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New Member

2 adsl lines 1841 2 ethernet ports want to use same subnet

hi all,

ive 2 adsl lines terminating to my 1841 2 ADSL wic cards are installed.now

my issue is my lan ip scheme is 172.16.0.0/16, its a flat network and i cannot do a vlan in my network bcoz of some issues. so in 1841 ive 2 ethernet ports. now my issue is both the ethernet ports should have ip's from same subnet

my ip for fa0/0 is 172.16.5.1/16 and for fa0/1 i need 172.16.11.1/16 as my ip. but as you all know 2 ethernet ports of the router will not support ip's from same subnet.

is there anyway i can achieve this, so that i can do some sort of redundancy

Binoy

15 REPLIES

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Hall of Fame Super Gold

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Ah IRB configuration will let you have one single Ip subnet on two ethernet ports (with one blocked by STP), but not overlapping subnets.

Hall of Fame Super Gold

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Since it's a flat network, once you attach one single ethernet port to your switch, and configure secondary address on it, the result will be the same.

New Member

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

hi thanks for your reply,

my prb is for some reason some people want to use 172.16.5.1/16 and some of them 172.16.11.1/16 so thats where im stuck.so you mean to say its not possible at all.

Hall of Fame Super Gold

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

It is possible, but using a single interface, not two. That is, if you configure two interfae via IRB, one will become blocked by spanning-three. In any case, configure a regular address and a secondary on the interface.

Hope this helps, please rate post if it does!

New Member

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

but my issue is like ive 1 adsl line then how can i utilise these 2 lines then

can you suggest a better solution to me

Hall of Fame Super Silver

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Binoy

It seems like this posting changes the question significantly. Originally the question was about interface(s) to connect to your LAN. Now the question seems to become about connecting to and using both adsl lines. It seems to me (without having sufficient knowledge of your environment) that if each adsl has its own IP address, that you connect to both of the adsl and that you configure 2 default routes (one default route to each adsl). This will send some of your traffic out one adsl and will send other traffic out the other adsl.

If you want to be more deterministic and send certain tyypes of traffic out one adsl and send other types of traffic out the other adsl then you would want to configure Policy Based Routing which can do this.

HTH

Rick

Hall of Fame Super Silver

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Binoy

We have 2 reasonable solutions suggested so far. Each of them is a reasonable solution for a certain type of problem. If we can get some clarity about what your requirements really are then we can figure out which of the solutions is the preferable one.

Your original post mentions wanting to achieve some redundancy. Implementation of IRB (with bridging configured on both physical interfaces which do not have IP addresses configured and the BVI having the IP address) will allow one interface to back up the other interface. As others have mentioned with both interfaces bridging in the same broadcast domain Spanning Tree will put one interface in blocking mode and one interface will be active. If the active interface suffers a failure then the other interface becomes active. If redundancy is your real requirement then IRB is the way to go. But note that this does not allow you to put a different address on each interface. And my personal opinion is that the risk of a router interface failure is fairly remote and I do not believe that the redundancy achieved by IRB is really worth the complexity that it introduces into the configuration.

In further posts in this thread it begins to look like your requirement is really to be able to have 2 IP addresses active in the flat network. If this is your requirement then secondary addressing is a better solution.

So if you can provide some clarity about what the requirement really is, then the best solution will become more evident.

HTH

Rick

Hall of Fame Super Gold

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Hi Rick,

I guess, what Binoy wants to do, is to send traffic via one or the other adsl interface depending which "default router" the client is using.

We know that this is not possible, or at least, not easily.

What can be done, is to send traffic out one or another interface depending on the IP address of the client. As we know, this is done via PBR and route-maps in NAT, but it may be to complicated for a "beginner" installation, so just using two default routes and tr to load balance would be probably be much easier.

New Member

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

hi roburts thanks for your reply

i will explain my scenario, actually in my customer place their ip scheme is 172.16.0.0 /16 and it is a flat network.they are not ready to go for a vlan network so it will be flat network.

rt now they have 2 ADSL connections they are using 2 localy available modem's d-link or linksys something like that.

now they have purchased 1841 cisco router with 2 no's of WIC1-ADSL and as u know it has got 2 ethernet ports.

and the exisitng ip addresses for the adsl are 172.16.5.1/16 and 172.16.11.1/16 he want to use the same ip's in cisco 1841 also, the present condition is the person who's supporting the IT divn is not experienced so he is not agreeing for any change he want everything exactly as before.

ive configured the first adsl line and it is working, ive used fa0/0 of router for the first adsl line and ip as the person suggested its 172.16.5.1/16 and he want us to install the 2nd adsl line and put 172.16.11.1/16 for fa0/1 which is not possible bcoz router will say overlaps with fa0/0 due to same ip subnet on fa0/0.

so my scenario is this if i create a subinterface on fa0/0 will it allow me to put 172.16.11.1/16 which is from the same subnet?

and the routing i can say like i will create 2 dialer interfaces and

ip route 0.0.0.0 0.0.0.0 dialer1

ip route 0.0.0.0 0.0.0.0 dialer2

where both dialers are representing 2 adiff adsl line is it? please correct me if i'm wrong.

hope to hear back from you soon

New Member

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

to be honest with you, ive done PBR's in diff places but with the person whos supporting this particular customer is not experienced and not ready to listen anything, even he doesnt know what needs to be done who need to access through 5.1/16 or 11.1/16 so we are stuck; as its a one time implemenataion i'm not in a position to go and do the survey i just want to give both the lines in working condition.

that was the reason i was asking abt a simple solution so that i can finish it off at the earliest.

Hall of Fame Super Silver

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Binoy

I understand the desire for a quick and simple solution. However I believe that this situation does not have a quick and simple solution. Especially insisting that the network be completely flat and use 172.16/16.

You mention configuring a subinterface but this will not work. If you configure a subinterface the router will treat the FastEthernet as a trunk and will expect VLAN information on the subinterface. But your flat network does not do VLANs. The suggestion made in previous post to use secondary addressing is the best way to get 2 addresses onto the interface.

There are a few things in your description that I do not understand. If the first ADSL is on FA0/0 and the second ADSL will go on FA0/1 then where is the customer network connected (since the 1841 comes with 2 FastEthernet interfaces? I am also not understanding using 172.16 addresses on the ADSL. I would have thought that the ADSL would have used public addresses given by the provider. Can you clarify these things?

If the customer does not have a requirement that certain types of traffic use a particular ADSL then there is no particular need for PBR and your 2 default routes should work just fine.

HTH

Rick

New Member

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

ok i will explain; 1841 has got 2 ADSL WIC cards and the isp termination is coming to these 2 interfaces, and these interfaces will get DHCP ip from ISP,that i'm not bothered about. so this is my WAN and from 1841's ethernet ports i need to connect to the LAN switch is it, i'm talking about that part.

so rt now as i told u they are using 2 dlink router's one has got 172.16.5.1/16 and 2nd router got 172.16.11.1/16 as their default gateway's. hope you are with me.

so rt now may be 100 pc's having gateway as 172.16.5.1/16 and may be other 120 pc's having gateway as 172.16.11.1/16

the same thing they want to achieve, thats where i'm stuck.

(172.16.11.1/16)

------ -----Fa0/1

2 ADSL lines ------Router -----Fa0/0

(172.16.5.1/16)

and Fa0/0 and Fa0/1 connecting to a 2950 cisco switch (layer 2 switch) now for the network (172.16.0.0/16) there are 2 gateways

which are indicated above. so now this scenario is working bcoz they have 2 diff dlink routers so just need to change default gateway in pc( no DHCP server in network) but how van i integrate this in one equipment which is cisco 1841.

Hall of Fame Super Gold

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Well, this is what I was telling you before, since it is not possible (at least in my knowledge) to have the router decide to use one or the other ADSL depending on the GW configured on the PC, you will have to configure access-list/PBR on the router to make the decision based on PC IP address.

Hall of Fame Super Silver

Re: 2 adsl lines 1841 2 ethernet ports want to use same subnet

Binoy

This explanation does help to clarify some aspects of the situation.

I would make this suggestion for the most simple solution for this customer. Attempting to connect a 2950 switch to each FastEthernet is a significant complication and would require configuration of IRB. I suggest that you uplink one 2950 to the other 2950 and connect 1 2950 to the 1841. On the 1841 interface where the switch is connected configure 172.16.5.1/16 as the primary address and configure 172.16.11.1/16 as a secondary address.

When you get the ADSL interfaces configured and obtaining addresses via DHCP then your 2 default routes should take care of outbound traffic.

I do not see a way to make it more simple than this. (Though I believe that the customer would be much better off if they were to consider getting away from the flat network.)

HTH

Rick

HTH

Rick

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