From firewalls-gc Thu Jul 10 10:30:29 1997 From: Paul Ferguson Date: Thu, 10 Jul 1997 10:30:29 +0000 To: firewalls-gc Subject: Re: Two ISP's to one DMZ X-MARC-Message: https://marc.info/?l=firewalls-gc&m=87619474410463 At 12:29 AM 07/10/97 -0500, mikech@avana.net wrote: > >Nope, as I stated previously, how do you route one ISP's CIDR addresses >through another ISP? Are you saying I can grab a chunk of Sprint's CIDR >(Classless Inter-Domain Routing) address space and reroute it thorugh MCI? >Will it be added to the MCI routing tables as a separate entry? How will >Sprint remove the class C from its CIDR block? Won't this fragment the hell >out of the backbone routing tables? > Yes, multihoming does inject more specific prefixes into the global routing table, and punches holes in CIDR blocks. This is something that aggregation purists have belabored for several years. However,the fact of the matter is that in order to effectively multihome in an environment where your IP address space comes from with one of the upstream provider's CIDR blocks, this is how it works, and is perfectly feasible. In fact, there are many organizations doing this today. With regards to prefix announcement filtering, it is true that there are several larger ISP's which filter announcements on a /19 boundary. This is a political problem, not really a technical one. >> Exactly how does NAT and DNS provide for the announcement of AS's >> and/or prefixes into the global routing system? > >It doesn't. It is an *alternate* solution. You can remap Internal address >space to multiple external IPs. These IPs could even come from different ISPs. >The dynamic DNS allows you to remap inbound connections by changing the IPs a >domain name is associated with in real time. > Again, I do not view this as a technically feasible method of multihoming an organization. Use a hammer to pound nails, not a wrench. - paul -- Paul Ferguson || || Consulting Engineering || || Herndon, Virginia USA |||| |||| tel: +1.703.397.5938 ..:||||||:..:||||||:.. e-mail: pferguso@cisco.com c i s c o S y s t e m s