That is very common procedure. In doing that at least 2 entries will be in the global routing table. And that's exactly the intension of the customer so in case one path failed(thus withdrwan properly ) the other path will kick in. If the provider B lost both connections to the customer and provider A then it lost the FULL correct routing table then this is not the issue of filtering /20 anymore. If provider B configure their BGP sessions right then it should withdraw all the routes from provider A and the customer. Ping Lu Cable & Wireless USA Network Tools and Analysis Group W: +1-703-292-2359 E: plu@cw.net

-----Original Message----- From: BLOCKI,JACEK (HP-Poland,ex1) [mailto:jacek_blocki@hp.com] Sent: Thursday, October 11, 2001 2:12 PM To: Lu, Ping; lir-wg@ripe.net; routing-wg@ripe.net Subject: RE: Multihoming - Resilience or Independence

Hi, The customer will of course use 2 ISP - as requested. He will use addresses coming from a single PA block advertised by two providers. Of course you need to ensure 100% up connection between border routers. If connection between border routers is down one of them has to stop advertising PA block. Otherwise you have problem with return traffic. Imagine provider B lost connection to both customer and provider A. If B is still advertising common block some ASes may decide to reach customer over B, but B cannot reach customer network... In order to deliver proposed service providers A and B have to be connected with redundant network, which is quite common.

Vicious circle (sp) as I see it: *Providers want small routing tables *Customers 100% up service so they as for resilience *Providers usually have single POP in area *Customers go BGP creating problem for community (table grows) and themselves (BGP expert costs)

Solution: Providers enter into cooperation so they can advertise customer addresses over 2 independent BGP speakers. This idea is not very different from existing aggregation. Specific routing information is hidden in a group of ASes instead of single AS. Of course you need some administrative effort (e.g. consistent billing) but from technical point of view it seems feasible. I may be wrong, it won't be for the first time ;-) Regards, Jacek

-----Original Message----- From: Lu, Ping [mailto:PLu@cw.net] Sent: Thursday, October 11, 2001 5:02 PM To: 'BLOCKI,JACEK (HP-Poland,ex1)'; lir-wg@ripe.net; routing-wg@ripe.net Subject: RE: Multihoming - Resilience or Independence

...

-----Original Message----- From: BLOCKI,JACEK (HP-Poland,ex1) [mailto:jacek_blocki@hp.com] Sent: Wednesday, October 10, 2001 1:10 PM To: lir-wg@ripe.net; routing-wg@ripe.net Subject: RE: Multihoming - Resilience or Independence

Hi, It seems multihoming discussion is an example of vicious circle (sp): More specific routes result in larger routing table but we want those routes and small table... Let me suggest a blasphemy: CIDR being advertised from two ASes. They say it should be only one, but why? In my opinion there is nothing wrong in advertising CIDR from two ASes as long as you can guarantee each border router advertises CIDR only if it can reach it. Imagine the following construction: AS-A--\ +(OSPF) --(CIDR) AS-B--/

That's the problem. When customer want to be muitl-homed you can't tell them to use which ISP ? If the original prefix are not in the CIDR with the other ISP then the customer have to change all their IP address. If they don't then the second ISP have to leak the original prefix into global routing table.

The question is how do you convince your customer to change all their IP addresses to have a second link without the risk of losing its business ?

...

Each BGP speaker advertises CIDR if and only if it learned about it from OSPF. It can be done, if you don't know how I'll forward you a working example. Each border router generates a default router and injects it into OSPF. From technical point of view I see no reasons why it should not work. What you need is: * An agreement between ISPs * Change in procedure making such a union of ASes an officially blessed solution so nobody would dare to hinder cooperation with filters. * Optionally you may need a separate CIDR, since both ASes have to advertise same prefix. You need it if each ISP wants to have private customers in addition to shared ones.

The customer has "an independent connection to two ISPs" which is the quest item. I see more commercial than technical problems with such a solution. However my expertise is limited and somebody can point drawbacks I cannot see. Feel free to burn me on a stake, that's the right way of treating a blasphemy ;-) Regards, Jacek

It is a good idea but not easy to implement under the pressure of making revenue.

Ping Lu Cable & Wireless USA Network Tools and Analysis Group W: +1-703-292-2359 E: plu@cw.net