Dynamic routing and frame relay

Posted: October 13, 2010 in Infrastructure Management and Disaster Recovery

Had an interesting evening building (emulating) a resilient, dynamic routing LAN. That was quite straightforward, using Cisco’s venerable EIGRP protocol. This was simple enough to configure and test, and was strangely satisfying to see packets reaching their intended destination no matter what I did to the route in between (so long as there was always a possible route). Mesh LAN. Nice.

The rest of my time was spent trying to get frame relay encapsulation working properly, so I can emulate the internet cloud in Packet Tracer. This works fine for the directly-connected devices, but at the moment I can’t get once-or-more-removed subnets to understand the routing path. So R1 > Cloud > R2 is fine but R1 > Cloud > R2 > R3 is not fine. Watching the packets in simulation mode is very interesting, because contrary to what you might expect, the packets do actually reach the destination (R3) but then they just disappear. As far as I can tell R3 makes no attempt to respond. Some sort of encapsulation error probably. Either that or a bug in Packet Tracer.

Tomorrow I’ll be simulating stretching out a VPN over the Cloud. And, I suspect, negotiating the release of some additional Cloud IP addresses with my lecturer! The brief calls for 9 geographically disparate sites to talk to each other. I want to do this using VPNs, but we only have two public IP addresses available to us (or at least only two specified in the brief). Negotiating hat on.


  1. Stephen says:

    Hi Steve
    Use the ‘show frame-relay map’ to check the DLCI number maps.
    It would be worth checking again that each interface has frame-relay encapsulation and the right DLCI number assigned.
    It’s been a while since I messed around with this, have fun ;-)
    Cheers, Stephen

    • rymixxx says:

      Thanks Stephen. All seems fine to me. I am successfully communicating over the frame relay, but only if the traffic starts from a device ‘directly’ connected to the relaying routers. Whack another router in between somewhere and the route just fails.

      The thing is, packets DO reach their destinations from left to right and from right to left but never are they returned. I’m pretty sure it’s broken encapsulation somehow. Getting a bit suspicious of Packet Tracer, especially its fairly dodgy frame relay emulation.

      I’m going to try this in the physical lab tomorrow and see if I get the same symptoms. That is, if I can get three routers all to myself…

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