IP ADVERTISEMENTS BY ISPs

Unanswered Question
Aug 26th, 2009

Hi

we own AS NUMBER and about few subnets (/24).I told my ISP to advertise two (/24) but ISP is saying to advertise all of them.I told them That I do not need all of them right now but they are insisting as to why I am not advertising all of them.My question is

1.Do I have to advertise all my subnets with ISP who I am using as Multihoming ?

2.What happens If I change my ISP in future,Is it a pain to readvertise my subnets again at different ISP ?WHAT IS involved in readvertisements ?

I have this problem too.
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Paolo Bevilacqua Wed, 08/26/2009 - 12:56

1 No. But if the subnets are adjacent, it make sense to advertise a single aggregate.

2. Depends if the addresses are PI or PA. If the latter, ISP can ask that you stop using them when terminating service.

CCDECCDE9 Wed, 08/26/2009 - 13:18

We are PI.How does this advertsing works ?Now ISP1 is advertising our PI and tomorrow I got o ISP2 for advertisement,what happens with ISP1 advertisments ?

CCDECCDE9 Wed, 08/26/2009 - 13:18

We are PI.How does this advertsing works ?Now ISP1 is advertising our PI and tomorrow I got o ISP2 for advertisement,what happens with ISP1 advertisments ?

gthermaenius Thu, 08/27/2009 - 11:21

Well it's routing. Best path wins the race. If a customer in ISP1 are going to your network it should go through isp1. If your connection to ISP1 goes down ISP1 learns the routes from ISP2 and traffic goes that way instead.

Giuseppe Larosa Thu, 08/27/2009 - 22:59

Hello CCDE,

>>2.What happens If I change my ISP in future,Is it a pain to readvertise my subnets again at different ISP ?WHAT IS involved in readvertisements ?

nothing to worry about because you have your own AS number and your own public ip address blocks it is just the time to setup the new eBGP session with ISP2

the problem in changing provider comes when you are using a public address block borrowed from ISP1 space.

1) not sure but RIR like RIPE or ARIN may require this: if you have applied for public ip address blocks it is for using them on the public internet.

For example RIPE required real multihoming to confirm an AS number assignation (years ago before the new 4-byte AS numbers).

As Paolo has suggested it makes sense to advertise an aggregated route.

Hope to help

Giuseppe

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