CEF (Cisco Express Forwarding)

Just below the diagram showing the processing of an IP packet arriving at a multilayer switch and how it is processed is the sentence that states “The multilayer switch will check the routing table, notices that 192.168.20 /24 is directly connected…”. You will notice that the IP address in that sentence has only 3 octets!
Brad

Hello Brad

Thanks for the heads up, I will let Rene know to have that fixed.

Thanks again!

Laz

Just below the diagram showing the processing of an IP packet arriving at a multilayer switch and how it is processed is the sentence that states “The multilayer switch will check the routing table, notices that 192.168.20 /24 is directly connected…”. You will notice that the IP address in that sentence has only 3 octets!
Brad

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Thanks Brad, just fixed this typo.

Rene

Hi Laz,

Why do we need two hardware tables? Why not just construct a single hardware table containing IP dest, Prefix, Next Hop IP and MAC? Surely this would be more efficient as aren’t two table lookups required using the FIB and Adjacency Table?

Is the need for two tables because the two hardware tables are stored in separate types of memory in the ASIC (CAM & TCAM)? :thinking:

Thanks,

Gareth.

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Hey Gareth

This is an interesting question Gareth, and I think you will agree with me that the answer is also very interesting. First of all, let’s take a look at the similarities and differences between CAM and TCAM.

What makes both of these memory types fast is the fact that all entries can be searched for in parallel. So if you have hundreds of entries, you can search for a particular value in all the entries in just a single clock cycle. The difference between the two has to do with the search algorithm that each uses, an algorithm that is hardwired into the circuitry of the memory itself.

CAM requires an exact match to return a result. This means that the value that is being searched for (in binary) must contain either 0s or 1s.

TCAM does not require an exact match, and is queried using 0s, 1s, and Xs, where X essentially means “anything” (that is, either 0 or 1). It’s kind of like a wildcard state. The name ternary CAM comes from the fact that you use three states rather than two.

Now applying this to networking devices, when looking up an address in the MAC table, you always require an exact match, so CAM is used. When looking up a prefix in a routing table, you don’t need an exact match, as long as the destination is contained within the prefix in the routing table, and that is where TCAM is used.

I hope this has been helpful!

Laz

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Thanks Laz - by searching in parallel do yo mean searching in the CAM and TCAM at the same time?

This explanation helps a lot as to why two different tables are needed.

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Hello Gareth

Actually, what I mean is that all entries of the CAM or TCAM table can be searched simultaneously. If you have a table with a list of 100 entries stored in regular memory, in order to search for a particular entry, say a specific MAC address, you will have to search sequentially in each row of the table. This would take 100 CPU cycles, assuming one cycle per entry.

CAM and TCAM memory allow all of the entries to be searched in parallel, simultaneously. That means that only one CPU cycle is needed to do the same lookup.

I hope this has been helpful!

Laz

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Wow - the mind boggles at the speed in which hardware works at in our industry. Thanks Laz.

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I come across a Cisco documentation. It’s about troubleshoot CEF routing loop.
https://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/26083-trouble-cef.html
Since the FIB is based on RIB, how come in this article, the show ip route 10.10.34.0 result is different than the show ip cef 10.10.34.0 result?
Capture
In the article, it also says a packet forward decision consists of:
A routing table lookup for the longest prefix match.
A forwarding information base (FIB) lookup.
Since different prefix length will be considered as different destination network, which will all be written into FIB, should it be enough to just look at the FIB without RIB if there’s a routing issue?

Thanks,

Hello Helen

Remember that the FIB is created not only based on the RIB, but also on the ARP table. Remember this image?
image

In the example shown in the Cisco documentation, it is demonstrated that a specific sequence of events, with particular configuration parameters can result in an FIB that is incorrect and inconsistent with the RIB, for at least as long as the ARP entry exists in R2 for that particular host. In this particular case, if you look at the sequence of events at the end of the document that lead to this situation you can clearly see why there is a discrepancy in the RIB and FIB.

This is a little bit confusing and misleading. It is not saying that every time you forward a packet you check the routing table and then the FIB. That would be inefficient and would defeat the purpose of CEF. If the specific destination is not in the FIB, then we go to the routing table for routing, and the FIB is then populated appropriately.

We actually want to bypass the routing table to speed things up. So the example here is showing how the FIB can have conflicting information under particular circumstances. It is necessary to see both the RIB and the FIB during the troubleshooting procedures in order to pinpoint where the problem is. This is why we checked both the RIB and the FIB, saw the discrepancy, and the proceeded to see why the RIB was right and the FIB was wrong.

I hope this has been helpful!

Laz

3 Likes

Hi Laz,

Thank you for the explanation.
So as long as the ARP entry still exists in R2, it won’t update its fib table entry for this particular host even though the routing entry for this particular host has been already updated in its RIB, is this correct?

Hello Helen

Yes that is correct. But this is only the case because in this particular example, we have a floating static default route that uses only the egress interface without a next hop IP like so:

R2(config)#ip route 0.0.0.0 0.0.0.0 ethernet 0/0 10

Because there is no next hop IP in the route, an ARP request is sent out of that interface for the destination IP of the packet. Because proxy ARP is enabled on all routers, it receives the MAC address of R1 and sends the packet there. And because the FIB table uses the ARP table to populate it, it keeps the entry as long as the ARP entry exists…

In such a situation (and as a general best practice) it’s always a good idea to include the next hop IP in the floating static route.

I hope this has been helpful!

Laz

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Thank you very much. It helps a lot!

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Hi Laz ,

I am unable to understand cef and it’s purpose to use .
Kindly explain Tcam table ,process and fast switching in detail.

Its quite tough to understand this topic .

Rgds
Shivam Chaudhary

Hi
For recursive routing the next hop ip address is reachable or directly connected if we look up the touting table on R1 to reach Loopback of R3
Its look up 3 time in RIB ,which will create recursive behaviour .

Rgds
Shivam

Hello Shivam

When a router accepts a packet, it looks at the destination IP address to decide out of which interface it should be sent. This process of routing involves the destination address being loaded into memory, where the programming of the software will compare it with the routing table, which is also in memory, will process it using CPU power, and then decide out of which interface send it. All of this required the use of memory, CPU power, and software involvement. All of these components are in the Control Plane.

Higher end switches also contain dedicated hardware that contains various tables of information that will allow all of these routing processes to take place much faster. CEF takes advantage of this hardware so that routing can be performed at much higher speeds, allowing a device to provide even better performance. This specialized hardware is considered to be part of the Data Plane, and this is where CEF takes place.

We need CEF in order to be able to achieve the high throughput speeds that today’s devices can reach. How this works is described clearly within the lesson.
Take a look at this post to learn more about CAM and TCAM

Yes that is correct.

I hope this has been helpful!

Laz

3 Likes

Hi Laz ,

A) Would you please explain this term FIB table may or may not be held in TCAM Table . I can not decided the major difference between both Tcam and Fib table as both reside in data plan to forward ip packet

As go through with lesson you mentioned that Tcam table contain routing table , ACL and Qos detail and they do not choose the exact match of the ip packet to reach destination .

B)How to decide which one table is reside in which model of L2 , L3 switch and Router ?

C)Kindly confirm is data and control plane exit in Normal L2 switch

sir , why you mentioned about the CAM table while routing the ip packet as i know it only work for Layer 2 forwarding on the basis of destination Mac address when the traffic is transient but in routing the ip packet , we use interface of router as gateway . So how it fills it cam table during routing the ip packet

I am not relate the use of cam table as my understanding it is use for layer 2 forwarding when we talked about only transient traffic ,no use of interface mac address in L2Switch .
Please confirm how Layer 3 and router use Cam table as they use it interface mac address as a gateway How u relate cam during ip packet forwarding

Regards
Shivam Chaudhary

Hi Team ,
How L3 switch work when we have forward ethernet frame in terms of control plane and data plane and confirm is router use Cam table to forward ip packet .

Regards
ShivamChaudhary

Hello Shivam

I believe that your confusing comes from the use of the terms CAM and TCAM. CAM and TCAM are types of memory found in the hardware of the device:

  • CAM - content-addressable memory
  • TCAM - ternary content-addressable memory

They are not tables, but hardware memory that is used to store various pieces of information. This may be confusing because we often refer to the MAC address table as the CAM table, which is not correct.

So to clarify, the MAC address table, FIB, access lists, QoS information, and the routing table, are all data that are used to allow the operation of network devices.

CAM and TCAM are types of high-speed hardware memory that store these data.

Now as far as control and data plane go, where various types of data are stored depends upon the platform being used. High-end devices will have TCAM and CAM available in the data plane to include the FIB, and access-list information so that can be retrieved very quickly, and reduce data transmission delays that are introduced by going to conventional memory and software. Some lower-end devices won’t have that hardware available.

I hope this has been helpful!

Laz

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