Unresolved Adjacency; Central Cef Mode Operation; Distributed Cef Mode Operation - Cisco Catalyst 3850 Configuration Manual

Unresolved Adjacency

Unresolved Adjacency
When a link-layer header is prepended to a packet, the FIB requires the prepended header to point to an
adjacency corresponding to the next hop. If an adjacency was created by the FIB and not discovered
through a mechanism such as ARP, the Layer 2 addressing information is not known and the adjacency is
considered incomplete or unresolved. Once the Layer 2 information is known, the packet is forwarded to
the RP, and the adjacency is determined through ARP. Thus, the adjacency is resolved.

Central CEF Mode Operation

You can use central Cisco Express Forwarding mode when line cards are not available for Cisco Express
Forwarding switching, when you need to use features not compatible with distributed Cisco Express
Forwarding switching, or when you are running on a platform that is not a distributed platform. When
central Cisco Express Forwarding mode is enabled, the Cisco Express Forwarding FIB and adjacency
tables reside on the RP, and the RP performs the express forwarding.
The figure below shows the relationship between the routing table, the FIB, and the adjacency table during
central Cisco Express Forwarding mode operation. Traffic is forwarded from workgroup LANs to a device
on the enterprise backbone that is running central Cisco Express Forwarding. The RP performs the express
forwarding.
Figure 2
Cisco device
running CEF

Distributed CEF Mode Operation

For additional scalability, Cisco Express Forwarding runs in the distributed Cisco Express Forwarding form
on certain devices by spreading processing tasks across two or more line cards. When distributed Cisco
Express Forwarding mode is enabled, line cards maintain identical copies of the FIB and adjacency tables.
IP Switching Cisco Express Forwarding Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850
Switches)
6
Central Cisco Express Forwarding Mode Operation
Routing
table
Interface
card
E1 E2
Workgroup LAN
Route Processor
Adjacency
FIB table
table
Interface
card
E1 E2 E1
Workgroup LAN Workgroup LAN
Central CEF Mode Operation
Interface
card
E2