H3C Technologies H3C S12500 Series Switches - Manual (Page 14)

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Figure 6 Multicast data frame forwarding process
Flooding flow
An edge device handles floodings by frame type, as follows:
• Broadcast frame—Floods the frame out of all interfaces in the VLAN where the frame has been
received, including internal interfaces and EVI-link interfaces. For ARP packets, you can use the ARP
flooding suppression feature (see " ARP flooding suppression ") to reduce ARP broadcasts.
• Destination-unknown unicast or multicast frame—Floods the frame out of all internal interfaces in
the VLAN where the frame has been received. The edge device typically does not propagate
destination unknown frames to other sites. If a site-to-site flooding is desirable for a special MAC
address, use the selective flooding feature (see "
Selective flooding ").
If a frame must be flooded to remote sites, the EVI edge device replicates the frame on each EVI link,
encapsulates each replica in a unicast frame, and sends the unicast frames to their respective destination
sites.
Placement of Layer 3 gateways
For the hosts in an extended VLAN at a site, their Layer 3 gateway must be on the edge device at the
local site rather than a remote site.
ARP flooding suppression
ARP flooding suppression reduces ARP request broadcasts on the EVI network by enabling edge devices
to reply to ARP requests on behalf of remote-site hosts. As shown in
Figure 7 , this feature snoops ARP
replies on an EVI tunnel interface to cache ARP entries for remote MAC addresses. If an ARP request has
a matching ARP entry, the local edge device replies to the request on behalf of the remote-site host. If no
matching ARP entry is found, the edge device floods the request to the EVI network.
ARP flooding suppression uses the following workflow:
1. Host IP1 in site A sends an ARP request to obtain the MAC address of IP2.
(1) Multicast stream
(2) Replicate & encapsulate (3) Unicast (4) Decapsulate & replicate
EVI EVI
EVI
(5) Multicast stream
DR
Source
Receiver
Receiver
EVI
(3) Unicast
Eth1/1
EVI-Link1
EVI-Link2
Eth1/1
EVI-Link1
Eth1/1
(4) Decapsulate &
replicate
(5) Multicast stream
EVI-Link1
VLAN R-port H-port
100 Eth1/1 EVI-Link1
EVI-Link2
VLAN R-port H-port
100 EVI-Link1 Eth1/1
VLAN R-port H-port
100 EVI-Link1 Eth1/1
Site 1
Site 2
Site 3