Multiple ISP VLAN Trunking - Cisco 2620

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Dec 24th, 2009
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I am still a bit new to the Cisco products and I am not sure if this is even possible...  I have a situation where I have multiple ISP's and I have the following equipment for routing:  Cisco 3524XL Switch, 2620 Router, 506e PIX  What I need to know if this is even possible to have redundancy if one of the ISPs were to go offline.  My 2620 only has one Fast Ethernet port and so I was thinking about setting up multiple VLANs and Trunk them onto the 3524XL switch.  I have a picture attaced to show my layout that I was thinking...  Am I just out of my mind???


Doug Engle

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Peter Paluch Fri, 12/25/2009 - 05:38
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Hello Doug,

You have proposed a common scenario - yes, it would work. You can indeed create VLANs on the 3524XL switch so that one VLAN represents a link to ISP_A, another VLAN represent a link to ISP_B and yet another VLAN will represent your internal company network. The router will be configured with what we call subinterfaces that represent the router's virtual interfaces into individual VLANs and so it will be able to route and NAT between these VLANs accordingly.

What I did not completely understand is the placement of your PIX at the very right of the picture, without any other device connected to it. What did you mean by that?

There is a problem in your topology how should your router know that an ISP has failed and it should use the other ISP for redundancy. If the ISPs run some sort of routing protocol towards you then it should not be difficult - the ISP will be responsible for originating a default route to you so you will know which ISP is up and running. If, however, there is no dynamic communication between you and your ISPs, some other sort of ISP "liveness" has to be employed. A commonly used method is the IP SLA - an automatic probe that the router runs periodically (for example, pinging the provider's edge router), and tying the presence of a static default route through a particular ISP to the success of the IP SLA probe. If the probe fails, the route will not be present in the routing table. If the probe succeeds, the route will be reinstalled back. The 2620 routers with a recent IOS operating system should support at least the PING IP SLA mechanism which is sufficient for your needs.

Best regards,


1234Sprint Fri, 12/25/2009 - 21:56
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Thank you very much. I had everything else figured out but the IP SLA Feature. Like I said, I am still new to this game. That little information is all I needed and so I researched the syntax and now it works like a charm...


Peter Paluch Sat, 12/26/2009 - 00:53
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Hi Doug,

I am glad you got it working.

Best regards,



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