| 1. Redundancy | | | | limits and this limit is usually well below the interface |
| Redundant links can be installed on the LAN and WAN. | | | | speed. In other words, if you are passing packets |
| The question of whether redundant links are required is | | | | between networks at high speeds the type of |
| generally guided by a combination of key factors;a) | | | | hardware should be a major consideration. |
| Finance - is there sufficient money to cater for the | | | | Other hardware considerations include the size of the |
| additional equipment and any additional operational | | | | routing table, oversubscription rates, the number of |
| costs for a second link?b) Business Requirements - | | | | features required by each device (i.e. QOS, Neflow, |
| what is the business impact of lost network | | | | access lists, port security, multicast etc) and the |
| connectivity in terms of risk and revenue?c) Service | | | | effects of those features on the hardware. |
| Level Agreements - what are the existing service | | | | 4. Monitoring |
| level agreements with carriers and third party | | | | There is monitoring and then there is monitoring. Just |
| providers? Are the restoration times in line with | | | | knowing whether a device is up or down is usually not |
| business expectation? | | | | enough, additional protocols that can help to determine |
| 2. Congestion | | | | the performance of a router include;a) SNMP - by far |
| A heavily congested link is almost as bad as a down | | | | the most popular network management protocol. It can |
| link. The usual suspects causing network congestion | | | | be used to graph just about any counter including CPU |
| include;a) Misconfigured or Chatty applications - some | | | | utilization, memory utilization and interface utilization.b) |
| applications attempt to download large quantities of | | | | ICMP - still a useful protocol to determine uptime and |
| data over a link in order to update their software or | | | | round trip latency.c) Net flow - a fantastic protocol that |
| due to misconfiguration errors.b) Viruses and worms - | | | | captures traffic and allows you to keep records of |
| amongst other things, viruses and worms can launch | | | | conversations, volume and protocols across a |
| denial of service attacks which are designed to | | | | particular link.d) IPSLA - this protocol is proprietary to |
| overwhelm network services.c) Oversubscribed link - | | | | Cisco equipment. It is a very powerful tool for network |
| too many links aggregating into one point can cause | | | | performance monitoring. |
| congestion in much the same way that freeways and | | | | 5. Layer 8 of the 7 layer OSI model! |
| motorways become congested during peak hour. | | | | As a good friend of mine once wrote, layer 8 of the |
| One mitigation against heavy congestion is to | | | | OSI model is the user's expectation. High speed and |
| implement Quality of Service. This service will allow | | | | high availability are defined by business requirement |
| you to prioritise traffic based on relevance and | | | | and user expectation. Just as network response |
| importance to your business. | | | | speeds ten years ago would no longer be acceptable |
| 3. Hardware limits | | | | today, your network design should cater for the |
| Routers in particular are prone to high utilization under | | | | expectation that new networks will be responsive and |
| heavy loads. This varies, of course, depending on the | | | | available by virtue of the fact that they are new. |
| router model but they all have their route processing | | | | |