| Internet-Draft | AC-DF per EVI | May 2021 |
| Wang | Expires 8 November 2021 | [Page] |
The AC-influenced DF Election (AC-DF) per [RFC8584] is too dependent on EAD/EVI routes. For example, when no failures occured on an ESI, that AC-DF procedures will give incorrect results if no EAD/EVI routes are advertised. But in some light-weighted EVPNs (i.e. PBB EVPNs), no EAD/EVI routes will be advertised at all.¶
This draft proposes some new extensions to support AC-influenced DF Election without any EAD/EVI routes advertised in advance of any <ESI, EVI> failures.¶
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When the EAD/EVI route is not advertised before the corresponding ESI sub-interface fails, The AC-influenced DF Election procedures should elect the right DF before and after that failure.¶
Note that according to [RFC8584], the AC-influenced DF Election will be incorrect when no EAD/EVI route is advertised, even if no ESI sub-interface has failed at all.¶
This draft proposes some extension to DF-Capability negotiation and DF-Election procedures to support AC-influenced DF-election when no EAD/EVI routes are advertised.¶
Most of the terminology used in this documents comes from [RFC8584] and [RFC7623] except for the following:¶
The ethernet segment ES1's ESI is ESI1, AC1/AC2 is two sub-interfaces on ES1, and AC3/AC4 is two sub-interfaces on ES2. AC1 and AC3 are attached to EVPN Instance EVI1, while AC2 and AC4 are attached to EVI2. The redundancy mode of ES1 is all-active.¶
+----------+
PE1 | |
+-------------+ | |
| AC1 | | | PE3
/| AC2 |----| | +-------------+
/ | | | | | |
LAG / +-------------+ | | | AC3 |---CE2
CE1===== | PSN | | AC4 |
\ +-------------+ | |---| |
\ | | | | +-------------+
\| AC2 |----| |
| AC1 | | |
+-------------+ | |
PE2 | |
+----------+
EVI1 and EVI2 are two EVPN Instances, and there are no EAD/EVI routes advertised for them. Such EVPN Instances are called Light-weighted EVPNs in this draft. For example, EVI1 and EVI2 can be the I-Components of PBB EVPNs, because no EAD/EVI routes are advertised for these I-Components.¶
Initially PE1 is the DF for <ESI1, EVI1>, and PE2 is the DF for <ESI1, EVI2>. When the AC1 of PE1 fails (but ESI1 and AC2 of PE1 still works well), the DF for <ESI1, EVI1> should switch to PE2.¶
The DF-election should be done without any EAD/EVI routes advertised before any ESI sub-interface fails. Otherwise those EAD/EVI routes are advertised just for the adaptation of AC-influenced DF-election procedures per [RFC8584], but will do nothing good for the unicast packet forwarding in data plane (because data plane MAC learning don't use EAD/EVI routes).¶
We introduce a new bit named "AC-DF per EVI" in the "bitmap" field of the DF Election extended community. The "AC-DF per EVI" bit means that the DF-Election for an <ESI, EVI> will not take EAD/EVI routes into considerations untill a Reverse EAD/EVI route for that <ESI, EVI> is received from any of the PEs.¶
Only when all RT-4 routes of the same ESI indicate the "AC-DF per EVI" Capability, The DF Election will be executed in "AD-DF per EVI" mode. We can say that the DF Election mode for that ESI is negotiated as "AC-DF per EVI" mode.¶
Note that when any of the PEs on that ES is an old PE that don't support "AC-DF per EVI" mode, the RT-4 route from that PE will not indicate the "AC-DF per EVI" Capability, So the DF election will not be executed in "AD-DF per EVI" mode. This is for compatibility purpose.¶
When all RT-4 routes of the same ESI indicate both the "AC-DF per EVI" Capability and the old AC-DF Capability, The DF Election will be executed in "AD-DF per EVI" mode.¶
Note that when any of the PEs on that ES is an old PE that don't support "AC-DF per EVI" mode, the RT-4 route from that PE may indicate the "old AC-DF" Capability only, So the DF election will still be executed in old AD-DF mode per [RFC8584]. This is for compatibility purpose.¶
According to [RFC8584], the AC-influenced DF Election will be incorrect when no EAD/EVI route is advertised, even if no ESI sub-interface has failed at all.¶
But when the DF Election mode for an ESI is negotiated as AC-DF per EVI mode, no normal EAD/EVI routes can impact the DF Election procedures. The DF election will be done following that ESI's RT-4 routes only until at least one reverse EAD/EVI route is received.¶
In order to do AC-influenced DF election after a sub-interface of that ES fails, we introduce the "Reverse EAD/EVI Route". The reverse EAD/EVI Route is a special type of EAD/EVI Route that is advertised on the failure of corresponding <ESI, EVI>, not on the activation of that <ESI, EVI>.¶
Reverse EAD/EVI routes can use the same format as EAD/EVI Routes except for the following differences:¶
A Reverse EAD/EVI Route carries an EVPN Layer 2 Attributes Extended Community whose "Control Flags" field includes a new flag named "AC Failure". The "AC Failure" flag means that the corresponding AC (for which the Reverse EAD/EVI route is advertised) is in failure.¶
A Reverse EAD/EVI Route carries an ES-Import RT ([RFC7432]) and an EVI-RT ([I-D.ietf-bess-evpn-igmp-mld-proxy]), no traditional Route-Targets have to be carried.¶
A Reverse EAD/EVI Route should make its MPLS label field be set to zero.¶
When a Reverse EAD/EVI Route for an <ESI, EVI> is received from a remote PE X, the RT-4 Route of that PE x are expelled from that <ESI, EVI>'s DF election. Then the DF election for that <ESI, EVI> will be updated according to the corresponding DF Alg.¶
Note that the DF election for other <ESI, EVI>s will not be affected by that Reverse EAD/EVI Route.¶
When PE Y receives a reverse EAD/EVI route from PE X, and the ES-Import RT of that route can't match any local ES of PE Y, PE Y will not import that route into the EVI that is identified by that route's EVI-RT.¶
When RT Constraints Mechanism is enabled, each reverse EAD/EVI route will be distributed to the adjacent PEs of its ES only. Because that only the ES-Import RT are visible to the RT Constraints Mechanism, The EVI-RT is not visible to the RT Constraints Mechanism.¶
When no RT-2 Routes advertised, no EAD/EVI routes need to be advertised either. PBB EVPN is an example of that. In PBB EVPN, the C-MACs are learnt in the data plane.¶
Other light-weighted EVPNs also do data plane C-MAC learning, so they don't have to advertise EAD/EVI routes either. In such EVPNs, AC-DF per EVI will help.¶
Security considerations will be added in future versions.¶
IANA will be requested to allocate a new DF Election Capability in the "DF Election Capabilities" Registry. This capability is called "AC-DF per EVI Capability".¶
Bit Name Reference
---- ---------------- -------------
0 Unassigned
1 AC-DF Capability [RFC8584]
4 AC-DF per EVI Capability This draft
IANA will be requested to allocate a new Control Flag in the "EVPN Layer 2 Attributes Control Flags" Registry. This Control Flag is called "F" Flag, where "F" means AC-Failure.¶
Bit Name Reference ---- ---------------- ------------- P Advertising PE is the primary PE. [RFC8214] B Advertising PE is the backup PE. [RFC8214] C Control word [RFC4448] MUST be present. [RFC8214] F AC-Failure on Advertising PE This Draft
The authors would like to thank the following for their comments and review of this document:¶
Chunning Dai.¶