Information Technology Reference
In-Depth Information
Figure 2-5
CEF Components
Routing
Ta b l e
ARP/MAP
Ta b l e
Adjacency
Ta b l e
FIB Table
CEF
Distributed CEF
To increase scalability, FIB can also be distributed to line cards for Cisco 7500 and 12000
routers. In fact, Distributed CEF (dCEF) is the only switching mechanism supported in the
12000 series routers (Gigabit Switch Routers [GSRs]). The route processor (RP), RSP in
7500 or Gigabit RP (GRP) in 12000, uses the information from the IP routing table to build
the master FIB table, which is used to fully repopulate the line cards that are booted, inserted,
or cleared. When a line card is synchronized with the RP, the RP sends only incremental
updates to the individual line cards. Updates are sent only when changes in the routing
topology occur.
When dCEF is enabled, line cards maintain an identical copy of the FIB and adjacency
table. The line cards perform the express forwarding between port adapters, relieving the
RP of involvement in the switching operation. dCEF uses Inter-Process Communication
(IPC) to ensure synchronization of FIBs and adjacency tables between the route processor
and line cards. The IPC mechanism provides a reliable and orderly delivery of messages.
The delivery mechanism is a simple sliding-window protocol, with a window size of 1.
Figure 2-6 shows the components of dCEF.
dCEF is enabled by default on the Cisco 12000 routers and should not be disabled;
otherwise, packets will be dropped. The CLI keyword
distributed
is not supported on GSR
line cards, because it is the default. Depending on the implementation, forwarding in a line
card can be done by software or hardware.
NOTE
Even though IPC is a reliable communication mechanism for CEF, databases can get out of
sync between the RP and line cards during large updates. Inconsistency in CEF can cause
forwarding problems. Since Cisco IOS software Releases 12.0(15)S, 12.0(14)ST1, and
12.1(7), a CEF inconsistency checker has been implemented. This feature is on by default
but can be disabled with
no ip cef table consistency-check
. Various parameters of the
checker can be modified with
ip cef table consistency-check type
type
[
period
seconds
]
[
count
count
], where
type
is the type of consistency checker to be modified,
seconds
is how
often the FIB table is scanned, and
count
is the number of prefixes to be sent to the RP.
