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Role of Fabric Edge Nodes in Cisco SD-Access

Role of Fabric Edge Nodes in Cisco SD-Access

Let's talk about the Fabric Edge nodes in the Cisco SD-Access environment. You may relate this with your traditional environment in order to understand clearly.

An access layer switch in a traditional branch design is an edge node in a fabric-based design. The fabric edge nodes implement a Layer 3 access design plus fabric functions.

Fig 1.1- Fabric Edge Nodes in SDA Environment

Endpoint registration:
A fabric edge device will add a newly detected endpoint to a local host tracking database called the EID-table. The edge device will also issue a LISP map-register message to inform the control plane node of the endpoint detected so that the HTDB can be populated. 

Mapping of user to virtual network:
Endpoints get placed into virtual networks by assigning the endpoint to a VLAN, which is mapped to a LISP instance. The mapping of endpoints into VLANs can done statically or dynamically using 802.1X. An SGT can also be assigned to provide segmentation and policy enforcement at the fabric edge. 

Anycast Layer 3 gateway:
A common gateway (IP and MAC addresses) can be used at every node that shares a common EID subnet in order to provide for optimal forwarding and mobility across different RLOCs. 

LISP forwarding:
Instead of a typical routing-based decision, the fabric edge nodes query the map server in order to determine the RLOC associated with the destination IP and use that information to encapsulate the traffic in VXLAN. In case of a failure to resolve the destination RLOC, the traffic is sent to the fabric border in which the global routing table is used for forwarding. 

The response received from the map server is stored in the LISP map-cache, which is merged to the CEF table and installed in hardware. If VXLAN-encapsulated traffic is received at the fabric edge for an endpoint not locally connected, a LISP solicit map request is sent to the sending fabric edge in order to trigger a new map request; this addresses the case where the endpoint may be present on a different fabric edge switch.


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