ISP network design

[mohammad.y]

Hi,

we are trying to desgin isp network,we are thinking of having two aggregation routers to connect customers to them and another two gateway routers that will be connected to tier-1 ISP like level3.my question is what are the disadvantages of having the customers and the internet connections connected to the same router?what is cisco recommended design?

below is the suggested design:

customer1---------->aggragation-Router1 ------------------->GW-Router1------------->tier-1 isp

                                        |                                             |

customer1---------->aggregation-Router2-------------------->GW-Router2------------->tier-1 isp

what is the disadvantages of the below design?

customer1---------->aggragation-Router1 ------------------->tier-1 isp

                                        |                                            

customer1---------->aggregation-Router2-------------------->tier-1 isp

many thanks

[Mohamed Sobair]

Hello Mohamed,

When you design an ISP Network, you should consider multiple factors:

1- Fault tolerance

2- Redundancy

3- Loadsharing

Your Customer Access/Aggregation shouldnt be your Gateway router to the upstream, if you take any of the above points, you will find that it doesnt satisfy any of them.

Another important point is that, normally those Aggregation routers connects to your ISP Services Like DNS, Caches, Filters. The Gateway router doesnt apply any kind of filtering of traffic its just recieving routing table of the Internet. And the major Point is you would minimize too much of CPU & Memory Load from combining both into a single router additional of course to the above points.

Let me know if this answers your question,

Regards,

Mohamed

[mohammad.y]

Hello Mohamed,

many thanks for your reply

I agree with your second point

 Another important point is that, normally those Aggregation routers connects to your ISP Services Like DNS, Caches, Filters. The Gateway router doesnt apply any kind of filtering of traffic its just recieving routing table of the Internet. And the major Point is you would minimize too much of CPU & Memory Load from combining both into a single router additional of course to the above points.

but  I'm not sure how this design will affect fault tolerance,redundancy and loadsharing.

Regards

[Mohamed Sobair]

Hello Mohamed,

Ok, In many cases for enterprise customers, they require redundancy & loadsharing at the ISP Core, So, Lets assume you have combined both at one router, and this router fails OR the Upstream connected to this router fails, Both Internet and the Customer are Isolated Now. but if you have Access/Aggregation + Gateway, if the Internet Link fails at one gateway, the traffic could be redirected to the Second Gateway, Like wise, Many Customers Could be Multihoming to your ISP and they require High Availability and Redundancy at the ISP, would it be right to connect them to One router combining all functions, surely, NO.

If you Look at it from both the Internet (Tier-1 ISP) side, or from the Customer Side, you should maintain high availability and redundancy at the ISP Core.

Does this make sense?

Regards,

Mohamed

[mohammad.y]

Hello Mohamed,

I'm not talking about one aggragation router ,the design includes two aggregation routers and these routers will be used also to connect to tier-1 ISP .? in this case I think the loadsharing and redundancy point will not be valid.

I agree that having two aggregation routers and two internet gateway routers is a better design but I want to justify this??

[Mohamed Sobair]

Mohamed,

As I mentioned, its not a design to be followed for an ISP Network, interms of technical concept. you shouldnt have a single router connects your customers, services & Internet Links. its not sufficent interms of hardware Resource & doesnt Provide the functionality recommended for Seperate devices/layers.

Regards,

Mohamed