Tag: routing

(Part II) Virtual Routing and Forwarding

This is the second part in the series of posts dedicated to network virtualization and path isolation.

Ever needed one extra router? It’s possible to split the router into more logical routers by using VRF. How? Here’s how!

Virtual Routing and Forwarding or VRF allows a router to run more that one routing table simultaneously. When running more routing tables in the same time, they are completely independent. For example, you could use overlapping IP addresses inside more VRFs on the same router and they will function independently without conflict (You can see this kind of overlap in the example below). It is possible to use same VRF instance on more routers and connect every instance separately using VRF dedicated router port or only a sub-interface.

You can find VRFs to be used on ISP side. Provider Edge (PE) routers are usually running one VRF per customer VPN so that one router can act as a PE router for multiple Customer Edge (CE) routers even with more customers exchanging the same subnets across the VPN. By running VRF per customer, those subnets will never mix in-between them.

VRFs are used to create multiple virtual routers from one physical router.

Every VRF is creating his own Routing table and CEF table, basically a separate RIB and FIB.

How it works?

If you have two routers / two Layer3 switches connected with two L3 links (two paths) you can route with two equal static routes towards the same prefix and the router will load balance traffic across both links.

The idea is to make two same static routes on the same router but with different next-hops. The question was: Which link or which route will be used? And if the traffic will be load balanced, which mechanism will be used to share the traffic across both of links.

 

ip route 10.0.0.0 255.0.0.0 192.168.10.2
ip route 10.0.0.0 255.0.0.0 192.168.11.2

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.

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

Main 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: