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My Cisco 3825, (ISR?) is running fine now, thanks to a lot of help. We've now gotten another WAN link from a different ISP for redundancy.

What I need is to know how to configure the router to be able to Fail Over if the main link has an issue. I don't want to load balance.

  1. Is this possible?
  2. If so, how, e.g. commands/code to implement this

I've seen something called HSRP, but the article seemed to only reference Catalyst devices, so I'm not sure if it's a thing on my router, and also, it seems to deal more with multiple routers, not a single one with two WAN links.

I will also need to purchase another card to add a port for this new connection, could someone tell me exactly what I'd need?

EDIT: added topology per request Office Network Topology

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  • @RonMaupin Hi, yeah i know. It was just available at the right price at a time when our brand new, yet consumer grade, router was just not cutting it. There's a local guy here who basically has old cisco stuff stacked floor to ceiling so I can likely find what I need if someone can specify.
    – Bruce
    Commented Sep 18, 2017 at 20:21
  • In terms of public IP addresses, what's that look like? Do you have a /28 from each ISP, or what? That's gonna be key to redundancy (inbound). Outbound routing is easy as pie, it's the updating of PE routers that's potentially the challenge (BGP).
    – Ronnie Smith
    Commented Sep 19, 2017 at 1:31
  • do have a basic topology map that you could include? Also, we need to know if you are hosting any Internet applications, e.g. email, multimedia gateways, web server(s), etc. that require the use of the failover ISP link.
    – Ronnie Smith
    Commented Sep 20, 2017 at 4:04
  • Added Topology per request. It's really basic. Again, there's no hosting going on, that's all Cloud. We've had some internet drops which leaves us basically able to do nothing, so we're adding a link for backup. I just need to have a main link, and one that it fails over to when the primary dies.
    – Bruce
    Commented Sep 21, 2017 at 12:59

2 Answers 2

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Couple of things. First, the link may not go down so depending on link down to failover may not actually work. Second is NAT/PAT. Cisco has published a how to for exactly what you are looking to do. See ISP Failover with Default Routes using IP SLA Tracking

Introduction

This document describes how to configure WAN (or ISP) redundancies, wherein multiple WAN links terminate on the same end router. This document also explains how to configure Network Address Translation (NAT) when there are multiple ISP's for internet connectivity and you want seamless failover i.e. when Primary ISP goes down then Secondary takes over with correct NAT with the use of the secondary ISP's public IP address.

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  • Hi Ron,Thanks, that document IS exactly what I've been looking for. However, like you said, the link might go down. Say for instance there's a DNS outage so no traffic can go through. Is there a way to make the router detect that kind of thing?
    – Bruce
    Commented Sep 21, 2017 at 18:11
  • IPSLA'ing to ISP-A's gateway address should be able to detect network layer outage (vs link layer outage). That's the point behind the IPSLA feature (in this network layer context), to have connectivity visibility at the network (vs link) layer. Also, don't forget to enable CBAC or Firewall feature set if you have that licensed/available to you
    – Ronnie Smith
    Commented Sep 21, 2017 at 18:42
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HSRP is for your LAN, not your WAN.

Normally it is very simple if you use static default routes.

On your WAN, you probably have a default route:

ip route 0.0.0.0 0.0.0.0 <next hop>

A static route such as this has a really low AD (default is 1, and lower is more preferred). You can add a second default route pointing to your backup next hop, but assign it a higher AD (for example 10):

ip route 0.0.0.0 0.0.0.0 <backup next hop> 10

The routing table will only install the most preferred default route, and if the interface to that route goes down, then the other default route will automatically be installed in the routing table, and if the primary route comes back up, it will be installed in the routing table in the backup's place.

If you use a routing protocol (eBGP, but that is unlikely, although not unheard of, with your router), you can also use AD to prefer one over the other, or there are other methods using the routing protocol.

Unfortunately, that router and the interface cards for it have reached not only End-of-Sale, but also End-of-Life. You may be able to find a used HWIC-1GE-SFP card.

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  • Both wan links are just incoming ethernet links, nothing special. BGP is definitely not being used :) I'll definitely try the AD thing. What command would I use to be able to view the AD values on the links?
    – Bruce
    Commented Sep 18, 2017 at 20:28
  • Simply show ip route. You are not already using one of the WAN slots for your WAN? You should have one of those cards for your current WAN connection.
    – Ron Maupin ♦
    Commented Sep 18, 2017 at 21:01
  • By the way, the AD is not on the links, it is on the routes. You will only see the one currently in the routing table. The backup route will not be in the routing table unless the primary route goes down. AD lets a router select the most trusted route (lowest AD), but any route through a down interface will be withdrawn from the routing table, makeing the route with the higher AD the preferred route.
    – Ron Maupin ♦
    Commented Sep 19, 2017 at 0:40
  • Ron what about inbound packets? How will they know to take an alternate path?
    – Ronnie Smith
    Commented Sep 19, 2017 at 1:32
  • They will need to respond to outbound packets. He is not advertising his site. Yes, connections will be broken, but simply resending something, e.g. browser refresh page button, outbound should restore the connection. Unless someone is advertising the site, there really is no way around this, but it does keep the network able to use the Internet when the primary ISP connection goes down.
    – Ron Maupin ♦
    Commented Sep 19, 2017 at 1:38

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