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

Hannes Gredler hannes at juniper.net
Wed May 17 05:38:15 EDT 2006


to mitigate the problem while diagnosing
you could configure a firewall that discards
traffic from non-local-subnet sources.

but lets focus on the loop:
   what is the mac-adress of the mystery node ?


Frances Albemuth wrote:
>  Hi Hannes,
>  Thanks for your response.  When I'm sniffing on the segment I see a
> massive stream of ICMP TTL exceeded messages being returned by the
> "mystery node".  The topology is definitely loop-free and the
> "loop-ish" behavior that we see only seems to occur when data is
> transmitted to unreachable destinations.
> I assume by forwarding loop you mean an Ethernet loop? I would agree
> that it behaves this way in some respects.  Of course, if I had a
> genuine loop the problems would be more serious and would occur
> regardless of routed traffic (the Ethernet topology with a handful of
> hosts would cripple itself).
>  Also interesting: the node returning the TTL exceeded "storm" lives
> behind a link with a maximum synchronous capacity of 10M.  The "storm"
> itself results in 10M of traffic pushing consistently over all ports
> where the VLAN lives.  It thusly only cripples other devices with a
> 10M maximum synchronous bandwidth.
> Thanks!
>  -FC
> On 5/16/06, Hannes Gredler <hannes at juniper.net> wrote:
>> frances,
>> 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.
>> /hannes
>> 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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