CSS Line Device Approach: Path Selection

CSS Line Device Approach: Path Selection

Hi All,

  In the previous post I explained how Line Device CSS approach works, but I also mentioned:

• ·        The Device CSS decides through which gateway/trunk call will go out of our network.
• ·        Line CSS defines (Class of Service) calling privileges.

In this post we will try to understand how device CSS helps us to select the right gateway. Although Cisco has introduced a concept of local gateway, but we will discuss that in a different post.

Now we know that device CSS is allowed to allow a call and Line CSS is used to block a call. We will not discuss anything about blocking a call now. Now we will discuss when a Route Pattern is matched how it will select the correct gateway.

Let’s take an example of two Site A and B. In this case we need to create Route Patterns for each site. Here is the configuration for Route Pattern and CSS.

Route Pattern Site A	Partition		Route Pattern Site B	Partition
9.[2-9]XX.XXXX	PT-Local-Site-A		9.[2-9]XX.XXXX	PT-Local-Site-B
9.1.[2-9]XX.[2-9]XX.XXXX	PT-Long Distance-Site-A		9.1.[2-9]XX.[2-9]XX.XXXX	PT-Long Distance-Site-B
9.XXXXXXXXXXX	PT-International-Site-A		9.XXXXXXXXXXX	PT-International-Site-B

Call Routing:

Route Pattern Site A                                                                                           Route Pattern Site B

  ->Route Group                                                                                                   ->Route Group

     ->Route List                                                                                                         ->Route List

          ->Gateway or Trunk (Site A)                                                                              ->Gateway or Trunk (Site B)

CSS	Partition
CSS-Device-Site-A	PT-Local-Site-A, PT-Long Distance-Site-A, PT-International-Site-A
CSS-Device-Site-B	PT-Local-Site-B, PT-Long Distance-Site-B, PT-International-Site-B

Device	CSS
Device-Site-A	CSS-Device-Site-A
Device-Site-B	CSS-Device-Site-B

When a device in Site A calls a local number it checks its CSS: CSS-Device-Site-A, when it matches Rote Pattern 9.[2-9]XX.XXXX  it has two options PT-Local-Site-A & PT-Local-Site-B. As CSS of Site A device contains partition PT-Local-Site-A it will match the route pattern for Site A and as per the call routing diagram call will go out using gateway at site A.

  Similarly it will work for site B.

  In order to make this work simple when number of sites increases Cisco has introduced the concept of Local Route Gateways, which we will discuss in next post.

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