Information Technology Reference
In-Depth Information
Example 2-6 shows a MAC header for Ethernet. In this example, 00044EB31838 is the
MAC destination address, 0003E4BB2000 is the source MAC address, and 0800 is the
EtherType for IP.
Adjacency Information
Example 2-6
router#show adjacency detail
Protocol Interface Address
IP FastEthernet0/0 10.0.4.2(11)
0 packets, 0 bytes
00044EB31838
0003E4BB20000800
ARP 01:31:21
As soon as a route is resolved, it points to an adjacent next hop. If an adjacency is found in
the adjacency table, a pointer to the appropriate adjacency is cached in the FIB element. If
multiple paths (that is, multiple next hops or adjacencies) exist for the same destination, a
pointer to each adjacency is added to the load-sharing structure. With CEF, load sharing per
packet is available at the interrupt level.
Several types of exception adjacencies exist. When prefixes are added to the FIB, prefixes
that require exception handling are cached with special adjacencies. The following are
some special adjacencies:
•
Null
—For packets destined for Null 0 interfaces that are to be dropped.
•
Glean
—For destinations that are attached via a broadcast network but for which
MAC rewrite strings are unavailable. Consider the router directly connected to a sub-
net with several hosts. The FIB table on the router maintains a prefix for the subnet
instead of individual host prefixes. This subnet prefix points to a glean adjacency.
When packets need to be forwarded to a specific host, the adjacency database is
gleaned for the specific prefix. This incurs the cost of an additional lookup.
•
Punt
—Packets are forwarded for handling by the next-slower switching path if CEF
is not supported for these packets.
•
Drop
—Drops the packets because they cannot be CEF-switched or cannot be punted
to other paths.
•
Discard
—Similar to drop adjacency but applies only to Cisco 12000 routers.
Figure 2-5 ties all the CEF components together.
