In the previous blog post, we looked at a few fundamental OSPF concepts, including neighbor and adjacency formation. As we continue through the basics of OSPF, this post will examine router roles, timers, and metric calculation.
A designated router (DR) is the router interface that wins an election among all routers on a multiaccess network segment such as Ethernet. A backup designated router (BDR) is the router that becomes the designated router if the current designated router has a failure on the network. The BDR is the OSPF router with the second highest priority at the time of the last election. OSPF uses the DR and BDR concept to assist with efficiencies in the operations of OSPF.
Keep in mind that a given OSPF speaker in your network can have some interfaces that are designated and others that are backup designated, and others that are non-designated. If no router is a DR or a BDR on a given...
The OpenShortest Path First (OSPF) dynamic routing protocol is one of the most beloved inventions in all of networking, widely adopted as the Interior Gateway Protocol (IGP) of choice for many networks. In this blog series, you'll be introduced first to the basic concepts of OSPF and learn about its various message types and neighbor formation.
Where does the interesting name come from when it comes to OSPF? It is from the fact that it uses Dijkstra's algorithm, also known as the shortest path first (SPF) algorithm. OSPF was developed so that the shortest path through a network was calculated based on the cost of the route. This cost value is derived from bandwidth values in the path. Therefore, OSPF undertakes route cost calculation on the basis of link-cost parameters, which you can control by manipulating the cost calculation formula.
As a link state routing protocol, OSPF maintains a link state database. This is a form of a network...
This post is the 6th and final in a series of Border Gateway Protocol (BGP) posts. If you missed any of the first five parts, here are the links:
In this post, we're going to take a look at how we can work with BGP in IPv6.
You will recall from this chapter that BGP was constructed to support many different protocols and NLRI right out from its creation. As a result, we have robust support for such technologies as IPV6, MPLS VPNs, and more.
You will also relish in the fact that once you master the basics of BGP that we have covered in this , working with BGP in IPv6 is much more similar than it is different!
BGP is so remarkably flexible, as discussed earlier in this chapter, you can use IPv4 as the “carrier” protocol for IPv6...
This post is the 5th in a series of Border Gateway Protocol (BGP) posts. If you missed any of the first four, here are the links:
In this post, we're going to take a look at BGP scalability mechanisms and related concepts.
Just as IP address depletion has been a concern with the Internet, so has the depletion of available autonomous system numbers. To help solve this, the engineers turned to a familiar solution. They marked an AS number range as private-use only. This permits you to experiment with AS construction and policy in a lab (for example) and use AS numbers that are guaranteed not to conflict with Internet-based systems.
Remember, the AS number is a 16-bit number permitting up to 65,536 AS numbers. The private space is marked as 64512-65535.
This post is the 4th in a series of Border Gateway Protocol (BGP) posts. If you missed any of the first three, here are the links:
In this post, we're going to take a look at configuring BGP to advertise Network Layer Reachability Information (NLRI), and also the configuration of a BGP routing policy.
Before we even begin advertising NLRI using our various commands in this section, let’s take a moment to discuss an old feature of BGP that Cisco disables by default for you. The feature is called BGP synchronization. For proof that Cisco has disabled this feature on your device, just perform a show running-configuration on one of your lab BGP speakers and under the BGP process you will find the command no synchronization. If enabled, the synchronization feature prevents a BGP speaker from entering prefixes into BGP...
This post is the 3rd in a series of Border Gateway Protocol (BGP) posts. If you missed either of the first two, here are the links:
Now, in this post, you'll learn about how BGP neighborships are formed, within an autonomous system, between autonomous systems, and even between routers that are not directly connected. Also, we'll check out BGP authentication.
Given that BGP is an AS-to-AS routing protocol, it would make good sense that external BGP (i.e. eBGP) is a key ingredient in its operations. The very first thing that we need to keep in mind with eBGP is that the standards are built so that there is a requirement for a direct connection. This is something that we can work around (of course), but this point is worth consideration. Because a direct connection is assumed, the BGP protocol does two things:
Part 1 of our blog series on Border Gateway Protocol (BGP) gave you an overview of BGP and then delved into BGP message types and neighbor states. Now, in this post, you'll learn about one of the most challenging aspects of BGP, how it makes its path selection decision. While routing protocols such as RIP, OSPF, and EIGRP each have their own metrics used to pick the "best" path to a destination network, BGP uses a collection of path attributes (PAs).
When your BGP speaker receives a BGP prefix, there are going to be many path attributes tagged to it, and we know that these are going to be critical when it comes to BGP doing things like choosing a very best path to a destination. Interestingly, not all of these path attributes are created equal.
All BGP path attributes fall into one of four main categories. Note that this list also provides example attributes in each category. Do not be too concerned with these specific attribute values now, as you will...
One of the most intimidating topics for Cisco certification candidates in the Route/Switch track is Border Gateway Protocol (BGP). To help remove the FUD (Fear, Uncertainty, and Doubt) surrounding BGP, I'll be sharing a series of blog posts with you to help demystify this routing protocol. In this first post of the series, you'll be introduced to the very basics of BGP and learn about its various message types and states.
Let’s face it - Border Gateway Protocol is just incredibly unique, especially when we compare it to other routing protocols. The very first thing that makes BGP so unique, is what it does for us. It is our only Exterior Gateway Protocol (EGP) in major use today. We know we have our Interior Gateway Protocols (IGPs), and that would be like OSPF running inside of an autonomous system. But BGP is an EGP, which means that it is (usually) going to take prefixes that are inside an autonomous system and send those to other autonomous systems....
This post is the fourth in a series of posts on route redistribution. If you haven't yet read the first three, here are the links:
Up until now in this series, we’ve seen the need for route redistribution, looked at a basic configuration, saw how we could filter specific routes from being redistributed, and learned how to prevent a routing loop by tagging redistributed routes. In this final route redistribution post, we want to check out route redistribution with IPv6, and how that configuration varies a bit from what we’ve done previously with IPv4 networks.
First, consider a router running a routing protocol; let’s say it’s OSPF in this instance. Also, let’s say that router has several interfaces that are participating in the OSPF routing protocol. On that same router, imagine we’re running...
This post is the third in a series of posts on Route Redistribution. If you didn’t yet read the first two, here are the links:
So far in this series, the route redistribution examples we’ve worked through used a single router to do all of the redistribution between our autonomous systems. However, from a design perspective, we might look at that one router and realize that it's potential single point of failure.
For redundancy, let’s think about adding a second router to redistribute between a couple of autonomous systems. What we probably don’t want is for a route to be advertised from, let’s say, AS1 into AS2, and then have AS2 advertise that same route back into AS1, as shown in the figure.
The good news is, with default settings, that probably won’t be an issue. For example, in the above graphic, router BB2 would learn two ways to get to Network A. One way would...