Cisco: Simple configuration of IP multicast dense mode

Until now, I did not meet a network engineer to be really “in love” with multicast. Some are considering challenging, interesting, useful but most of the network engineers (I repeat myself, from the persons that I discussed with) are rather preferring do other topics that multicast. If you are curious about my position in regarding multicast, well I’m somewhere in the middle. I consider it challenging, a topic for the future (like IPv6), I MUST know it, but let’s say that is not my strongest topic in networking.

I think that I can share some of my experience here with you, and maybe discuss a little bit on th multicast subject. And since I do not want to enter more complex multicast configuration from the start I said I should take it slow with the easiest one: multicast dense mode configuration. As you know there is also sparse mode configuration which can be configured in a more complex way, and I will do it in next tutorials. For now I only hope that you know what is multicast and words like PIM, IGMP, dense-mode, sparse-mode does not sound like alien descriptions for you. If you are feeling insecure about this topic, please check this link as you might get very good information about IP multicast.

For this tutorial I have a simple point to point router connection (R1 and R2) with a subnet of 10.0.12.0 /24 in between, and each router has an additional host (R1 has the multicast sender connected and R2 has the multicast reveiver connected). Since I have no possibility to test the IP multicast traffic in this configuration, the explanation above is just to have a picture about the environment. I hope that for the multicast sparse mode I will find a way to generate some multicast traffic, when I will write the tutorial somewhere next weeks.

Please see the tutorial by clicking the image below:

Multicast dense mode

Cisco: BGP path selection for inbound traffic

In some previous post we saw how we can manipulate BGP paths using attributes for outgoing traffic. Today we will see how to use another BGP attribute, but this time for manipulating inbound traffic. First I would like to ask you to have a look into the topology file and also to check the config files (if you have a basic idea about how BGP is configured that you don’t need the config files).

From the table above:
we see that the attributes needed for inbound traffic manipulation are AS-Path and MED. Today we will use AS-Path for traffic manipulation (MED sometime in closer future).

Beside using BGP attributes, there are other ways to manipulate traffic and paths in a BGP environment, but usually this need that the provider will support your actions. As an example to understand, you can do route tagging in your network (for example in MPLS on your CE) and your ISP will apply rules based on different tags (on PE side), but this is out of the scope of this tutorial and will be discussed maybe in another tutorial.

For our test environment, that you see in the topology, we advertise everything into BGP domain, exactly as draw, so there will be no problems of reachability.

Please see the tutorial and explanations below:

Cisco: Traffic manipulation with Policy Based Routing

Cisco Policy Based Routing provides a flexible mechanism for network administrators to customize the operation of the routing table and the flow of traffic within their networks. Policy Based Routing (PBR) offers many advanced features, including selection and forwarding of traffic to discreet Virtual Routing and Forwarding (VRF) instances, as well as Enhanced Tracking of the availability of next-hops. More about PBR you can read here.

Before we start this tutorial please download the topology file which contain the routers connection, BGP topology and some explanations. As you can see in the topology file, traffic going from R4 to R5 will choose the shortest path through R1-R2. For this tutorial let’s assume that we want the traffic from R4 to R5 (and viceversa) to flow through path R1-R3-R2 (BGP is full mesh configured among these devices). For this we need to apply Policy Based Routing (PBR) on R1 and R2. What is PBR basically doing? It will change the next-hop for interesting traffic to R3 on this devices.

Please see below the tutorial about how to configure PBR:

Cisco: Very simple NTP configuration

NTP (Network Time Protocol) is usually very simple to configure on Cisco devices. Of course you can reach complex configuration, but since I work in this field I didn’t saw somebody to push the things to extreme in NTP configuration.

NTP is based on server – client relation. It is recommended that in a Cisco network environment, you should use online the client part of the NTP, and to choose some external NTP sever to synchronize with.  This is because using a NTP server (master) on your networ  violates NTP’s hierarchical trust model. You should use a NTP master only is there is no possibility to reach an external one or if some corporate policies dictate this.

NTP can operate in 4 different modes. See a short explanation below:
-> Client:  A NTP client is configured to let its clock be set and synchronized by an external NTP timeserver. NTP clients can be configured to use multiple servers to set their local time and are able to give preference to the most accurate time sources. They will not, however, provide synchronization services to any other devices.
-> Server: A NTP server is configured to synchronize NTP clients. Servers can be configured to synchronize any client or only specific clients. NTP servers, however, will accept no synchronization information from their clients and therefore will not let clients update or affect the server’s time settings.
-> Peer: With NTP peers, one NTP-enabled device does not have authority over the other. With the peering model, each device shares its time information with the other, and each device can also provide time synchronization to the other.
-> Broadcast / Multicast: Broadcast/multicast mode is a special server mode with which the NTP server broadcasts its synchronization information to all clients. Broadcast mode requires that clients be on the same subnet as the server, and multicast mode requires that clients and servers have multicast access available and configured.

For our simple NTP configuration we will use the server and  client mode. For this tutorial we will use the same topology like in the post “Cisco: BGP path selection for outgoing traffic” where we have already a working BGP environment. If you do not have the topology, you can download it here. Please see the tutorial below: