Reading through O'Reilly's Network Warrior in the chapter "Ethernet Resiliency he describes the following Scenario (Trying to keep this in the limits of fair use):

alt text http://www.kbrandt.com/files/HSRP_Limitation.png

"Assume that the link between New York and Los Angeles suffers a partial outage. Something has happened to cause the serial interface on NY-Primary to enter a state of up/down, but the serial interface on LA-Primary has stayed up/up. I’ve seen this more than once on different kinds of circuits.

HSRP responds to the down interface on the New York side by making the NY- Backup router active because we’re tracking the serial interface on NY-Primary. Packets are forwarded to NY-Backup, and then across the T1 to LA-Backup, which forwards them to their destinations. The return packets have a problem, though. As the LA-Primary router does not recognize the link failure on the primary T1, it has remained the active router. "

It then goes on to say:

"A more robust solution to a link-failover scenario is to incorporate an interior gateway protocol running on all of the routers. A protocol like OSPF or EIGRP establishes neighbor adjacencies across links. When a link fails, the routing protocol knows that the remote neighbor is unavailable, and removes the link from the routing table."

My Situation, and Question:
This seems like what I might be looking for. I will have to links from Datacenter provider and I have to routers I would like to have be redundant. HSRP can help me on the LAN side, but since their routers can't talk to mine, HSRP from them won't help with WAN redundancy because if one of routers fails the provider's won't know about it. Is EIGRP a possible solution in my scenario like the one described (Assuming a EIGRP ASN can be shared between my router and the datacenter, this normal?) ? Can anyone explain how this works with an example, or have a link to something I can read that shows examples of how this works?

In my case, the provider will own the address and not myself, so they advertise them on the internet and have redundant providers for the ip addresses. Also, my goal is failover, not balancing.

Here are some related questions on my recent exploration of network redundancy:
Network Redundancy Example
Getting Started with NIC and Switch Redundancy


I don't think that you will be able to talk your data center provider into sharing an EIGRP ASN, think of them as your ISP and an external network. Also if you have overlapping ip's that could cause an issue especially if you broadcast the wrong network(s) into the ASN.

Now that being said, you are probably getting an Ethernet drop from your data center correct? I don't believe you would see this failure condition (UP/DOWN on one side, UP/UP on the other) with a strait Ethernet drop. The author specifically points out SONET networks - due to link integrity being local on both sides - for doing this. Since link integrity with Ethernet is not local to each side, you would be more likely to see an Up/down & Up/down condition, which would cause both routers to fail over to the secondary links preventing this kind of issue.

BTW: Network Warrior is an excellent book - one of the few tech books i've read cover-to-cover and not just used as a reference.

Your comment got me thinking, they could have you setup in 1 of 2 ways. The first is actually less likely.

  1. You are directly connected to a router - in that case the their router would detect your router going down and adjust accordingly.

  2. You are connected to a edge or core switch (most likely) - In this case you are going to have to talk to them to see how they handle fail-over if your link goes down and work from there. Thinking about it a little more, they would have to be running some sort of routing protocol to detect this kind of failure. My guess though is it is more likely to be BGP or OSPF than EIGRP. But in any case they should have a "way they do that" already.

  • I see how that situation is different. My datacenter will give me two drops, and I have two routers, so I am picturing one drop per router. The problem with HSRP on the WAN side is that if one of my routers goes down, the datacenter's routers won't know it. So with HSRP if my router that goes down is their active one, they won't know to switch over to my other router. – Kyle Brandt Feb 8 '10 at 18:58
  • So on my routers with this setup, I would need the LAN to be dependant on the WAN (Interface Tracking), and then I also need my WAN side to track their active link... – Kyle Brandt Feb 8 '10 at 19:01
  • @Kyle: Yes, you would need interface tracking. See edit for response to 1st comment. – Zypher Feb 8 '10 at 19:28
  • For typical long-haul Ethernet (at least in the UK), you have a local box with a copper Ethernet, then haul it across SDH or PDH, then convert it on the other end. Alternatively, you have switches in-between, so you tend not to have end-to-end link integrity. – Vatine Feb 10 '10 at 12:37

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