Category: Routing

For a long time I was confused about this. It was not clear to me what is the difference between setting the static route using next hop interface IP address instead of exit interface (outgoing interface) syntax. It seemed that both methods are the same and that you have basically two different ways to define static route on specific device for no particular reason.

It was not clear to me why would someone do this kind of option on networking device OS if there were no reason for doing so. In other words I was strongly convinced that there must be some difference between two configs and learning more detail about the process of routers recursive searches and Proxy ARP function did answer all my doubts.

So now is time to put it all on paper for you to see it:

You can define static route like this:

R1(config)#ip route 10.0.0.0 255.255.255.0 10.10.2.1

This means that all packets from R1 with destination address from 10.0.0.0/24 subnet will be forwarded out the interface leading to next hop device with 10.10.2.1 address on its interface.

Other way is to define the same static route like this:

R1(config)#ip route 10.0.0.0 255.255.255.0 fastEthernet 0/0

If fastEthernet 0/0 is the interface on R1 router that leads to next hop router with best path to 10.0.0.0/24

What’s the difference, which is better?

If you use next-hop address, you can conclude that your router will not have the information which interface must he use in order to route those packets out towards destination. R1 must then find an interface that is having 10.10.2.1 on other side.

I can say at the beginning that more and more fellows are visiting this blog. Finally networking knowledge project did succeed to get some fans from out there so I felt the need to thank you all for visiting this place. As you can see here from the blog post list I didn’t follow this movement myself these days. I actually didn’t come here for some time now. My last few weeks were very intense. One of the reasons was my learning sessions that took me at least few hour a day and the most interesting reason was my new assignment at work. I got an assignment to carry out and even more interesting to secure IPv6 in our company for a whole network segment. At least one segment for now, but it was a nice thing to get me into IPv6 deep dive learning. And I make the last step of the learning like this:

What is better than document the networking job that you did until is fresh in your head. So I write it down for the future implementations but rather than put it into my notepad I do it here on my blog, so next time is easier to search for it and something even better than that, you can use it too.

RA – router advertisement message format from RFC 4861

This time I will write about some of the most important things in IPv6 networking that you needed to know before you go on and carry out IPv6 implementation. RA router advertisements and some strange special bits in the RA packet header that are configuration option flags. Good thing is that there are only a few of them, four to be precise. The bad thing is that at first, I needed a couple of days to stop mixing them up.

Static floating route is static route like any other but with added administrative distance in the configuration

R1(config)#ip route 172.16.10.0 255.255.255.0 10.10.10.2 200

Static Floating Routes are very interesting so I decided to give you a short description and an example.

A static floating route is the same as normal static route except that this kind of static route has administrative distance configured to some value higher than 1.

Remember that if we configure normal static route like this:

R1(config)#ip route 172.16.10.0 255.255.255.0 10.10.10.2

It will send all packets destined for 172.16.10.0/24 network to the neighbour with interface address 10.10.10.2

Of course, this static route will have administrative distance (AD) of 1 by default.

If we make the configuration like this:

R1(config)#ip route 172.16.10.0 255.255.255.0 Serial 0/0

In this case, the AD will be zero (0). Pretty cool right? There’s another article about that here.

In either case, this is normal because this is the way we configure routing on our router or if we use a dynamic routing protocol, this is then the way of overriding routing protocol decisions for some exceptions.

But what if we want to use a static route to make something completely opposite?

If we want to use static route only to be a backup route. Then we will need to give the precedence to the path learned by some IGP (Interior Gateway Protocol) like OSPF for example. We know that most paths (routes) learned by OSPF protocol have Administrative Distance of 110. In that case, the Administrative distance of a static route needs to be bigger than 110 if we don’t want to kick out the OSPF route from routing table.

About Policy-Based Routing

Policy-Based Routing – PBR gives you very simple way of controlling where packets will be forwarded before they enter in the destination-based routing process of the router.

It’s a technology that gives you more control over network traffic flow because you will not always want to send certain packets by the obvious shortest path. That is the job of routing protocol. If you want to send some traffic to the destination using some other path, you will need to use a method that will catch the packet as soon as they enter into router and decides where to send packets before they enter destination-based routing process. That’s Policy-Based routing all about.

There is so much about MPLS and how MPLS works. Here I wrote some simple introductory lines about it but only from one perspective. The costumer side one. There is nothing here about BGP and all the things that need to be done and configured in order for MPLS to function in ISP cloud. As an introductory in MPLS this text will take you to the central office and branch side of the MPLS configuration and in this way it will be simpler to explain and enter in the world of MPLS networking technology.

MPLS

In MPLS networks, packets are sending with special MPLS prefix before IP packet data. With an MPLS header that is sometimes mentioned as a label stack. In MPLS header there are labels, every label with some value:

• Traffic-class field, important for quality of service – QoS
• Bottom-of-stack flag
• 8-bit time-to-live – TTL field