Cisco Nexus 9000 Series Configuration Manual page 134

EVPN Multihoming BUM Flows
Designated Forwarder
It is important that only one of the switches in the redundancy group decaps and forwards BUM traffic over
the ESI links. For this purpose, a unique Designated Forwarder (DF) is elected on a per Ethernet Segment
basis. The role of the DF is to decap and forward BUM traffic originating from the remote segments to the
destination local segment for which the device is the DF. The main aspects of DF election are:
• DF Election is per (ES, VLAN) basis. There can be a different DF for ES1 and ES2 for a given VLAN.
• DF election result only applies to BUM traffic on the RX side for decap.
• Every switch must decap BUM traffic to forward it to singly homed or orphan links.
• Duplication of DF role leads to duplicate packets or loops in a DHN. Therefore, there must be a unique
DF on per (ES, VLAN) basis.
Split Horizon and Local Bias
Consider BUM traffic originating from H2. Consider that this traffic is hashed at L1. L1 encapsulates this
traffic in Overlay Multicast Group and sends the packet out to the core. All switches that have joined this
multicast group with same L2VNI receive this packet. Additionally, L1 also locally replicates the BUM packet
on all directly connected orphan and ESI ports. For example, if the BUM packet originated from ES1, L1
locally replicates it to ES2 and the orphan ports. This technique to replicate to all the locally attached links is
termed as local-bias.
Remote switches decap and forward it to their ESI and orphan links based on the DF state. However, this
packet is also received at L2 that belongs to the same redundancy group as the originating switch L1. L2 must
decap the packet to send it to orphan ports. However, even through L2 is the DF for ES1, L2 must not forward
this packet to ES1 link. This packet was received from a peer that shares ES1 with L1 as L1 would have done
local-bias and duplicate copies should not be received on ES2. Therefore L2 (DF) applies a split-horizon filter
for L1-IP on ES1 and ES2 that it shares with L1. This filter is applied in the context of a VLAN.
Figure 27: BUM traffic originating at L1. L2 is the DF for ES1 and ES2. However, L2 must perform split horizon check here
as it shares ES1 and ES2 with L1. L2 however
Ethernet Segment Route (Type 4)
Cisco Nexus 9000 Series NX-OS VXLAN Configuration Guide, Release 7.x
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Configuring VXLAN EVPN Multihoming