a
move. Existing MAC mobility defined in [RFC7432] may be leveraged to
apply to corresponding MAC+IP route to support this mobility
scenario.
3.2.2. Host IP Move to new MAC
This is the case, where a host move results in VM IP moving to a new
MAC binding.
3.2.2.1. VM Reload
A host reload or an orchestrated host move that results in a host
being re-spawned at a new location may result in host getting a new
MAC assignment, while maintaining its existing IP address. This
results in a host IP move to a new MAC binding:
IP-a, MAC-a ---> IP-a, MAC-b
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3.2.2.2. MAC Sharing
This takes into account scenarios, where multiple hosts, each with a
unique IP, may share a common MAC binding, and a host move results in
a new MAC binding for the host IP.
As an example, hosts running on a single physical server, each with a
unique IP, may share the same physical server MAC. In yet another
scenario, an L2 access network may be behind a firewall, such that
all hosts IPs on the access network are learnt with a common firewall
MAC. In all such "shared MAC" use cases, multiple local MAC-IP ARP
entries may be learnt with the same MAC. A host IP move, in such
scenarios (for example, to a new physical server), could result in
new MAC association for the host IP.
3.2.2.3. Problem
In both of the above scenarios, a combined MAC+IP EVPN RT-2
advertised with a single sequence number attribute implicitly assumes
a fixed IP to MAC mapping. A host IP move to a new MAC breaks this
assumption and results in a new MAC+IP route. If this new MAC+IP
route is independently assigned a new sequence number, the sequence
number can no longer be used to determine most recent host IP
reachability in a symmetric EVPN-IRB design OR the most recent IP to
MAC binding in an asymmetric EVPN-IRB design.
+------------------------+
| Underlay Network Fabric|
+------------------------+
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
| PE1 | | PE2 | | PE3 | | PE4 | | PE5 | | PE6 |
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
\ / \ / \ /
\ ESI-1 / \ ESI-2 / \ ESI-3 /
\ / \ / \ /
+\---/+ +\---/+ +\---/+
| \ / | | \ / | | \ / |
+--+--+ +--+--+ +--+--+
| | |
Server-M1 Server-M2 Server-M3
| | |
VM-IP1, VM-IP2 VM-IP3, VM-IP4 VM-IP5, VM-IP6
Figure 1
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As an example, consider a topology shown in Figure 1, with host VMs
sharing the physical server MAC. In steady state, IP1-M1 route is
learnt at PE1, PE2 and advertised to remote PEs with a sequence
number N. Now, VM-IP1 is moved to MAC Server-M2. ARP or ND based
local learning at PE3, PE4 would now result in a new IP1-M2 route
being learnt. If route IP1-M2 is learnt as a new MAC+IP route and
assigned a new sequence number of say 0, mobility procedure for VM-
IP1 will not trigger across the overlay network.
A sequence number assignment procedure needs to be defined to
unambiguously determine the most recent IP reachability, IP to MAC
binding, and MAC reachability for such a MAC sharing scenario.
3.2.3. Host MAC move to new IP
This is a scenario where a host move or re-provisioning behind a new
gateway location may result in the host getting a new IP address
assigned, while keeping the same MAC.
3.2.3.1. Problem
The complication with this scenario is that MAC reachability could be
carried via a combined MAC+IP route while a MAC only route may not be
advertised at all. A single sequence number association with the
MAC+IP route again implicitly assumes a fixed mapping between MAC and
IP. A MAC move resulting in a new IP association for the host MAC
breaks this assumption and results in a new MAC+IP route. If this
new MAC+IP route independently assumes a new sequence number, this
mobility attribute can no longer be used to determine the most recent
host MAC reachability.
+------------------------+
| Underlay Network Fabric|
+------------------------+
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
| PE1 | | PE2 | | PE3 | | PE4 | | PE5 | | PE6 |
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
\ / \ / \ /
\ ESI-1 / \ ESI-2 / \ ESI-3 /
\ / \ / \ /
+\---/+ +\---/+ +\---/+
| \ / | | \ / | | \ / |
+--+--+ +--+--+ +--+--+
| | |
Server1 Server2 Server3
| | |
VM-IP1-M1, VM-IP2-M2 VM-IP3-M3, VM-IP4-M4 VM-IP5-M5, VM-IP6-M6
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Figure 2
As an example, consider a host VM IP1-M1 that is learnt locally at
PE1, PE2 and advertised to remote hosts with a sequence number N.
Consider a scenario where this VM with MAC M1 is re-provisioned at
server 2, however, as part of this re-provisioning, assigned a
different IP address say IP7. IP7-M1 is learnt as a new route at
PE3, PE4 and advertised to remote PEs with a sequence number of 0.
As a result, L3 reachability to IP7 would be established across the
overlay, however, MAC mobility procedure for M1 will not trigger as a
result of this MAC-IP route advertisement. If an optional MAC only
route is also advertised, sequence number associated with the MAC
only route would trigger MAC mobility as per [RFC7432]. However, in
the absence of an additional MAC only route advertisement, a single
sequence number advertised with a combined MAC+IP route may not be
sufficient to update MAC reachability across the overlay.
A MAC-IP sequence number assignment procedure needs to be defined to
unambiguously determine the most recent MAC reachability in such a
scenario without a MAC only route being advertised.
Further, PE1/PE2, on learning new reachability for IP7-M1 via PE3/PE4
MUST probe and delete any local IPs associated with MAC M1, such as
IP1-M1 in the above example.
Arguably, MAC mobility sequence number defined in [RFC7432], could be
interpreted to apply only to the MAC part of MAC-IP route, and would
hence cover this scenario. This interpretation could be considered a
clarification to [RFC7432] and one of the reasons for the common
sequence number assignment procedure across all MAC-IP mobility
scenarios detailed in this document.
3.3. EVPN All Active multi-homed ES
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+------------------------+
| Underlay Network Fabric|
+------------------------+
+-----+ +-----+
| PE1 | | PE2 |
+-----+ +-----+
\\ //
\\ ESI-1 //
\\ /X
+\\---//+
| \\ // |
+---+---+
|
CE1
Figure 3
Consider an EVPN-IRB overlay network shown in Figure 2, with hosts
multi-homed to two or more PE devices via an all-active multi-homed
ES. MAC and ARP entries learnt on a local ES may also be
synchronized across the multi-homing PE devices sharing this ES.
This MAC and ARP SYNC enables local switching of intra and inter
subnet ECMP traffic flows from remote hosts. In other words, local
MAC and ARP entries on a given ES may be learnt via local learning
and / or via sync from another PE device sharing the same ES.
For a host that is multi-homed to multiple PE devices via an all-
active ES interface, local learning of host MAC and MAC-IP at each PE
device is an independent asynchronous event, that is dependent on
traffic flow and or ARP / ND response from the host hashing to a
directly connected PE on the MC-LAG interface. As a result, sequence
number mobility attribute value assigned to a locally learnt MAC or
MAC-IP route at each device may not always be the same, depending on
transient states on the device at the time of local learning.
As an example, consider a host VM that is deleted from ESI-2 and
moved to ESI-1. It is possible for host to be learnt on PE1
following deletion of the remote route from PE3, PE4, while being
learnt on PE2 prior to deletion of remote route from PE3, PE4. If
so, PE1 would process local host route learning as a new route and
assign a sequence number of 0, while PE2 would process local host
route learning as a remote to local move and assign a sequence number
of N+1, N being the existing sequence number assigned at PE3, PE4.
Inconsistent sequence numbers advertised from multi-homing devices
introduces:
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* Ambiguity with respect to how the remote PEs should handle paths
with same ESI and different sequence numbers. A remote PE might
not program ECMP paths if it receives routes with different
sequence numbers from a set of multi-homing PEs sharing the same
ESI.
* Breaks consistent route versioning across the network overlay that
is needed for EVPN mobility procedures to work.
As an example, in this inconsistent state, PE2 would drop a remote
route received for the same host with sequence number N (as its local
sequence number is N+1), while PE1 would install it as the best route
(as its local sequence number is 0).
There is need for a mechanism to ensure consistency of sequence
numbers advertised from a set of multi-homing devices for EVPN
mobility to work reliably.
In order to support mobility for multi-homed hosts using the sequence
number mobility attribute, local MAC and MAC-IP routes learnt on a
multi-homed ES MUST be advertised with the same sequence number by
all PE devices that the ES is multi-homed to. There is need for a
mechanism to ensure consistency of sequence numbers assigned across
these PEs.
4. Design Considerations
To summarize, sequence number assignment scheme and implementation
must take following considerations into account:
* MAC+IP may be learnt on an ES multi-homed to multiple PE devices,
hence requires sequence numbers to be synchronized across multi-
homing PE devices.
* MAC only RT-2 is optional in an IRB scenario and may not
necessarily be advertised in addition to MAC+IP RT-2.
* A single MAC may be associated with multiple IPs, i.e., multiple
host IPs may share a common MAC.
* A host IP move could result in host moving to a new MAC, resulting
in a new IP to MAC association and a new MAC+IP route.
* A host MAC move to a new location could result in host MAC being
associated with a different IP address, resulting in a new MAC to
IP association and a new MAC+IP route.
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* Local MAC-IP learn via ARP would always accompanied by a local MAC
learn event resulting from the ARP packet. MAC and MAC-IP
learning, however, could happen in any order.
* Use cases discussed earlier that do not maintain a constant 1:1
MAC-IP mapping across moves could potentially be addressed by
using separate sequence numbers associated with MAC and IP
components of MAC+IP route. Maintaining two separate sequence
numbers however adds significant overhead with respect to
complexity, debugability, and backward compatibility. Hence, this
document addresses these requirements via a single sequence number
attribute.
5. Solution Components
This section goes over the main components of the EVPN IRB mobility
solution specified in this document. Later sections will specify
exact sequence number assignment procedures resulting from concepts
described in this section.
5.1. Sequence Number Inheritance
The main idea presented here is to view a local MAC-IP route as a
child of the corresponding local MAC route within the local context
of a PE, such that the local MAC-IP route inherits the sequence
number attribute from the parent local MAC only route:
Mx-IPx -----> Mx (seq# = N)
As a result, both parent MAC and child MAC-IP routes share one common
sequence number associated with the parent MAC route. Doing so
ensures that a single sequence number attribute carried in a combined
MAC+IP route represents sequence number for both a MAC only route as
well as a MAC+IP route, and hence makes advertisement of the MAC only
route truly optional. As a result, optional MAC only route with its
own sequence number is not required to establish the most recent
reachability for a MAC in the overlay network. Specifically, this
enables a MAC to assume a different IP address on a move, and still
be able to establish the most recent reachability to the MAC across
the overlay network via the mobility attribute associated with the
MAC+IP route advertisement. As an example, when Mx moves to a new
location, it would result in local Mx being assigned a higher
sequence number at its new location as per [RFC7432]. If this move
results in Mx assuming a different IP address, IPz, local Mx+IPz
route would inherit the new sequence number from Mx.
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Local MAC and local MAC-IP routes would typically be sourced from
data plane learning and ARP learning respectively, and could get
learnt in the control plane in any order. Implementation could
either replicate the inherited sequence number in each MAC-IP entry
OR maintain a single attribute in the parent MAC by creating a
forward reference local MAC object for cases where a local MAC-IP is
learnt before the local MAC.
5.2. MAC Sharing
Further, for the shared MAC scenario, this results in multiple local
MAC-IP siblings inheriting a sequence number attribute from the
common parent MAC route:
Mx-IP1 -----
| |
Mx-IP2 -----
. |
. +---> Mx (seq# = N)
. |
Mx-IPw -----
| |
Mx-IPx -----
Figure 4
In such a case, a host-IP move to a different physical server would
result in IP moving to a new MAC binding. A new MAC-IP route
resulting from this move must now be advertised with a sequence
number that is higher than the previous MAC-IP route for this IP,
advertised from the prior location. As an example, consider a route
Mx-IPx that is currently advertised with sequence number N from PE1.
IPx moving to a new physical server behind PE2 results in IPx being
associated with MAC Mz. A new local Mz-IPx route resulting from this
move at PE2 must now be advertised with a sequence number higher than
N and higher than the previous Mz sequence number M. This is so that
PE devices, including PE1, PE2, and other remote PE devices that are
part of the overlay can clearly determine and program the most recent
MAC binding and reachability for the IP. PE1, on receiving this new
Mz-IPx route with sequence number say, N+1, for symmetric IRB case,
would update IPx reachability via PE2 in forwarding, for asymmetric
IRB case, would update IPx's ARP binding to Mz. In addition, PE1
would clear and withdraw the stale Mx-IPx route with the lower
sequence number.
This also implies that sequence number associated with local MAC Mz
and all local MAC-IP children of Mz at PE2 must now be incremented to
N+1 or to M+1 if the previous Mz sequence number M is greater than N,
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and re-advertised across the overlay. While this re-advertisement of
all local MAC-IP children routes affected by the parent MAC route is
an overhead, it avoids the need for two separate sequence number
attributes to be maintained and advertised for IP and MAC components
of MAC+IP RT-2. Implementation would need to be able to lookup MAC-
IP routes for a given IP and update sequence number for it's parent
MAC and its MAC-IP children.
5.3. Multi-homing Mobility Synchronization
In order to support mobility for multi-homed hosts, local MAC and
MAC-IP routes learnt on a shared ES MUST be advertised with the same
sequence number by all PE devices that the ES is multi-homed to.
This also applies to local MAC only routes. local MAC and MAC-IP may
be learnt natively via data plane and ARP/ND respectively as well as
via SYNC from another multi-homing PE to achieve local switching.
Local and SYNC route learning can happen in any order. Local MAC-IP
routes advertised by all multi-homing PE devices sharing the ES must
carry the same sequence number, independent of the order in which
they are learnt. This implies:
* On local or SYNC MAC-IP route learning, sequence number for the
local MAC-IP route MUST be compared and updated to the higher
value.
* On local or SYNC MAC route learning, sequence number for the local
MAC route MUST be compared and updated to the higher value.
If an update to local MAC-IP sequence number is required as a result
of the above comparison with SYNC MAC-IP route, it would essentially
amount to a sequence number update on the parent local MAC, resulting
in inherited sequence number update on the MAC-IP route.
6. Requirements for Sequence Number Assignment
Following sections specify sequence number assignment procedure
needed on local and SYNC MAC and MAC-IP route learning events in
order to accomplish the above.
6.1. Local MAC-IP learning
A local Mx-IPx learning via ARP or ND should result in computation OR
re-computation of the parent MAC Mx's sequence number, following
which the MAC-IP route Mx-IPx would simply inherit parent MAC's
sequence number. The parent MAC Mx Sequence number MUST be computed
as follows:
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* MUST be higher than any existing remote MAC route for Mx, as per
[RFC7432].
* MUST be at least equal to corresponding SYNC MAC sequence number
if one is present.
* If the IP is also associated with a different remote MAC "Mz",
MUST be higher than the "Mz" sequence number.
Once the new sequence number for MAC route Mx is computed as per
above, all local MAC-IPs associated with MAC Mx MUST inherit the
updated sequence number.
6.2. Local MAC learning
The local MAC Mx Sequence number MUST be computed as follows:
* MUST be higher than any existing remote MAC route for Mx, as per
[RFC7432].
* MUST be at least equal to the corresponding SYNC MAC sequence
number if one is present.
* Once the new sequence number for MAC route Mx is computed as per
above, all local MAC-IPs associated with MAC Mx MUST inherit the
updated sequence number.
Note that the local MAC sequence number might already be present if
there was a local MAC-IP learnt prior to the local MAC, in which case
the above may not result in any change in local MAC's sequence
number.
6.3. Remote MAC or MAC-IP Update
On receiving a remote MAC OR MAC-IP route update associated with a
MAC Mx with a sequence number that is
* either higher than the sequence number assigned to a local route
for MAC Mx,
* or equal to the sequence number assigned to a local route for MAC
Mx, but the remote route is selected as best because of lower VTEP
IP as per [RFC7432],
following handling IS REQUIRED on the receiving PE:
* the PE MUST trigger probe and deletion procedure for all local IPs
associated with MAC Mx.
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* the PE MUST trigger deletion procedure for local MAC route for Mx.
6.4. REMOTE (SYNC) MAC update
On receiving a REMOTE SYNC, the corresponding local MAC Mx (if
present) sequence number should be re- computed as follows:
* If the current sequence number is less than the received SYNC MAC
sequence number, it MUST be increased to be equal to received SYNC
MAC sequence number.
* If a local MAC sequence number is updated as a result of the
above, all local MAC-IPs associated with MAC Mx MUST inherit the
updated sequence number.
6.5. REMOTE (SYNC) MAC-IP update
Receiving a SYNC MAC-IP for a locally attached host results in a
derived SYNC MAC Mx route entry, as MAC only RT-2 advertisement is
optional. The corresponding local MAC Mx (if present) sequence
number should be re-computed as follows:
* If the current sequence number is less than the received SYNC MAC
sequence number, it MUST be increased to be equal to received SYNC
MAC sequence number.
* If a local MAC sequence number is updated as a result of the
above, all local MAC-IPs associated with MAC Mx MUST inherit the
updated sequence number.
6.6. Interoperability
In general, if all PE nodes in the overlay network follow the above
sequence number assignment procedures, and the PE is advertising both
MAC+IP and MAC routes, sequence numbers advertised with the MAC and
MAC+IP routes with the same MAC would always be the same. However,
an inter-op scenario with a different implementation could arise,
where a PE implementation non-compliant with this document or with
[RFC7432] assigns and advertises independent sequence numbers to MAC
and MAC+IP routes. To handle this case, if different sequence
numbers are received for remote MAC+IP and corresponding remote MAC
routes from a remote PE, sequence number associated with the remote
MAC route MUST be computed as:
* Highest of all the received sequence numbers with remote MAC+IP
and MAC routes with the same MAC.
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* MAC sequence number would be re-computed on a MAC or MAC+IP route
withdraw as per above.
A MAC and / or IP move to the local PE would now result in the MAC
(and hence all MAC-IP) sequence numbers being incremented from the
above computed remote MAC sequence number.
If MAC only routes are not advertised at all, and different sequence
numbers are received with multiple MAC+IP routes for a given MAC, the
sequence number associated with the derived remote MAC route should
still be computed as the highest of all of the received MAC+IP
sequence numbers with the same MAC.
6.7. MAC Sharing Race Condition
In a MAC sharing use case described in section 5.2, a race condition
is possible with simultaneous host moves between a pair of PEs. As
an example, consider PE1 with local host IPs I1 and I2 sharing MAC
M1, and PE2 with local host IPs I3 and I4 sharing MAC M2. A
simultaneous move of I1 from PE1 to PE2 and of I3 from PE2 to PE1,
such that I3 is learnt on PE1 before I1's local entry has been probed
out on PE1 and/or I1 is learnt on PE2 before I3's local entry has
been probed out on PE2 may trigger a race condition. This race
condition together with MAC sequence number assignment rules defined
in section 6.1 can cause new mac-ip routes I1-M2 and I3-M1 to bounce
a couple of times with an incremented sequence number until stale
entries I1-M1 and I3-M2 have been probed out from PE1 and PE2
respectively. An implementation MUST ensure proper probing
procedures to remove stale ARP, ND, and local MAC entries, following
a move, on learning remote routes as defined in section 6.3 (and as
per [RFC9135]) to minimize exposure to this race condition.
6.8. Mobility Convergence
This sections is optional and details ARP and ND probing procedures
that MAY be implemented to achieve faster host re- learning and
convergence on mobility events.
* Following a host move from PE1 to PE2, the host's MAC is
discovered at PE2 as a local MAC via a data frames received from
the host. If PE2 has a prior remote MAC-IP host route for this
MAC from PE1, an ARP/ND probe MAY be triggered at PE2 to learn the
MAC-IP as a local adjacency and trigger EVPN RT-2 advertisement
for this MAC-IP across the overlay with new reachability via PE2.
This results in a reliable "event based" host IP learning
triggered by a "MAC learning event" across the overlay, and hence
faster convergence of overlay routed flows to the host.
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* Following a host move from PE1 to PE2, once PE1 receives a MAC or
MAC-IP route from PE2 with a higher sequence number, an ARP/ND
probe MAY be triggered at PE1 to clear the stale local MAC-IP
neighbor adjacency or to re-learn the local MAC-IP in case the
host has moved back or is duplicate.
* Following a local MAC age-out, if there is a local IP adjacency
with this MAC, an ARP/ND probe MAY be triggered for this IP to
either re-learn the local MAC and maintain local l3 and l2
reachability to this host or to clear the ARP/ND entry in case the
host is indeed no longer local. Note that this accomplishes
clearance of stale ARP entries, triggered by a MAC age-out event
even when the ARP refresh timer was longer than the MAC age-out
timer. Clearing of stale IP neighbor entries in-turn facilitates
traffic convergence in the event that the host was silent and not
discovered at its new location. Once the stale neighbor entry for
the host is cleared, routed traffic flow destined for the host can
re-trigger ARP/ND discovery for this host at the new location.
6.8.1. Generalized Probing Logic
The above probing logic may be generalized as probing for an IP
neighbor anytime a resolving parent MAC route is "inconsistent" with
the MAC- IP neighbor route, where being inconsistent is defined as
being not present or conflicting in terms of the route source being
local OR remote. The MAC-IP to MAC parent relationship described
earlier in this document in section 5.1 MAY be used to achieve this
logic.
7. Routed Overlay
An additional use case is possible, such that traffic to an end host
in the overlay is always IP routed. In a purely routed overlay such
as this:
* A host MAC is never advertised in the EVPN overlay control plane.
* Host /32 or /128 IP reachability is distributed across the overlay
via EVPN route type 5 (RT-5) along with a zero or non- zero ESI.
* An overlay IP subnet may still be stretched across the underlay
fabric, however, intra-subnet traffic across the stretched overlay
is never bridged.
* Both inter-subnet and intra-subnet traffic, in the overlay is IP
routed at the EVPN PE.
Please refer to [RFC7814] for more details.
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Host mobility within the stretched subnet would still need to be
supported for this use. In the absence of any host MAC routes,
sequence number mobility Extended Community specified in [RFC7432],
section 7.7 may be associated with a /32 OR /128 host IP prefix
advertised via EVPN route type 5. MAC mobility procedures defined in
[RFC7432] can now be applied as is to host IP prefixes:
* On local learning of a host IP, on a new ESI, the host IP MUST be
advertised with a sequence number attribute that is higher than
what is currently advertised with the old ESI.
* On receiving a host IP route advertisement with a higher sequence
number, a PE MUST trigger ARP/ND probe and deletion procedures on
any local route for that IP with a lower sequence number. A PE
would essentially move the forwarding entry to point to the remote
route with a higher sequence number and send an ARP/ND PROBE for
the local IP route. If the IP has indeed moved, PROBE would
timeout and the local IP host route would be deleted.
Note that there is still only one sequence number associated with a
host route at any time. For earlier use cases where a host MAC is
advertised along with the host IP, a sequence number is only
associated with a MAC. Only if the MAC is not advertised at all, as
in this use case, is a sequence number associated with a host IP.
Note that this mobility procedure would not apply to "anycast IPv6"
hosts advertised via NA messages with 0-bit=0. Please refer to
[RFC9161].
8. Duplicate Host Detection
Duplicate host detection scenarios across EVPN IRB can be classified
as follows:
* Scenario A: where two hosts have the same MAC (host IPs may or may
not be duplicate).
* Scenario B: where two hosts have the same IP but different MACs.
* Scenario C: where two hosts have the same IP and host MAC is not
advertised at all.
Duplicate detection procedures for scenario B and C would not apply
to "anycast IPv6" hosts advertised via NA messages with 0-bit=0.
Please refer to [RFC9161].
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8.1. Scenario A
For all use cases where duplicate hosts have the same MAC, the MAC is
detected as duplicate via the duplicate MAC detection procedure
described in [RFC7432]. Corresponding MAC-IP routes with the same
MAC do not require duplicate detection and MUST simply inherit the
duplicate property from the corresponding MAC route. In other words,
if a MAC route is in duplicate state, all corresponding MAC-IP routes
MUST also be treated as duplicate. Duplicate detection procedure
need only be applied to MAC routes.
8.2. Scenario B
Due to misconfiguration, a situation may arise where hosts with
different MACs are configured with the same IP. This scenario would
not be detected by [RFC7432] duplicate MAC detection procedures and
would result in incorrect forwarding of routed traffic destined to
this IP.
Such a situation, on local MAC-IP learning, would be detected as a
move scenario via the following local MAC sequence number computation
procedure described earlier in section 6.1:
* If the IP is also associated with a different remote MAC "Mz",
MUST be higher than "Mz" sequence number.
Such a move that results in sequence number increment on local MAC
because of a remote MAC-IP route associated with a different MAC MUST
be counted as an "IP move" against the "IP" independent of the MAC.
Duplicate detection procedure described in [RFC7432] can now be
applied to an "IP" entity independent of MAC. Once an IP is detected
as duplicate, corresponding MAC-IP route should be treated as
duplicate. Associated MAC routes and any other MAC-IP routes
associated with this MAC should not be affected.
8.2.1. Duplicate IP Detection Procedure for Scenario B
The duplicate IP detection procedure for such a scenario are
specified in [RFC9161]. What counts as an "IP move" in this scenario
is further clarified as follows:
* On learning a local MAC-IP route Mx-IPx, check if there is an
existing remote or local route for IPx with a different MAC
association, say, Mz-IPx. If so, count this as an "IP move" count
for IPx, independent of the MAC.
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* On learning a remote MAC-IP route Mz-IPx, check if there is an
existing local route for IPx with a different MAC association,
say, Mx-IPx. If so, count this as an "IP move" count for IPx,
independent of the MAC.
A MAC-IP route SHOULD be treated as duplicate if either of the
following two conditions are met:
* The corresponding MAC route is marked as duplicate via existing
duplicate detection procedure.
* The corresponding IP is marked as duplicate via extended procedure
described above.
8.3. Scenario C
For a purely routed overlay scenario described in section 7, where
only a host IP is advertised via EVPN RT-5, together with a sequence
number mobility attribute, duplicate MAC detection procedures
specified in [RFC7432] can be intuitively applied to IP only host
routes for the purpose of duplicate IP detection.
* On learning a local host IP route IPx, check if there is an
existing remote or local route for IPx with a different ESI
association. If so, count this as an "IP move" count for IPx.
* On learning a remote host IP route IPx, check if there is an
existing local route for IPx with a different ESI association. If
so, count this as an "IP move" count for IPx.
* With configurable parameters "N" and "M", if "N" IP moves are
detected within "M" seconds for IPx, treat IPx as duplicate.
8.4. Duplicate Host Recovery
Once a MAC or IP is marked as duplicate and frozen, corrective action
must be taken to un-provision one of the duplicate MAC or IP. Un-
provisioning a duplicate MAC or IP in this context refers to a
corrective action taken on the host side. Once one of the duplicate
MAC or IP is un-provisioned, normal operation would not resume until
the duplicate MAC or IP ages out, following this correction, unless
additional action is taken to speed up recovery.
This section lists possible additional corrective actions that could
be taken to achieve faster recovery to normal operation.
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8.4.1. Route Un-freezing Configuration
Unfreezing the duplicate or frozen MAC or IP via a CLI can be used to
recover from duplicate and frozen state following corrective un-
provisioning of the duplicate MAC or IP.
Unfreezing the frozen MAC or IP via a CLI at a PE should result in
that MAC or IP being advertised with a sequence number that is higher
than the sequence number advertised from the other location of that
MAC or IP.
Two possible corrective un-provisioning scenarios exist:
* Scenario A: A duplicate MAC or IP may have been un-provisioned at
the location where it was NOT marked as duplicate and frozen.
* Scenario B: A duplicate MAC or IP may have been un-provisioned at
the location where it was marked as duplicate and frozen.
Unfreezing the duplicate and frozen MAC or IP, following the above
corrective un-provisioning scenarios would result in recovery to
steady state as follows:
* Scenario A: If the duplicate MAC or IP was un-provisioned at the
location where it was NOT marked as duplicate, unfreezing the
route at the frozen location will result in the route being
advertised with a higher sequence number. This would in-turn
result in automatic clearing of local route at the PE location,
where the host was un-provisioned via ARP/ND PROBE and DELETE
procedure specified earlier in section 6 and in [RFC7432].
* Scenario B: If the duplicate host is un-provisioned at the
location where it was marked as duplicate, unfreezing the route
will trigger an advertisement with a higher sequence number to the
other location. This would in-turn trigger re-learning of local
route at the remote location, resulting in another advertisement
with a higher sequence number from the remote location. Route at
the local location would now be cleared on receiving this remote
route advertisement, following the ARP/ND PROBE.
Note that the probes referred to in the above scenarios are event
driven probes resulting from receiving a route with a higher sequence
number. Periodic probes resulting from refresh timers may also occur
in addition as completely independent probes.
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8.4.2. Route Clearing Configuration
In addition to the above, route clearing CLIs may also be used to
clear the local MAC or IP route, to be executed AFTER the duplicate
host is un-provisioned:
* clear MAC CLI: A clear MAC CLI can be used to clear a duplicate
MAC route, to recover from a duplicate MAC scenario.
* clear ARP/ND: A clear ARP/ND CLI may be used to clear a duplicate
IP route to recover from a duplicate IP scenario.
Note that the route unfreeze CLI may still need to be run if the
route was un-provisioned and cleared from the non-duplicate / non-
frozen location. Given that unfreezing of the route via the un-
freeze CLI would any ways result in auto-clearing of the route from
the "un- provisioned" location, as explained in the prior section,
need for a route clearing CLI for recovery from duplicate / frozen
state is truly optional.
9. Security Considerations
Security considerations discussed in [RFC7432] and [RFC9135] apply to
this document. Methods described in this document further extend the
consumption of sequence numbers for IRB deployments. They are hence
subject to same considerations if the control plane or data plane was
to be compromised. As an example, if host facing data plane is
compromised, spoofing attempts could result in a legitimate host
being perceived as moved, eventually resulting in the host being
marked as duplicate. Considerations for protecting control and data
plane described in [RFC7432] are equally applicable to such mobility
spoofing use cases.
10. IANA Considerations
None.
11. Acknowledgements
Authors would like to thank Vibov Bhan and Patrice Brissette for
feedback the process of design and implementation of procedures
defined in this document. Authors would like to thank Wen Lin for a
detailed review and valuable comments related to MAC sharing race
conditions. Authors would also like to thank Saumya Dikshit for a
detailed review and valuable comments across the document.
12. References
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12.1. 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>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007, <https://www.rfc-editor.org/rfc/rfc4861>.
[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, <https://datatracker.ietf.org/doc/html/rfc7432>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", RFC 8174, May 2017,
<https://www.rfc-editor.org/rfc/rfc8174>.
[RFC826] Plummer, D., "An Ethernet Address Resolution Protocol",
RFC 826, November 1982,
<https://www.rfc-editor.org/rfc/rfc826>.
[RFC9135] Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
Rabadan, "Integrated Routing and Bridging in EVPN",
RFC 9135, DOI 10.17487/RFC9135, October 2021,
<https://www.rfc-editor.org/rfc/rfc9135>.
[RFC9161] Rabadan, J., Sathappan, S., Nagaraj, K., Hankins, G., and
T. King, "Operational Aspects of Proxy-ARP/ND in EVPN
Networks", RFC 9161, DOI 10.17487/RFC9161, January 2022,
<https://www.rfc-editor.org/rfc/rfc9161>.
12.2. Informative References
[RFC7814] Xu, X., Jacquenet, C., Raszuk, R., Boyes, T., and B. Fee,
"Virtual Subnet: A BGP/MPLS IP VPN-Based Subnet Extension
Solution", RFC 7814, DOI 10.17487/RFC7814, March 2016,
<https://tools.ietf.org/html/rfc7814>.
Authors' Addresses
Neeraj Malhotra (editor)
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States of America
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Email: nmalhotr@cisco.com
Ali Sajassi
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States of America
Email: sajassi@cisco.com
Aparna Pattekar
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States of America
Email: apjoshi@cisco.com
Jorge Rabadan
Nokia
777 E. Middlefield Road
Mountain View, CA 94043
United States of America
Email: jorge.rabadan@nokia.com
Avinash Lingala
AT&T
3400 W Plano Pkwy
Plano, TX 75075
United States of America
Email: ar977m@att.com
John Drake
Juniper Networks
Email: jdrake@juniper.net
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