Leaf switches peer with external routers, and spine switches distribute the learned external routes to leaf switches in the same tenant.
Summary
Router peering and route distribution involve the following components:
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Leaf switches establish routing protocol peering sessions with external routers and maintain forwarding tables containing the learned routes.
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External routers establish routing protocol peering sessions and advertise WAN routes.
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Spine switches act as route reflectors and distribute external routes throughout the fabric.
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Tenant endpoints send packets to the default gateway in the Cisco ACI fabric.
Workflow
Router peering and route distribution occur as follows:
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The leaf switch interface is configured to establish a routing protocol peering session with the external router.
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The leaf switch sends the routes learned through the peering session to the spine switches.
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The spine switches act as route reflectors and distribute the external routes to all leaf switches that have interfaces belonging to the same tenant.
The leaf switches install the routes as longest prefix match (LPM) entries in their forwarding tables. Each entry identifies the virtual tunnel endpoint (VTEP) IP address of the remote leaf switch to which the external router is connected.
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WAN routes do not use a forwarding proxy. If the WAN routes exceed the capacity of a leaf switch forwarding table, the leaf switch drops the traffic.
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Because the external router is not the default gateway, tenant endpoints send their packets to the default gateway in the Cisco ACI fabric.