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Beginner

Need Help for Redundant Link Config

Hello,

I have a 1751 Router which connect to ISP1(Lease Line). For the Internet Connection, I have faced many problem with the connection, sometimes disconnect(not stable). So that's why I decided to connect to one more ISP( ISP2, SDSL). What am i going to do is to link the router 1751 to both ISPs.

In this case,could anybody help me to config the router for redundant link, meaning that when ISP1 disconnect, the router will route to ISP2 automatically.

If you need some more detail info. plz let me know. Thanks in advance.

Regards,

Sakdavin

8 REPLIES 8
Advisor

Re: Need Help for Redundant Link Config

In this case, since the primary line is a leased line, its line protocol will go down, if the connection to ISP 1 breaks. So you can simply set two default routes

ip route 0.0.0.0 0.0.0.0 serial 0

ip route 0.0.0.0 0.0.0.0 150

Beginner

Re: Need Help for Redundant Link Config

Hi Sakdavin/Shanky,

Adding default route only is not going to help. You need to confirm what IP addresses you are using in your network. If these IP address subnets are provided by ISP1, then backup is not going to work with ISP 2 as they will not entertain request coming from source IP of ISP1. You need to do NAT/PAT in your router, if you are using this link only for Internet access.

Beginner

Re: Need Help for Redundant Link Config

Hi Gaurav/Shanky,

Thanks for your both reply.

Gaurav's solution is absolutely right. Coz now we have a Firewall which connects directly to the router(connect to ISPs) also use public IP address of ISP1. In this case if ISP1 disconnect, so the public ip of ISP1 become private ip address when we perform NAT to ISP2. Am i correct?

Could you plz take a look at my attachment file and if possible, plz advise me how to config NAT/PAT, and route to each ISP. Coz I need to implement this system very soon.

Thanks in advance and your help would be appreciated.

Regards,

Sakdavin

Highlighted
Beginner

Re: Need Help for Redundant Link Config

Hi Sakdavin,

The simplest config is to do PAT using WAN Interface IPs of the links of respected ISP & add two default routes to both ISP links (one with highr metric) in router. Firewall IP will be RFC 1918.

Advisor

Re: Need Help for Redundant Link Config

Gaurav,

I know you have to use two nat pools appropriately,but I was too lazy to find a link and post it along with my post.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093fca.shtml

Re: Need Help for Redundant Link Config

Hi,

Use the routemap method to choose between two pools via two different interfaces.

There is a match criteria based on ip next-hop

match ip next-hop

Please try yourself with the help of following link:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093fca.shtml

regards

Krishnamurthy Suresh

Beginner

Re: Need Help for Redundant Link Config

Hi all,

Thank you so much for your help, and now everthing works fine after re-configure as your advice.

Regards,

Sakdavin

Contributor

Re: Need Help for Redundant Link Config

Last time I tried this, I discovered that the NAT policy routes were only applied when the NAT is initially set up. As a result, when a failure forced traffic from one ISP to the backup ISP, all open connections would be hung and only new connections would work. The problem would repeat when going back to the primary ISP (for any NATs set up while on the backup).

Are you sure everything is working fine? Policy maps for NAT/PAT are finally getting applied on every packet rather than only at translation setup time would be a major advance. If this is actually the case, could you share with us what IOS release, what feature set, and what router platform you are using?

Note that NAT/PAT policy routing works fine if only one path is "outside". But I got the impression that you were NATting in the one router for both ISPs.

Vincent C Jones

www.networkingunlimited.com

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