Design Case Study 1 – Problem Overview

I thought it would be useful to document my design process using a case study. It is better (for me anyway) to force myself to write down my thought process so I more thoroughly understand the decisions involved. I believe the process of documenting, although sometimes tedious, is educational and gives me a much deeper understanding of the problem and solutions. With that said, let’s begin Design Case Study 1 – Problem Overview.

The customer wants to provide network services to remote users using satellite communications equipment. You do not have to concern yourself with the RF (radio frequency) portion of the network, but should have a basic understanding of how layer 3 and layer 2 services map to the RF network. The customer is a satellite network provider (worldwide) that provides the RF network management and connectivity to the customer network via landline (T1 or greater speeds).

Your objective is to document the current state of the network and propose changes that will scale the solution and provide the following services: IP Data and Voice. The network should be manageable, scalable, and secure.

The provider has purchased and installed equipment, but has little expertise in configuring and managing the network gear. As the network designer you should consider this when planning for operational expenses (OPEX).

Let’s begin with a high-level diagram. The basic design consists of N remote sites and a single Headquarters location. The remotes are backhauled using low-speed T1 (or lower) links.

What information do you need in order to make a proper assessment of the high-level design? I would probably start with the following questions:

  1. What business requirements are imposed on any network design? Does the business need a 24x7x365 operational network? Or is the availability constrained to an 8×5 model?
  2. What are the recovery requirements, and does it vary by site? How quickly must service be restored in the event of a failure, and how difficult is it to get parts to the site?
  3. Does the business expect the bandwidth requirements to grow? If so, will the growth be predictable in any sort of fashion? For example: if one site covers the Los Angeles area, and another site covers a less densely populated area, should we expect the LA site to support more growth?
  4. How deterministic must the network performance be? Will this network be required to support low-latency traffic like voice and video?
  5. What are the security requirements for the network? Must we simply protect portions like management access to devices or are we required to provide protection for data as well?

I think this covers the basics, I would drill down into specifics as I received answers from the customer.

What questions would you ask? Do you see anything I missed?


Multicast Notes

The amount of multicast on the outline is a bit sparse, so to speak. Sorry, I just couldn’t resist. Anyway, with this in mind, there are just a few things to note with respect to multicast design.

  1. Don’t use Dense mode. Only use Sparse.
  2. When using Anycast-RP, or any design where you’re using multiple loopbacks, it’s a best practice to hardcode the router id in your IGP.
  3. If you can use Anycast-RP and AutoRP together, do it.
  4. A new alternative to the RP of last resort is no ip pim dm-fallback

Sparse mode requires a Rendezvous Point (RP) to hold the shared tree information. There are several ways of assigning RPs. You can use AutoRP, BSR, or Anycast-RP.

AutoRP requires candidate RPs and mapping agents, which can be set up with the commands:

ip pim send-rp-announce scope #
ip pim send-rp-discovery scope #
ip pim autorp listener (required since you’ll be using sparse mode)
no ip pim dm-fallback (so it won’t fall back to dense mode)

Also note that the interface or IP address you use must be reachable via IGP.

Some outputs:

R101 is the RP and Mapping Agent:

R101#sho ip pim rp map
PIM Group-to-RP Mappings
This system is an RP (Auto-RP)
This system is an RP-mapping agent (Loopback0)

RP (?), v2v1
Info source: (?), elected via Auto-RP
Uptime: 01:04:06, expires: 00:02:13

BSR is another RP mechanism – BSR stands for BootStrap Router. You can configure it like so:

ip pim bsr-candidate
ip pim rp-candidate
no ip pim dm-fallback

Anycast RP is an RP mechanism where you configure the same RP address on multiple routers. You then need to ensure all routers in your topology have this IP address configured statically as their RP.

On the RP’s:
ip pim rp-address
interface loopback10
ip address
interface loopback0
ip address
ip msdp peer connect-source loopback0
ip msdp originator-id loopback0

You can also combine AutoRP and Anycast RP to avoid the configuration of an RP address on every router.


Cisco IOS 12.4T IP Multicast Configuration Guide

Verifying IP Multicast Operation