Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 13 Configuring Cisco Express Forwarding
Information About Cisco Express Forwarding
Cisco Express Forwarding Adjacency Tables Overview
A node is said to be adjacent to another node if the node can be reached with a single hop across a link
layer (Layer 2). Cisco Express Forwarding stores forwarding information (outbound interface and MAC
header rewrite) for adjacent nodes in a data structure called the adjacency table. Cisco Express
Forwarding uses adjacency tables to prepend Layer 2 addressing information to packets. The adjacency
tables maintain Layer 2 next-hop addresses for all FIB entries.
The following sections provide additional information about adjacencies:
• Adjacency Discovery, page 13-4
• Adjacency Types That Require Special Handling, page 13-4
• Unresolved Adjacency, page 13-5
Adjacency Discovery
Each adjacency table is populated as adjacencies are discovered. Adjacencies are added to the table
either through indirect manual configuration or dynamically—discovered through a mechanism like
Address Resolution Protocol (ARP) or added through the use of a routing protocol, such as Border
Gateway Protocol (BGP) or Open Shortest Path First (OSPF), which forms neighbor relationships. Each
time an adjacency entry is created, a link-layer header for that adjacent node is computed and stored in
the adjacency table.
The adjacency information is subsequently used for encapsulation during Cisco Express Forwarding
switching of packets.
Adjacency Types That Require Special Handling
In addition to adjacencies associated with next hop interfaces (host-route adjacencies), other types of
adjacencies are used to expedite switching when certain exception conditions exist. Prefixes requiring
exception processing or special handling are cached with one of the special adjacencies listed in
Table 13-1.
Table 13-1 Adjacency Types That Require Special Handling
Packets of This Adjacency Type Receive This Processing
Null adjacency Packets destined for a Null0 interface are dropped. Null adjacency
can be used as an effective form of access filtering.
Glean adjacency When a router is connected to a multiaccess medium, the FIB table
on the router maintains a prefix for the subnet rather than for the
individual host prefixes. The subnet prefix points to a glean
adjacency. A glean adjacency entry indicates that a particular next
hop should be directly connected, but there is no MAC header
rewrite information available
. When the router needs to forward
packets to a specific host on a subnet, Cisco Express Forwarding
requests an ARP entry for the specific prefix, ARP sends the MAC
address, and the adjacency entry for the host is built.
Punt adjacency The router forwards packets that require special handling or packets
sent by features that are not yet supported in conjunction with
Cisco Express Forwarding switching paths to the next higher
switching level for handling.