Understanding Frame Relay ...
    29 June 08

    Understanding Frame Relay Mappings to

    Posted byBrian McGahan

    Hi Brian,

    I ran into these nasty frame relay mappings during an initial lab set-up and was wondering why they are there, (even with inverse-arp disabled), and what they are actually doing. I was able to remove them only after writing my configuration to memory, and then performing a reload of the router.

    Router(config-if)#do show frame map
    Serial0/0 (up): ip dlci 113(0x71,0x1C10)
    CISCO, status defined, inactive
    Serial0/0 (up): ip dlci 105(0x69,0x1890)
    CISCO, status defined, active
    Serial0/0 (up): ip dlci 104(0x68,0x1880)
    CISCO, status defined, active
    Serial0/0 (up): ip dlci 103(0x67,0x1870)
    CISCO, status defined, active



    Hi Josh,

    This is actually an error relating to AutoInstall over Frame Relay. When the router boots up and does not have a configuration file saved in NVRAM, it attempts to run autoinstall to automatically find an IP address and download a config. The first thing the router does is to detect the encapsulation on its WAN interfaces, which in this case is Frame Relay. Once the router finds that it's running Frame Relay, it attempts to send a config request via TFTP. In order to do this it needs an IP address, so it sends a BOOTP request out all DLCIs. Since the router doesn't know what the unicast IP addresses are on the other ends of the circuits, it creates IPv4 mappings to for all circuits and includes the "broadcast" keyword on them. This allows the router to encapsulate the BOOTP request out all DLCIs.

    If you haven't actually configured IP helper-address or a BOOTP server, the operation will fail. The result of this that we see is that when Frame Relay is re-enabled on the interfaces the mappings to reappear. In some versions of IOS this can be fixed by removing Frame Relay and re-applying it, for example:

    router#config t
    router(config)#interface s0/0
    router(config-if)#encapsulation ppp
    router(config-if)#encapsulation frame-relay

    In most versions however this does not work. Therefore the way to fix this is just to have the router not do autoinstall on bootup. Since the router does autoinstall because it doesn't have a config saved in memory, the only way to 100% fix it is to save your config to NVRAM (wr m), and to reload.

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