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

Frances Albemuth frances.cincinattus at gmail.com
Tue May 16 00:18:18 EDT 2006


 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).



More information about the juniper-nsp mailing list