Redundancy on 1841 router (Two ISP's)

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May 3rd, 2009
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Hello Guys

my network is illustrated in the figure bellow.

i need to connect two ISP's to cisco "1841 router". one comes form serial cable and the other comes from Ethernet cable and make redundancy between them. if ISP 1 goes down then automatically the router will transfer the traffic to ISP 2.

also, the PC's shown bellow needs remote access from internet and routers (Router1, Router2, Router3) all needs public IP's.

note: all traffic will go through one ISP, and no load balance is implemented.

can you please tell me if this solution is possible or not on (1841 router)?


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Overall Rating: 4 (1 ratings)
Collin Clark Mon, 05/04/2009 - 08:32
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Do you have your own public address space or do you have two different IP blocks, one from each provider?

diyar_eng Tue, 05/05/2009 - 00:59
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Yes of course, Every ISP has its own Public IP family. also, i need to know if this will succeed on 2811 router.

thanks in advance

Collin Clark Tue, 05/05/2009 - 05:14
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They run the same IOS, so the platform depends on how much traffic & services you will be running.

diyar_eng Tue, 05/05/2009 - 23:39
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they run the same IOS and they handle the same traffic but the bandwidth is different

ISP 1 delivers 1024/256 kbps

ISP 2 delivers 512/128 kbps

i just need to know if this solution is applicable in routers 2811 and 1841 and how?


Collin Clark Wed, 05/06/2009 - 05:12
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Either platform can handle that throughput. I don't know what you mean by how?

diyar_eng Wed, 05/06/2009 - 05:21
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I meant the redundancy applied in the router 2811 Or 1841.

Collin Clark Wed, 05/06/2009 - 05:23
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You can load balance OUTBOUND, but not inbound. You can create tow default routes.

ip route ISP_A

ip route ISP_B

diyar_eng Wed, 05/06/2009 - 07:49
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This default routes you configuration in the router redundancy:

redundancy_router(config)#ip route serail ISP2

redundancy_router(config)#ip route faEth0/0 IPS1

Thanks for help

kplunkett512 Wed, 05/06/2009 - 13:28
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The way I'd do this is to get a /24 address block from your primary ISP (whichever you choose). Use BGP routing so that remote access to your PC's will use the same public IP. To do so, you need the following:

1. AS number from carrier A and carrier B

2. Your own ASN. You can get this from ARIN by going to Get an org ID, then a POC (point of contact), then request the ASN. You'll need to provide this to the carriers.

3. Let the carriers know you're multihomed.

4. install the following configuration.

router bgp {your AS number}

no synchronization

bgp log-neighbor-changes

network {network /24 from primary) mask

neighbor {serial IP to primary} remote-as {ASN of primary carrier}

neighbor {serial IP to primary} route-map SET-LOCAL in

neighbor {serial IP to backup} remote-as {ASN of backup carrier}

neighbor {serial IP to backup} route-map foo out

no auto-summary


ip route {network /24 from primary} Null0



route-map SET-LOCAL permit 10

set local-preference 5000


route-map foo permit 5

set as-path prepend {your AS number} {your AS number} {your AS number} {your AS number}

You should be good from there. This way, if carrier A fails, the same IP block will be routed (both inbound and outbound) through carrier B. This allows easier access to your public IP's that are either NAT'd, or directly on your PC's. Let me know if you need any help or clarification. We use the config quite a bit.


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