This article is intended for those new to CEF and its impact on the way Multilayer Switching is done in Cisco hardware. Of course, this article can also serve as a review for those familiar with Cisco Express Forwarding but are looking for a refresher. In this first article we are going to go over the components that make up this switching architecture followed by some fundamental examples to illustrate these components and concepts at work. Before we get started be sure to download the topology we are going to be using in the lab examples for clarity.

Modern Catalyst Multilayer switches utilize CEF-based Multilayer Switching. The terminology and architecture of this switching model can be tough to understand at first, but trust me, it really isn’t that difficult to grasp after you start working with it.

There are a couple different functions provided by a CEF-based MLS. The first function is building routing information. This routing information is built by the Layer 3 engine within the control plane and includes both static and dynamic routes. This information can be seen in the routing table. The second function provided by CEF is hardware switching of packets. Hardware switching of packets is done in the Layer 3 Forwarding Engine within the data plane. The data plane is where CEF works its magic. The control plane is where layer 3 decisions are made, when those layer 3 packets can NOT be switched in hardware.

Since Cisco Explress Forwarding’s magic is provided within the data plane, we will begin with it. It is the most fun anyway. The Layer 3 Forwarding Engine within the data plane has two components of its own.

CEF Forwarding Information Base is the first component, and the second is the Cisco Express Forwarding Adjacency table. The CEF Forwarding Information Base is basically just a reformatted routing table ordered such that the most specific routes are found first. The Forwarding Information Base contains next hop information for each prefix. The routing and next-hop information is built in software in the control plane, and then passed to the Layer 3 forwarding engine and placed in the FIB. It is really important to understand that this is basically a reordered routing table with some additional entries in it. When a packet enters the switch, the switch consults the FIB and finds the longest match prefix and obtains the next hop address. I know this doesn’t sound like magic yet, but stay with me, there is more and this stuff pretty neat.

The second component, the adjacency table, contains and maintains layer 2 addresses for every entry in the FIB. This table is built the same way the FIB is built. It is built from the ARP table that is built with the Layer 3 engine in the control plane and then passed to the Layer 3 Forwarding Engine and placed in the Cisco Express Forwarding Adjacency table. If you know how packets are encapsulated and rewritten as they make their way across a layer 3 network, you are probably beginning to develop an idea of what is going to happen with the adjacency table.

Since the FIB and Adjacency tables are both handled in hardware, we’re starting to see how CEF can improve the performance of layer 3 forwarding operations. It receives a copy of the work the Layer 3 Engine does in software, and the Layer 3 Forwarding Engine uses it to make multilayer switching decisions in hardware. Between the Forwarding Information Base having next hop layer 3 information, and the adjacency table having both the layer 3 and layer 2 information, Cisco Express Forwarding has at its disposal everything it needs to forward packets without consulting a routing table running in software. It is all in hardware and it all happens at line speed. I sure love it when tidbits of information come together.

Now, let’s take a look at two scenarios to see the paths packets take through a CEF-enabled MLS. In scenario 1, we have a valid Forwarding Information Base entry and associated adjacency table entry. A packet comes in the ingress interface, the Forwarding Information Base is consulted and an entry is found. The Forwarding Information Base is matched on the longest prefix. The layer 2 information is retrieved from the adjacency table and the packet is then forwarded through the packet rewrite engine, which rewrites the appropriate packet and frame header information at line speed and sends the packet out the egress interface. Notice that no ARP requests are made, no software based processing is performed, and frame information is written in hardware.

In scenario two, as a packet comes ingress on an interface, the Forwarding Information Base is consulted and is unable to be CEF switched because of one of several different reasons. At this point the packet is sent to the Layer 3 engine for further processing, which is known as a Cisco Express Forwarding punt. We aren’t going to cover all the scenarios in which a CEF Punt occurs here. We’ll save those more in depth scenarios for Part 2.

It should be obvious, but it is worth mentioning here for clarity. As changes happen in the routing and ARP tables that are maintained by the Layer 3 Engine, those changes are automatically sent to the Layer 3 Forwarding Engine. This updates the Cisco Express Forwarding FIB and the Adjacency tables instantaneously.

Since we have all of the fundamentals out of the way, let’s begin looking at the relationships between the routing table, arp table, the Cisco Express Forwarding FIB table, and the Cisco Express Forwarding adjacency table. We’ll begin by having a look at the IP addresses of the connected interfaces of the two devices used in these demonstrations.

MPLS1#show ip interface brief

Interface                  IP-Address      OK? Method Status                Protocol

FastEthernet0/0            unassigned      YES NVRAM  administratively down down    

FastEthernet1/0172.16.13.1YESNVRAMupup

FastEthernet1/1            172.16.12.1     YES NVRAM  up                    up      

FastEthernet2/0            172.16.15.1     YES NVRAM  up                    up      

FastEthernet2/1            unassigned      YES NVRAM  administratively down down    

FastEthernet3/0            unassigned      YES NVRAM  administratively down down    

FastEthernet3/1            unassigned      YES NVRAM  administratively down down    

Loopback0                  10.0.0.1        YES NVRAM  up                    up      

Tunnel710.0.0.1YESTFTPupdown

Tunnel702                  10.0.0.1        YES TFTP   up                    down    

Tunnel70310.0.0.1YESTFTPupdown

MPLS2#show ip interface brief

Interface                  IP-Address      OK? Method Status                Protocol

FastEthernet0/0unassignedYESNVRAMadministrativelydowndown

FastEthernet1/0172.16.12.2YESNVRAMupup

FastEthernet1/1            172.16.23.2     YES NVRAM  up                    up      

FastEthernet2/0172.16.24.2YESNVRAMupup

FastEthernet2/1            172.16.25.2     YES NVRAM  up                    up      

FastEthernet3/0unassignedYESNVRAMadministrativelydowndown

FastEthernet3/1            unassigned      YES NVRAM  administratively down down    

Loopback010.0.0.2YESNVRAMupup

Now it is time to look at the routing table on MPLS1:

MPLS1#show ip route

Codes:C-connected,S-static,R-RIP,M-mobile,B-BGP

D-EIGRP,EX-EIGRPexternal,O-OSPF,IA-OSPFinterarea

       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

       E1 - OSPF external type 1, E2 - OSPF external type 2

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

o-ODR,P-periodicdownloadedstaticroute

Gatewayoflastresortisnotset

     172.16.0.0/28 is subnetted, 6 subnets

O172.16.24.0[110/2]via172.16.12.2,01:12:32,FastEthernet1/1

O172.16.25.0[110/2]via172.16.12.2,01:12:32,FastEthernet1/1

O       172.16.23.0 [110/2] via 172.16.12.2, 01:12:32, FastEthernet1/1

C172.16.12.0isdirectlyconnected,FastEthernet1/1

C       172.16.13.0 is directly connected, FastEthernet1/0

C       172.16.15.0 is directly connected, FastEthernet2/0

     10.0.0.0/32 is subnetted, 2 subnets

O       10.0.0.2 [110/2] via 172.16.12.2, 01:12:32, FastEthernet1/1

C10.0.0.1isdirectlyconnected,Loopback0

On to the FIB FIB on MPLS1. Be aware of the similarities and in particular the next hop addresses.

MPLS1#show ip cef

Prefix              Next Hop             Interface

0.0.0.0/0dropNull0(defaultroutehandlerentry)

0.0.0.0/8           drop

0.0.0.0/32          receive

10.0.0.1/32receive

10.0.0.2/32172.16.12.2FastEthernet1/1

127.0.0.0/8         drop

172.16.12.0/28      attached             FastEthernet1/1

172.16.12.0/32      receive

172.16.12.1/32      receive

172.16.12.2/32172.16.12.2FastEthernet1/1

172.16.12.15/32     receive

172.16.13.0/28      attached             FastEthernet1/0

172.16.13.0/32      receive

172.16.13.1/32      receive

172.16.13.15/32receive

172.16.15.0/28attachedFastEthernet2/0

172.16.15.0/32      receive

172.16.15.1/32receive

172.16.15.15/32receive

172.16.23.0/28      172.16.12.2          FastEthernet1/1

172.16.24.0/28172.16.12.2FastEthernet1/1

172.16.25.0/28      172.16.12.2          FastEthernet1/1

224.0.0.0/4         drop

224.0.0.0/24        receive

240.0.0.0/4         drop

255.255.255.255/32  receive

Next, is the ARP table on MPLS1..followed by the Cisco Express Forwarding Adjacency table.

MPLS1#show ip arp

Protocol  Address          Age (min)  Hardware Addr   Type   Interface

Internet  172.16.13.1             -   ca00.0bd0.001c  ARPA   FastEthernet1/0

Internet  172.16.12.1             -   ca00.0bd0.001d  ARPA   FastEthernet1/1

Internet  172.16.12.2            73   ca01.0bd0.001c  ARPA   FastEthernet1/1

Internet  172.16.15.1             -   ca00.0bd0.0038  ARPA   FastEthernet2/0

MPLS1#show adjacency detail

ProtocolInterfaceAddress

TAG      FastEthernet1/1           172.16.12.2(7)

0packets,0bytes

                                   CA010BD0001C

                                   CA000BD0001D8847

                                   TFIB       02:48:53  

Epoch:0

IP       FastEthernet1/1           172.16.12.2(17)

0packets,0bytes

                                   CA010BD0001C

CA000BD0001D0800

ARP02:48:53

Epoch:0

The correlations between the above tables should all be apparent. Notice the last four digits on the line under the bolded MAC addresses. These are ethertype codes. 8847 is MPLS-IP. 0800 is Ethernet.

That about brings Cisco Express Forwarding Part I to a conclusion. That should provide you with a foundational knowledge of what CEF does and how it works. There are quite a few more details to be covered in later articles. Right now I just to get this introduction out there because we will be needing it for MPLS Part III.