Blog posts

Multi-Area OSPF

Submitted by rayc on Tue, 11/30/2021 - 08:45

An OSPF area is a logical grouping of routers into a specific OSPF area (Area 0, the backbone area by default). Using a single area OSPF design has it's benefits in that it can simplify your network, however there are some drawbacks to using a single area when your OSPF network gets too large. 

OSPF Part 2

Submitted by rayc on Tue, 11/23/2021 - 19:44

In this part of the OSPF article, I will talk about OSPF Area types and also the specific LSA messages and where/how they're used. 

OSPF uses the concept of Areas to logically group routers together inside an AS. There are 5 different area types on Cisco OSPF routers.

  1. Backbone Area (This is always Area 0)
  2. Non-Backbone Area (Any other are other than 0)
  3. Stub Area 
  4. Totally Stubby Area
  5. Not-So-Stubby-Area (NSSA)


OSPF Part 1

Submitted by rayc on Tue, 11/23/2021 - 19:44

OSPF (Open Shortest Path First) is the IETFs replacement IGP for RIP and is a link state routing protocol. Link state routing protocols advertise all of the information about all links to all link state routers inside the AS. OSPF is a link state protocol thst uses Dijkstra's SPF algorithm to find the shortest path to a network. 

EIGRP Part 2

Submitted by rayc on Mon, 11/22/2021 - 21:24

Now that I've discussed the ins and outs of how EIGRP works, let's talk about the configuration of EIGRP.

Cisco routers support two methods of configuring EIGRP. There is the traditional method that supports IPv4 only, and the named EIGRP method that supports both IPv4 and IPv6 route using address families. Both the traditional and Named EIGRP configurations are backwards compatible with each other so you can configure each router in your topology in either method without any issues. 


Submitted by rayc on Wed, 11/17/2021 - 08:41

RIP is probably one of the oldest routing protocols and if i'm honest, it should stand for Rest In Peace instead of Routing Information Protocol. In saying that, at one stage it was a great little lightweight protocol. RIP is a distance vector routing protocol that uses hop count as a metric. Each router in the path to a network is considered a hop. 

EIGRP Part 1

Submitted by rayc on Tue, 11/16/2021 - 08:09

EIGRP is considered a hybrid distance vector protocol. While EIGRP does keep track of hop count as a metric, it doesn't so much use it as a metric like RIP does. Instead EIGRP uses path attributes to determine the best route. Don't worry too much about these path attributes as I will go through them later.

Let's start with how EIGRP sends routing information. EIGRP uses the multicast address to send its messages using the EIGRP protocol number 88. There are 5 types of EIGRP messages that a router will send.

Routing Protocols

Submitted by rayc on Mon, 11/15/2021 - 14:37

This article brings us to the Layer 3 part of the Network world and routing in general. In order for a packet to be sent from its source to its destination, our network devices, either a Router or a MLS, need to know how to get there. And to do that, Routers need to know which next-hop device the packet should be forwarded to and in which direction (Interface) that next-hop device is. There are several ways for routers to learn this information.

Layer 3 etherchannels

Submitted by rayc on Sat, 11/13/2021 - 06:44

In my last few articles, I've talked about etherchannels and what they are and how they work as far as being a layer 2 etherchannels goes. But what if we want to load balance across layer 3 interfaces and route traffic? Well, Cisco routers and switches (MLS switches at least) support both Layer 2 and Layer 3 etherchannels. 


Submitted by rayc on Mon, 10/25/2021 - 10:51

PAgP or Port Aggregation Protocol, is a Cisco proprietary link aggregation protocol. This protocol works in a similar way to LACP in that it uses PAgP messages to negotiate and establish the etherchannel bundle. Like LACP, PAgP has two modes for configuration:


Submitted by rayc on Mon, 10/25/2021 - 10:51

In my previous article, I talked about the basic concept of etherchannels and how they work to provide additional redundancy and bandwidth in our networks. I also talked about how to configure an always on etherchanel. In this article, I will talk about how to dynamically establish an etherchannel using LACP (Link Aggregation Control Protocol)