Tag: routing

What is route recursion

We are going back to networking basics with this post. In few lines below you will find most important theory that makes network gear do its job.

The main router job is to making routing decisions to be able to route packets toward their destination. Sometimes that includes recursive lookup of routing table if the next-hop value is not available via connected interface.

Routing decision on end devices

Lets have a look at routing decision that happens if we presume that we have a PC connected on our Ethernet network.

If one device wants to send a packet to another device, it first needs to find an answer to these questions:

  • Is maybe the destination IP address chunk of local subnet IP range?
    • If that is true, packet will be forwarded to the neighbour device using Layer 2 in the ARP example below.
    • If that is not the case, does the device network card configuration include a router address through which that destination can be reached? (default gateway)
  • Device then looks at his local ARP table. Does it include a MAC address associated with the destination IP address?
    • If the destination is not part of the local subnet, does the local ARP table contain the MAC address of the nearest router? (MAC address to IP address mapping of default gateway router)

TCAM and CAM memory usage inside networking devices

As this is networking blog I will focus mostly on the usage of CAM and TCAM memory in routers and switches. I will explain TCAM role in router prefix lookup process and switch mac address table lookup.

However, when we talk about this specific topic, most of you will ask: how is this memory made from architectural aspect?

How is it made in order to have the capability of making lookups faster than any other hardware or software solution? That is the reason for the second part of the article where I will try to explain in short how are the most usual TCAM memory build to have the capabilities they have.

CAM and TCAM memory

When using TCAM – Ternary Content Addressable Memory inside routers it’s used for faster address lookup that enables fast routing.

In switches CAM – Content Addressable Memory is used for building and lookup of mac address table that enables L2 forwarding decisions. By implementing router prefix lookup in TCAM, we are moving process of Forwarding Information Base lookup from software to hardware.

When we implement TCAM we enable the address search process not to depend on the number of prefix entries because TCAM main characteristic is that it is able to search all its entries in parallel. It means that no matter how many address prefixes are stored in TCAM, router will find the longest prefix match in one iteration. It’s magic, right?

CEF Lookup

Image 1 shows how FIB lookup functions and points to an entry in the adjacency table. Search process goes through all entries in TCAM table in one iteration.


Router

In routers, like High-End Cisco ones, TCAM is used to enable CEF – Cisco Express Forwarding in hardware. CEF is building FIB table from RIB table (Routing table) and Adjacency table from ARP table for building pre-prepared L2 headers for every next-hop neighbour.

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How can router decide so fast?

Cisco created all sorts of different magic inside their boxes that optimize forwarding processing of packets.

CEFMain router function is fairly self-explanatory. Router performs IP forwarding more often called IP routing. IP routing is process of deciding where to send the packet after it was received.

 

 

 

IP Routing explained in detail

Logic behind IP forwarding is listed in steps here with the assumption it will be an IPv4 packet that was received. This is process switching explained in 11 steps:

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INE v5 Full-Scale Practice Lab1 TS GNS3 topology

Few days ago I added an article with Config GNS3 topology for newly published INE Routing and Switching Workbook v5 Full-Scale LAB1. Here’s now the topology with starting config of TS section for LAB1.

I will not insert here any of my stories today as the same article was published before but with other topology files so if you would like more info, just go to previous post INE R&Sv5 Workbook Full-Scale Practice Lab1 made in GNS3

LAB1 TS WBv5

DOWNLOAD

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INE R&Sv5 Workbook Full-Scale Practice Lab1 made in GNS3

UPDATE on 27 Dec 2016:
This post was updated in order to include Tom’s reply in the comments mentioning opening errors with GNS3 for MAC
UPDATE on 21 Jul 2015:
This post was updated on 21th of July 2015 with GNS3 version 1.3.7 INE  Full-Scale Practice Lab1 download. Just scroll to the bottom for download link..

 

Yesterday INE finally added a Full-Scale LAB in their new CCIE Route and Switching blueprint 5 workbook.
I realized this morning that you maybe don’t want to spend half of your day (like me) configuring this topology in GNS3. Better to just take it from here and start your lab right away.

In my study process for the last year I made almost all my labs from INE on GNS3. In that way I was able to run the labs for more days in a row and not think about the money I would spent on rack rentals. Of course, you will still need some rack rentals particularly for troubleshooting sessions. For troubleshooting you need preconfigured rack because if you configure those topologies by yourself there is a big chance that you will see ticket answers and that will break the point of troubleshooting study process.

All my config sessions were done on GNS3 and this one in the next few days will be also done in GNS3. If you want to spare some time and get the topology ready, up and running in few minutes you can download it at the end of this article.

INEv5GNS3

Read more and download files!

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