[nsp] Multihoming with Two ISP without BGP

[Kristofer Sigurdsson]

Chintan Shah, Tue, May 18, 2004 at 10:42:04PM -0700 :
> 
> Dear All,
> 
> My Customer is having two different link with two different ISP for internet connectivity. Customer is having two different department one is Manufaturing and one software and he wants to keep dedicated line for Manufature department with ISP1 and Software department  with ISP 2.  He is having Different IP block from ISP1 and ISP2.
> 
> Lex say ISP 1 - a.b.c.d/28 and from ISP 2 - p.q.r.s/28 block.
> 
> He is having one cisco 1700 series router with two WIC card (WAN card) and one Fast Ethernet card. Ethernet card is having multiple IP address each from a.b.c.d//28 and p.q.r.s/28 block so that request from any IP block will come to FastEthernet and then forwarded to WAN card.
> 
> To utilize dedicated connection with ISP 1 and ISP 2 for different department as per customer requirement, we have opted to use IP routing policy to define next hop using route-map with IP accesslist so that any request from block a.b.c.d/28 will be forwarded to ISP1 (Seria0) and from p.q.r.s/28 will be forwarded to ISP2 (serial1).
> 
> Now, customer wants some redudancy, as if any link goes down , let say ISP1 then Manufature department should not suffer as already second link is working. Now, its require BGP Mulithoming as  request for internet from IP which is allocated by ISP1 to be forwarded to ISP2 and for that customer has to do BGP with both provider. But customer does not have AS number and don't want to go with this despite of lots of discussion and still wants redudancy.
> 
> So, we agin suggested to do NAT with WAN IP allocated by ISP so that in case of ISP1 link will down , any request with source IP of ISP1 will be NATed through WAN IP of ISP2 and request will forwarded to ISP2 to provide uninteruppted Internet services and Return traffic will automatically come back via the same working link, because that link is the only link servicing that address range.
> 
> I hope above our solutions is workable, if you have any comment/suggestion , please most welcome.

You can indeed use the NAT solution, but I think you'd have to turn it on manually, I can't recall any Cisco
function that starts NAT'ing when a link goes down...

> 
> But , I want to know that if now customer wants to run any Internet services on his end like Webserver or DNS server then with above solution Of course, this approach won't work if you're providing services to the outside world, as the addresses associated with the failed link will disappear from the Internet. 
> 
> So what could be solution ,if possible for abvoe scenario.

Getting traffic from the user is not a problem.  You could even simply use two default routes, one for each ISP.  If the ISPs agree
to route both networks and announce them to the Internet, this will not be a problem (provided that the rest of the Internet accepts
the route, which it most likely will not, given the size of the networks).  However, making, say, ISP1, stop announcing the route
when it's connection to your client goes down is close to impossible, without running somekind of routing protocol between the ISPs
and your client.  You will have to do that.  Any routing protocol will suffice.

These speculations are highly hypothetical, as announcing /28 routes to the Internet will not work - they will simply be aggregated.
If ISP1 and ISP2 share upstream providers or peer between them, this is doable.  Do they?

-- 
Kristófer Sigurðsson			Tel: +354 525 4103 / MSN: ks at rhi.hi.is
Netsérfræðingur/Network specialist	Reiknistofnun HÍ/University of Iceland