[j-nsp] Strange behavior on directly connected interfaces?

Hannes Gredler hannes at juniper.net
Tue May 16 16:22:30 EDT 2006


looks like you have a forwarding loop in your setup;

for further troubleshooting attach a packet-sniffer to
the subnet in question and spot for the source MAC-adress
that is bouncing back your traffic.


Frances Albemuth wrote:
>  Hi,
>  This is my first post to the list and I would like to preface this by
> stating that I doubt this problem is actually related specifically to
> Juniper equipment (perhaps a configuration error involving Juniper
> equipment, however). I'm hoping the issue I'm working on right now
> might ring bells in the heads of others, and in any case I figure this
> is as good a place as any to find yourself beaten by the clue stick.
>   I have a directly connected interface facing a large, flat Ethernet
> infrastructure.  There are dozens of IP's mapped to the interface in
> question (this is a legacy aspect of the design, but migration to a
> more hierarchical infrastructure is a long process).  Periodically,
> when packets are transmitted with an unreachable destination IP
> residing on the directly connected interface,  a massive series of
> ICMP TTL exceeded packets is returned by a different host residing on
> a different logical interface.  Traceroutes to the unreachable IP
> similarly show a one-node loop (the same IP responds until the TTL=0).
>  The node is always the same, but if unmitigated ICMP traffic is
> permitted to and from addresses on the logical interface, sniffing the
> wire shows this behavior occurring to and from a number of nodes.  I
> haven't managed to duplicate the multi-node behavior in a
> semi-controlled environment.
>   When sniffing the segment in question, the ICMP is clearly visible,
> so for whatever reason it is universally broadcast, even though both
> nodes involved in the ICMP communication are legitimate unicast
> destinations.  If a ping is left running, these TTL exceeded messages
> will continue an accelerate ad nauseum until a de facto pseudo
> broadcast storm occurs, crippling access on every switching node where
> the VLAN in question is mapped.  Usually (but not always) the
> anomalies halt when the ping is killed.  The issue is largely
> mitigated by denying all ICMP to and from addresses mapped to the
> logical interface.
>   That's all I'm comfortable asserting about the issue at this time.
> What I'm really digging for here is an explanation as to why when the
> Juniper tries to transmit to an unreachable node, it doesn't discover
> the node is unreachable due to a lack of response from an ARP request
> and return ICMP unreachables on it's own.  I may have missed something
> obvious here (I'm sort of hoping so) and would appreciate any
> suggestions or experience from others.  If I've sent this message to a
> woefully inappropriate list I would greatly appreciate a suggestion as
> to a better place to bring my question(s).
>  Thanks,
>   -FC
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