Changing ISP

Unanswered Question
Jan 30th, 2007

Hello

I'm about to change ISP's. I connect to our current ISP #1 with a static route pointing my default route (0.0.0.0) to his dsl modem.

I will route to my new ISP #2 using BGP. I expect he will hand me a default route ( 0.0.0.0 ), and I assume the new BGP route will be of a lower priority than the static ? ( i.e. the static will be selected first ) Given this I have instructed ISP #1 to prepend the route that he advertises for us so that when we advertise to ISP #2 this new route will be higher priority than ISP #2. No dramas so far.

However, when I start advertising my route to ISP #2 via BGP, these routes will eventually filter to the internet and assume a higher priority than the prepended route of ISP #1.

My question is, what effect will my static route to ISP #1 have on the traffic that comes from ISP #2, particularly as it returns back to the source IP ? I see a situation whereby the route will filter to some sources who will com in via ISP #2 and then on returning to the source will pick up the static from ISP #1 and return via that link ?

Cheers - and thanks in advance

I have this problem too.
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vijayasankar Tue, 01/30/2007 - 21:03

Hi,

Im not sure about the exact configuration present on your router.

However looking at your initial post, here's what i can say.

You have mentioned that you will be accepting only the default route from the ISP 2.

You already have a static route in your router pointing to ISP1.

The static route in your router will always take precedence over anything else.

The effect of this is, the outbound traffic from your network to outside world will take the default static route, if no other specific route is present in your router.

Once you remove the static route to ISP1, the default route from ISP2 will take effect for your outbound traffic.

Hope this helps.

-VJ

mbstrauss Tue, 01/30/2007 - 21:50

Thanks VJ

That's pretty much as I expected.

Is there no way to route a packet back out to the source/originating interface ?

FYI the router is a 2811.

interface Fast 0/0 = ISP #1 -> with static route of 0.0.0.0 pointing to it

interface Fast 0/1 = ISP #2 -> accepting BGP default route

interface Fast 0/2 = internal network ( via pix)

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