On 03/09/2011 03:43 PM, Chris Adams wrote:
I have run into an odd issue with ARP on an EX switch that I think is a
bug in JUNOS, but I wanted to see what others thought before I tried
JTAC (maybe I'm missing something).
I have an EX2200 switch that cannot talk to one of my recursive DNS
servers. The switch is in subnet a.b.c.0/27, while the DNS IP is in
x.y.z.0/29. The DNS IP is anycasted, and the primary server serving it
is in the same a.c.b.0/27 subnet as the switch (the DNS IP is a
secondary IP on the same interface).
When the switch tries to reach the DNS IP, it sends the packet to the
default router. The router sends it to the server, and the server sends
an ARP request for the switch's IP. The sending IP address in the ARP
request is the DNS IP. As far as I can tell, JUNOS doesn't send a
response to the ARP request.
Yeah, we see this a lot. You need the following in /etc/sysctl.conf:
# These values make linux be sensible about making and replying
# to ARP requests - specifically they force ARP requests to come
# from an in-subnet IP, and ignore ARP replies for out-of-subnet
# addresses
net.ipv4.conf.all.arp_ignore = 1
net.ipv4.conf.all.arp_announce = 2
...and then you need to move the anycast /32 to the loopback (lo) interface.
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