Month: February 2015

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?

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.

Some time ago I was working on IPv6 implementation and in that period I wrote an article about NDP (you can read it here). After a while, I received some comments that it is not written very well so I reviewed a huge part of it. It looks my English was far worst two years ago that I was really aware of 🙂

In the reviewing process, I realised that NDP usage of Solicited-Node multicast addresses was not clearly explained. This is the follow-up article which should explain how and why Solicited-Node multicast address are used in NDP. After all, this kind of multicast addresses are there to enable IPv6 neighbour discovery function of NDP to work properly.

Let’s go!

Solicited-node multicast address is the IPv6 multicast address used on the local L2 subnet by NDP Network Discovery Protocol. NDP uses that multicast address to be able to find out L2 link-local addresses of other nodes present on that subnet.

NDP replaces ARP

As we know, NDP in IPv6 networks replaced the ARP function from IPv4 networks. In IPv4 world, ARP used broadcast to send this kind of discovery messages and find out about neighbours IPv4 addresses on the subnet. With IPv6 and NDP use of broadcast is not really a good solution so we use a special type of multicast group addresses to which all nodes join to enable NDP communication.