Two or more routers in a network

[VulgarDisplay]

I'm in the early stages of the cisco networking academy and I've come across a topic that I may be thinking entirely too hard about at this time, but maybe some of the guru's here could clarify this for me.

Why would you ever want two or more routers on a single LAN? Is it an issue of capacity?

Everything I know about networking at this point comes from small at home applications, and I know that with DHCP on a consumer router you do not want to connect two routers together unless you turn off DHCP on one and change it's IP address. Doing this essentially turns the second router into a switch.

What allows the routers used in my CCNA courses to continue to function as routers and not glorified switches?

Like I said I may be thinking entirely too hard about this, but it has basically blown my mind at this point and I'm lost.

 

[VirtualLarry]

I may be entirely wrong here, but I can't see a reason for there to be more than one DHCP server per L2 broadcast domain, since DHCP uses broadcasts I believe.

Maybe if you had multiple switches, each with a set of ports on seperate VLANs, and then had two routers, each handling DHCP for the separate VLANs, then I might be able to see it.

 

[Pheran]

First, you are at a disadvantage coming from the consumer space. DHCP and routers have absolutely nothing to do with one another. Those just happen to be two functions that are commonly combined on home devices.

It is very common to have two routers on a LAN for redundancy purposes. Typically they will share the gateway address using a redundancy protocol like HSRP.

When you move into the network core, it will probably be nothing but a mesh of routers talking to each other with a routing protocol like OSPF.

I'm not quite sure what question you are really trying to ask, but I suspect you haven't acquired quite enough knowledge yet to properly ask it.

 

[aarodav1]

I'd say its an issue of capacity and also to split up the network and have certain things routed by specific routers. If you only have layer 3 interfaces defined on one of the routers, only that router will route traffic to/from that interface, and the other router will simply forward traffic to that router to take care of. Say you have a core router and a separate router for just a single data center. It allows the networks to be more isolated from each other and on completely separate physical boxes.

I hope that makes sense somewhat. As far as routers talking to each other, there are many dynamic routing protocols where routers will share their routes with each other and each router can then build a map of the network and know where to send packets. You can also set up static routes that specify where to send certain packets.

 

[imagoon]

In a single "LAN" in this day and age more commonly defined as a single IP segment IE '192.168.1.0/24" you would use 2 routers for redundancy. In an environment where you have multiple IP segments, the router(s) are required to allow access to those segments. There you can use them for access, security [ACL's etc] redundancy, capacity etc.

As for DHCP while not related to the routers, you can have multiple DHCP networks on one segment as long as they do not have overlapping scopes. It is typical for a cheap form of DHCP redundancy to be setting one server's scope to say .10 -> .50 and another to .51 -> .91. Reservations and configurations need to be kept consistent otherwise.

 

[imagoon]

I may be entirely wrong here, but I can't see a reason for there to be more than one DHCP server per L2 broadcast domain, since DHCP uses broadcasts I believe.

Maybe if you had multiple switches, each with a set of ports on seperate VLANs, and then had two routers, each handling DHCP for the separate VLANs, then I might be able to see it.

That isn't a problem with DHCP. DHCP uses broadcasts but the clients in order to meet spec are required to only acknowledge and accept the first broadcast it receives with a unicast response.

 

[VulgarDisplay]

Moving through some more labs and seeing how default gateways are used really cleared this up for me. In the lab that confused me the router that confused me actually just represented the connection to the ISP/Internet.

 

[Danimal1209]

This question is kind of tough to answer.

But, in most networks that make use of CISCO hardware, DHCP is not really used. What makes routers different from switches is the ability for routers to run a routing protocol.

Routers basically command the network. Switches just allow for a junction of devices.

 

[MtnMan]

A LAN subnetted with a router between the two subnets will block broadcasts, creating 2 broadcast domains.

This can also provide security between departments, by denying/allowing traffic to cross the router based on type of traffic, or source or destination address.

 

[Pheran]

But, in most networks that make use of CISCO hardware, DHCP is not really used.

Maybe I'm misinterpreting what you are saying, but on the face of it this statement is one of the more ridiculous networking claims that I've read.

 

[VulgarDisplay]

Maybe I'm misinterpreting what you are saying, but on the face of it this statement is one of the more ridiculous networking claims that I've read.

Sounds like it would be quite difficult to get a wireless connection.....

 

[theevilsharpie]

Maybe I'm misinterpreting what you are saying, but on the face of it this statement is one of the more ridiculous networking claims that I've read.

I'll give him the benefit of the doubt and assume that he meant using Cisco devices as DHCP servers.