Get Creative: Static Routing with Catalyst 3750

Here's an interesting scenario:

We have two sites, each using a Catalyst 3750 switch, and routing between them using static routes. There's a primary fiber link between them and we're using twisted-pair-to-fiber converters due to port limitations on Cat3750. These converters do not report fiber link down status correctly (the carrier is still present on twisted pair even if fiber is down), so the primary Ethernet interfaces do not go down if the fiber link breaks and the primary static route is not removed, requiring manual action to switch over to the backup link.

The setup is summarized in this diagram:

Network diagram

Network diagram

My initial reaction was a polite answer explaining that the dynamic routing protocols were invented to handle scenarios like this one, but the poor guy responded that "his boss does not want to hear about a dynamic routing protocol."

I've got a few other ideas in the meantime (at least one of them working perfectly), but let's hear it from you first… what would be your solution to this problem?


  1. Gre tunneling + floating routes (if the 3750 supports it). When the Gre tunnel goes down, first route gets removed from routing table. (Ugly)


    SPT + Inter VLAN Switching(not sure).
    Every ethernet interface goes into this vlan, and routes points to the next hop(not interface). SPT should handle L2 connectivity issues.

    Are the same than yours?
  2. udld port aggressive
  3. 1. Object tracking using IP SLA, pinging the remote end of the fibre link and removing the static route if this link goes down. (...but would it cope with the interface being 'up' again after failure and routing over to the radio?)

    2. Spanning Tree, blocking the radio interface.

    3. LACP, bundle both and use all the bandwidth.
  4. Yeah. The best way is to have etherchannel between them. In case of failure port will get down, and then less preferable route over wireless will do the trick. :)
  5. BFD with floating static
  6. Ivan, its very interesting problem. Please give some comments about UDLD and BFD usability to detect failures of fiber channel between converters... its very common case.
  7. Mikhail an anon, AFAIK, there is no support for bfd on 3750, and it's definitly that bfd will not be supported for static routes on 3750 (6500 sxf still don't have it).
    so etherchannel, or udld. but udld needs additional configuration with errdisable stuff, that's why, imho, etherchannel is better :)
    and yes, it's a very common case :)
  8. Everything you do to cope with this issue will be dynamic routing. Maybe not named explicity. The simplest way is to change the boss.
  9. Actually, LACP over unequal bandwidth paths (not interfaces) may cause out-of-sequence frames, not packets, which will probably have a rather bad effect on throughput.
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  12. “his boss does not want to hear about a dynamic routing protocol.”

    The boss is an idiot and should be educated or removed. The questioner's time would be best spent finding out why the boss has an irrational fear against the right tool for the job.
  13. so, had anyone solve that problem with logical link checking on the 2nd lvl, coz i've the same problem? On the 3nd lvl the solvation is track+rtr+route-map.
  14. oh.. thanks. i'm investigating ur blog now. many new useful things for me.... thanks a lot Ivan.
  15. Why not configure the path cost and assign a priority port.
  16. Coming in a bit late here, but...

    Has the OP looked at how the media converter is set up? Many media converters have a test mode where a down link status is not passed from one side to another. Once you take it out of test mode, it will pass status. Usually it's a small push button, or a switch.

  17. Make the links L2 links on the same VLAN, with the backup link spanning tree's cost higher.

    Configure VLAN interface on both 3750, on on that VLAN use the smallest possible MAC address table timeout. Spanning tree will do the rest....

  18. This is a common issue when using media converters. Problem is that line protocol is not being passed from the fibre to the switch. Even if the fibre drops, the switch still (rightly) sees line-protocol UP between itself and the media converter.

    Usually there are some dip switches you can flip that will allow the unit to pass line-protocol. Check the documentation or manufacturers FAQ section.

    With the line-protocol issue fixed you just need to use static routes which include the interface syntax. Give the radio link a higher metric. Ethernet link drops, route gets removed and other link is used
  19. The original problem was that somehow they could not get the link status indication from the fiber side of the converter to the TP side of it. If you get that fixed, the rest is easy (static/floating static routes).
  20. Hi,
    I have the same network schema, but I have no problem using RIP. I'll to use RIP for automatic route change in case that one of them fails.
    The problem is that I want application traffic to work on the main line and the rest of the traffic on radio.
    Any idea how can i do that? QoS based on destination address?iZvyk
  21. c3750 support object tracking with ip SLA.the combination of both can let use use static routing without ugly dynamic routing protocol.u need cisco ios 12-2.40S or later to do the is the best.
  22. Well, i think that you should or do Etherchannel whith those links or use EIGRP, this is a dynamic routing protocol, i know, but this do a tracking of the neighborhood and can help when one device that should be there in the other side is not.
  23. I came across this post while searching for a similar problem of my own:

    Please consider the following:

    FWSM-6509 (Act) -> 3750E -> managed IPS -> 7206vxr-> ISP A
    | | |
    FWSM-6509 (Sty) -> 3750E -> managed IPS -> 7206vxr-> ISP B

    With the externally managed IPS devices (bridged interfaces that don't forward BPDU's) between the border router and the perimeter switch-stack, links on the 3750E's stay up even if a border router fails / reloads for maintenance, etc (mush like the media converters in the original problem). I have come across many of the same solutions that others have presented here only to discard them as:

    STP doesn't work since the IPS device discards BPDU's.
    LACP is not supported on the 7206vxr (even though lacp is listed as a supported feature on 12.4(24)T2). Etherchannel works fine but if only one link fails
    UDLD aggressive doesn't register the link loss

    The IPS devices have multiple in/out interfaces so I had created cross-stack etherchannels between the 3750 switch stack and each 7206vxr, attempting for greater redundancy - it seems that single links to the 7206 routers would make fault detection much simpler. I would prefer not to run a dynamic routing protocol in this area of the network, but I will if it is the "best" solution.

    Eagerly looking for the other 'in the meantime' solutions...
  24. This post is over two years old, the follow-up summary is here:
  25. cant we use bfd both side on static routes ...
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