Imagine you are migrating from one ISP provider to another. The current provider not only offers your web filtering but also your DNS, web mail, and other hosted services so you can't simply cut them off. You need to be able to split out the Web filtering to your new provider whilst maintaining access to the old ISP for the hosted services.
As part of this move you are migrating the internet access of 300 sites. Each site is getting a new firewall as well and the move to the new ISP is taking place as each site gets it's new hardware.
Due to this hardware change you will end up with some sites on the old hardware pointing to the old ISP and some sites on new hardware pointing to the new ISP.
My question is this, with the minimum of in put by your self how do you run both ISP's at the same time and have the new sites point to the new ISP for the web filtering whilst maintaining access to the old ISP for everything else?
Here is the solution, on the router that provides the connection to both the Old ISP and the New ISP:
1) ip route 0.0.0.0 0.0.0.0 10.10.10.254
2) access-list 111 deny ip any 10.10.10.0 0.0.0.255
3) access-list 111 permit ip 192.168.10.0 0.0.0.255 any
4) route-map new-isp permit 10
5) match ip address 111
6) set ip next-hop 172.16.1.1
7) interface GigabitEthernet0/0
description ***Inbound Interface towards the ISP's***
ip policy route-map new-isp
Breaking it down line by line the config is as follows:
1) This is the default route to the old ISP. This required for all sites that have not migrated, also your hosted services are located in this direction. DNS, web mail, Anti-virus are located on servers on the 10.10.10.0/24 subnet.
2) Extended ACL denies packets from any Host going to the old ISP for hosted services, this stops that packet being processed by the route map and therefore prevents the next hop from being changed. The denied packet will be processed as usual by the routing engine and as such you maintain access to your hosted services.
3) This permits the packet from this subnet, to any destination, to be processed by the Route-map. As each site is kitted out with it's new hardware you add the site's IP range ( a /24) to this ACL. This in turn is permitted to be processed by your route-map and the next hope for web filtering is changed.
Remember that if your newly migrated site still needs access to DNS, web mail etc, this will be caught in the first line, the DENY any statement and will not proceed to the route-map for processing.
4) This is the entry for your route-map
5) The match statement catches permitted IP's in ACL 111
6) Any matched IP's in ACL 111 are then directed to the new ISP at 172.16.1.1, the New ISP will provide your web filtering only.
7) Apply your route-map to the inbound interface that receives the traffic from all the sites that are being migrated. The route-map is porcessed at this point and the next-hop defined.
So what happens next?
Each time you want to cut over a new site you simply add a permit statement to ACL111.
Straight away the route-map will pick it up and web traffic will be diverted from the old ISP to the New ISP.
Ultimately you will end up with up to 300 sites listed in ACL111. Once all sites have been migrated you can change your default route, remove the route-map and run access to the internet as normal.