If you have dual upstream connections for Internet, traffic shaping would be one of major matter as a network administrator. Especially, company has one big pipe(Tier1) and mid-size connection with Tier2 ISP for redundant reason and those ISPs don't have any connections between. See Pic 1.

The company wants to use all outbound / inbound traffic on their major(big pipe). It's normal and understandable. However, mid-size secondary uplink pull(?) some traffic, even though they put higher preference on main connection. Why?

There would be few reasons, but main reason would be Tier2 ISP's routing policy. Most Tier2 ISP has many upstream connections as CUSTOMER BASED. It means they can have more control and could take advantage upon Tier 1 ISP's routing policy.

For example, if they are announcing their customer's IP block with local pref 120 to their upstream providers, at least upstream provider's router will pick his path for the route as best. (See Pic 2.) Due to most Tier 1 ISPs don't send any preference on their announcement to private and public peers, Tier 2 ISP will pull most of traffic and bring customer's redundant circuit which end customer doesn't want.

Why they do that? Simple answer, because of their business. In these days, a lot of competition on ISP / Internet connection market. One of competitive product tier 2 provides is usage based charge product. They only charge how much customer use bandwidth per monthly basis. Sounds cool and fair, but like I mention above example, they will suck all the traffic and push their subscribed line and charge more. End user might ask them why most traffic route to your (Tier 2) backbone? They would say "'our connection is fast and well optimized, we care of small company's traffic not like big ISPs ...........". I know it is not always true.

Most of Tier 1 ISP applies lower local pref for routes which is announcing from their private peer. It means they don't want to leave customer's traffic, if the customer has a connection with them. For the business, I understand(trying to).

Only ISP / Verizon Business (Former UUNET) I know doesn't have any preference on in / outbound traffic between peers. It would be not a best (in these days) for them, but at least good for end users.

I like old UUNET's routing policy. Verizon Business is still keeping their unique routing policy which let BGP decide best path and rely on routing algorithm. It makes customer easy to control their traffic after it left their network. Of course they will control IGP metric to internal traffic shape, we don't want nose into their business too deep. If ISPs start modifying a route preference to take advantage on other ISPs, it will eventually bring a disadvantage for all the customers. I hope no more dirty game with pure customer's traffic.

Solution:

1. If your ISP (Tier 2) provides a BGP community, use it to prevent the hassle. See our ALL BGP community strings of most ISP in the world.

2. If your ISP(Tier 2) provider doesn't help you at all, use Conditional BGP announcement. It will give you a bit flexibility. See our other article for "Conditional BGP announcement" page.

I hope this is informative for you.

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