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Question - Routing

Hi Guys,

I have 2 sites that are geographically separated but on the same LAN, site A and site B. These 2 sites are linked by backhaul radios. Site B gets its DHCP and other services from site A where all the servers are located. Traffic from site B together with other traffic from site A is routed to the internet via the Cisco 1841 router located at site A through ISP - 1. My task is to leave the setup the way it is (DHCP for site B to still come from the Cisco 1841 at site A) but just route traffic at site B through a different ISP - 2 and only get to be routed through the ISP – 1 when ISP-2 is down. Site B has been dedicated 2 subnets from the 6 configured at the Cisco 1841 router at site A .I ‘am very open to suggestions guys on how best I can achieve this.




Re: Question - Routing

Are ISP 1 and ISP 2 both terminated on the 1841?

You could try using policy based routing with object tracking. This guide below will get you started:

New Member

Re: Question - Routing

Hi sean,

Only ISP 1 terminats into the 1841, ISP - 2 will only be for site B.I' am  actually a Network Engineer for ISP - 2. These guys would like to continue getting their DHCP for site B from site A (from the 1841)but just route traffic to the internet for site B using a different ISP -2.

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Question - Routing


There are some inconsistencies in the description of this environment that puzzle me. You begin by saying that there is one LAN that is used by both sites. One LAN usually means one subnet. But then the description says that there are 6 subnets and that 2 subnets are to be dedicated to the second site. How can 2 subnets be dedicated if they truly are on the same LAN? How will the 1841 determine which site a DHCP request comes from so that it can allocate the appropriate subnet?

The description mentions one 1841 router and backhaul radios but does not tell us anything about what kind of network equipment is available at the second site and what feeds its data over the backhaul radio. If we knew that information we might be able to make better suggestions.

But based on the limited information I have a couple of suggestions.

-first suggestion is very simple. If the 1841 can detect which hosts are at the second site and will assign a different subnet in DHCP then it ought to be possible for the DHCP assignment to give a different default router address and assign an address that will point the users at the second site to your device as their gateway to the Internet.

- second suggestion is more complex and would probably require additional hardware at the second site. It could be possible to set up a small router in the LAN at the second site. This small router would connect to the backhaul radio for communication with the first site, would connect to your device for access to the Internet through you, and would connect to the other devices at the second site. The small router at the second site could have a primary default route which points to you as the next hop and could have a backup default route which points to the 1841 in the event that your route is not working.



Re: Question - Routing

on the same LAN? Do you mean the 2 sites are bridged? It's going to be tough to have one subnet with two default next-hops. It would seem easier to me to split the sites into 2 different subnets and route between the two. If you want site b to get dhcp from site a, then just use ip helper address. You could also implement some failover routing in case one of the ISP links fails as well.

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New Member

Question - Routing


I'm making some assumptions, as it makes it a little easier. The assumptions are that there are in total Six subnets, 4 at site A and 2 at site B. The DHCP server/s will be located at Site A. The radio link between the two will remain.

In the router at Site B, you'll need to put ip helper address statements in both the subnet interfaces, that have client machines. the address will be the address of the DHCP server.

You'll need to put a default route in site B to say route internet traffic to your ISP. This may get tricky if the two sites are have a routing protocol between them both. As I'd assume that site A will already have a route saying pretty much the same thing. Won't such be a problem however if they aren't redistributing Statics.

Things you'll probably want to consider, NAT/Firewalls. Routing protocols. Tunnels to do away with the wireless.


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