5
Figure 4 Layer 2 forwarding in a site
The following forwarding process (see Figure 5) takes place for unicast flows between sites:
1. The source edge device learns the source MAC address of the incoming Ethernet frame, and
looks up the destination MAC address in its MAC table for the outgoing interface.
2. If the outgoing interface is an EVI-Link interface instead of a physical port, the source edge
device encapsulates the frame in a GRE header, and then adds an IP header and a link layer
protocol header.
In the outer IP header, the source IP address is the source edge device's tunnel source IP
address, and the destination IP address is the destination edge device's tunnel source IP
address.
3. The source edge device forwards the encapsulated packet out of the EVI link to the destination
edge device across the IP transport network.
4. The destination edge device removes the headers of the original Ethernet frame, looks up the
destination MAC address in the MAC address table, and sends the frame out of the matching
outgoing interface.
EVI
Site 1
EVI
Site 2
MAC table
VLAN MAC Interface
200 MAC1 GE1/0/1
200 MAC2 GE1/0/2
MAC1 MAC2
EVI
Transport network
GE1/0/1
GE1/0/2
Host A Host B
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