Cisco Express Forwarding (CEF) is Cisco IOS's default packet forwarding method. CEF builds the forwarding table in advance based on the routing table, rather than building it on demand like fast switching. CEF is needed in MPLS networks because labeled packets are switched based on the label and IP packets are switched using the CEF table. CEF has two main components - the Forwarding Information Base (FIB) which makes forwarding decisions, and the adjacency table which provides Layer 2 rewrite information. Distributed CEF allows packet forwarding load to be distributed across distributed CPUs in routers for high performance.

1. CEF BASED SWITCHING
A Seminar
By
Israel Sylvester Umana
Chennai, India

2. What is CEF based switching?
• Cisco Express Forwarding (CEF) is a packet
forwarding or switching method that Cisco IOS
uses.
• It is the latest IP switching mmeetthhoodd ddeevveellooppeedd
in Cisco IOS, and it is the default packet
forwarding method being used now.
• CEF is needed in MPLS networks
2

3. Overview of Cisco IOS Switching
Methods
Packets can be forwarded through the router in three
basic ways:
• Process switching: a special Cisco IOS process that is
scheduled to run when the router receives packets.
• Interrupt Switching: whenpackets arrive ttoo tthhee rroouutteerr,,
the interface processor interrupts the central CPU and
asks it to switch the packet according to a route cache
or switching table.
• ASIC Switching: The router can program the switching
table into an ASIC so that the packets can be `switched
in hardware.
3

4. Fast Switching
• Builds an on-demand forwarding table
• The first packet for a destination that arrives is
process switched
• CPU builds a temporary cache ffrroomm tthhee ffiirrsstt
packet
• The cache is used to switch subsequent
packets for the same destination
• Cache is deleted when packets are no longer
switched to the same destination
4

5. CEF Switching
• Switching table is no longer built on demand,
but it is built in advance
• Each prefix in the routing table has an entry in
the CEF switching table aatt tthhee ssaammee ttiimmee..
• Only when the routing table changes does the
CEF switching table change
5

6. Why Is CEF Needed in MPLS
Networks?
Concerning MPLS, CEF is special for a certain
reason:
• Labeled packets that enter the router are
switched according to the LLFFIIBB oonn tthhee rroouutteerr..
• IP packets that enter the router are switched
according to the CEF table on the router.
• Thus, the outgoing packet can be a labeled
packet or an IP packet.
6

7. 7

8. What Are the Components of CEF?
CEF has two main data structures:
• the Forwarding Information Base (FIB)
• and the adjacency table.
8

9. 9

10. The Adjacency Table
• The adjacency table is the CEF component
that is responsible for the MAC or Layer 2
rewrite.
• Routers use a dynamic mechanism ttoo ddiissccoovveerr
each other.
10

11. The CEF Table
• The CEF table or FIB is the CEF component
that is responsible for the Layer 3 forwarding
decision that is made.
• Looks similar to the IP routing ttaabbllee oonn tthhee
router.
• Holds the essential information
• This information is the IP prefix, the
recursively evaluated next hop, and the
outgoing interface.
11

12. Operation of CEF
• When a packet enters the router, the router
strips off the Layer 2 information.
• makes a forwarding decision based on the
destination address iinn tthhee CCEEFF ttaabbllee
• The result points to one adjacency entry in the
adjacency table.
• The information retrieved from the adjacency
table is the Layer 2 rewrite string
12

13. Distributed CEF (DCEF)
• Some Cisco routers use a central CPU without any
form of decentralized or distributed intelligence
• Other hardware have distributed intelligence and
CPUs
• Therefore, the router can distribute the burden of
forwarding traffic through CCEEFF bbyy uussiinngg tthhee
distributed CPUs
• To achieve the distributed forwarding, both the
CEF and adjacency table have to be distributed to
these distributed CPUs
• To enable distributed CEF on a router, configure
the command ip cef distributed
13

14. 14

15. CEF Switching Packets in Hardware
• To achieve high rate packet forwarding, the
router can use ASICs on the boards or line
cards.
• ASICs are specially built chips tthhaatt ccaann ffoorrwwaarrdd
packets at the highest rate
15

16. Load Balancing in CEF
• CEF allows for load balancing or load sharing of
traffic among multiple outgoing links.
• Needs multiple outgoing links as next hops in the
routing table to perform load balancing.
• The command maximum-ppaatthhss ssppeecciiffiieess hhooww
many paths or next hops are allowed
• two main load balancing schemes are per-packet
or per-destination
• The per-packet load balancing is configured with
the interface command ip load-sharing per-packet
16

17. • The default CEF load balancing scheme is per-destination.
• CEF per-destination load balancing is done by
hashing the destination and ssoouurrccee IIPP aaddddrreessss..
• Per-destination load sharing is the default
load sharing scheme for CEF.
17

18. • Cisco IOS can load-balance in CEF by hashing
the source and destination IP address and
pointing the result of that hash to a load
sshhaarriinngg ttaabbllee..
• This table holds 16 hash buckets
• Each of the 16 hash buckets points to one
adjacency, and multiple buckets can point to
the same adjacency.
18

19. Source:
• Luc De Ghein - MPLS Fundamentals, Cisco
Press
19