1. EVPN and L2 Access Protocols: Single-Flow-Active load-balancing mode
draft-brissette-bess-evpn-l2gw-proto

2. Requirements
Support L2 Access Protocols into EVPN core (incl. EVPN-IRB).
(M)STP 802.1d, G.8032, MPLS-TP, MST-AG, REP-AG, etc.
L2 gateways largely unaware of the access network topology but
Access network is running an L2 protocol breaking loops
Access network is segmented by design
Solution MUST support:
Optimal forwarding at L2 and L3
Fast-convergence
Scalability
Anycast IRB support

3. Challenges with existing load-balancing modes
PE1
PE2
Sw-A
Sw-D
Sw-C
Sw-B
Possible location
of blocked port
Access networks
MPLS / EVPN
PE3
BVI /24
BLK*
FWD
L2 BUM: isolated switches
DF Election
ECMP
Untagged AC: EVPN-VPWS
All-Active
creates L2 and L3 aliasing resulting in remote ECMP towards “wrong” PE
L2 BUM isolated switches
TCN propagation blocked by ESI split-horizon label filtering
SHG Filter

4. Challenges with existing load-balancing modes
PE3
Single-Active
Isolates network segments
TCN propagation blocked by port state or ESI split-horizon label filtering
MPLS / EVPN
Untagged AC: EVPN-VPWS
PE1
PE2
BVI /24
BVI /24
BLK
FWD
DF Election
Sw-A
Sw-B
Isolated switches
Possible location
of blocked port
Access networks
Sw-D
Sw-C

5. Untagged AC: EVPN-VPWS
Single-Flow-Active
PE3
Active / Backup
MPLS / EVPN
Same ESI between peering PEs:
Ethernet-segment procedures still apply
ESI-Label BGP extcomm extension to signal
[2:0] = all-active,
001 = single-active,
010 = single-flow-active,
other = unused
[7:3]- Reserved
Peering PE in SFA mode, MUST:
not perform ESI Split-Horizon filtering
enable MAC-Mobility within same ESI
Remote PE for remote ESI in SFA mode:
MUST disable aliasing (at Layer-2 and Layer-3)
SHOULD pre-program backup per MAC entry
Untagged AC: EVPN-VPWS
PE1
PE2
BVI /24
BVI /24
FWD
FWD
No ESI Split-horizon
Common ESI means Ethernet-segment procedures other than DF still apply: such as MAC+IP sync, Mass-Withdraw, etc.
DF / Port state dictated by Access Protocol. PEs are behaving in Gateway mode, both PEs always Forwarding
The ESI-Label BGP extcomm is extended to signal new single-flow multi-homing mode, in the flags field contiguous to A/A and S/A flags
Redundancy mode field in flags.
Remote PE for remote ESI in S-F/A mode will receive the new mode in an ES/EAD update
SHOULD pre-program backup per MAC entry, this is same as for S/A mode
Sw-A
Sw-B
Possible location
of blocked port
Access protocol
STP, ERP, ...
Sw-D
Sw-C

6. Single-Flow-Active: TCN
PE3
Active / Backup
MPLS / EVPN
Peering PEs, upon receiving TCN from access
Perform local MAC flush
MAY result in MAC RT-2 withdraws
TCN propagation via dedicated PW
EVPN-VPWS well-suited for this
Relay TCN between peering PEs and also forward back into access
Peering access PEs MAY map TCN into EVI/EAD “Flush” extcomm with new Sequence ID
Untagged AC: EVPN-VPWS
PE1
PE2
BVI /24
BVI /24
FWD
FWD
No ESI Split-horizon
Peering PEs, upon receiving TCN from Access
Perform local MAC flush (MAY result in MAC RT-2 withdraws)
TCN Propagation via dedicated PW (i.e. EVPN-VPWS) on Untagged AC
Relay TCN between peering PEs and also forward back into Access
Remote PEs are unaware of TCN:
Peering Access PEs must map TCN into EVI/EAD “Flush” extcomm with new Sequence ID
The MAC may be re-learnt and advertised by the same PE or by another, in the same ESI
MAC-Move procedures MUST be used within same ESI in this case to refresh the MAC in BGP.
Sw-A
Sw-B
Possible location
of blocked port
Access protocol
STP, ERP, ...
Sw-D
Sw-C