[c-nsp] ARP and less specific interface entries

[Frank Bulk]

You're right, of course, there's just two types of ARP entries, and in my
case, they were incomplete.

interface FastEthernet0.5
 description George
 encapsulation dot1Q 5
 ip address a.b.c.1 255.255.255.0 secondary
 ip address 10.1.3.1 255.255.255.0 secondary
 ip address 10.1.4.1 255.255.255.0 secondary
 ip address 10.1.0.1 255.255.0.0 secondary
 ip address a.b.d.1 255.255.255.0
 ip access-group 101 in
 ip verify unicast reverse-path
 ip helper-address w.x.y.z
 no snmp trap link-status
!

Why won't overlapping subnets work on an interface?  What does that have to
do with the router's ability to ARP for an unknown MAC address?  It's the
clients that are key, right?  If they have the right mask and point to the
right gateway, the packets should be accepted by the router.  And as for the
router forwarding traffic to the clients, if they're locally connected,
whether they are more broadly or narrowly defined as being locally
connected, it just needs to ARP?

Regards,

Frank

-----Original Message-----
From: Peter Hicks [mailto:peter.hicks at poggs.co.uk] 
Sent: Monday, March 17, 2008 2:18 AM
To: frnkblk at iname.com
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] ARP and less specific interface entries

Frank Bulk - iNAME wrote:

> We have some devices with management IPs in the 10.1.0.0/16 range that I
> manage and I needed to split up into two groups.  All the devices were
> statically assigned an IP address in the form of 10.1.3.x/255.255.0.0, so
I
> added two more secondaries for router interface fa0.5: 10.1.3.1/24 and
> 10.1.4.1/24, the two desired groups.  We then re-IPed the devices from
> 10.1.3.x/16 to 10.1.3.x/24 and 10.1.4.x/24.
>
> NAGIOS went ballistic, claiming that devices from both 10.1.3.x and
10.1.4.x
>  were down.  If we attached a PC in that network with a 10.1.3.x/16
address
> it could ping them just fine.  If we rebooted the device we were sometimes
> able to ping it for a while, but not always.  That led me to believe it
was
> an ARP issue on the router.  The 1721 running 12.4(6)T
> (c1700-ipbase-mz.124-6.T.bin) had all kinds of incomplete ARP entries for
> those unpingable IPs.

There are only two types of ARP entry - complete and incomplete.

Can you post your interface configuration, please?  It's not clear whether
you're using secondary addresses or sub-interfaces.

It appears that you're trying to work with overlapping subnets on interfaces
the same router, which won't work (unless you're using different VRFs).

Peter

--
Peter Hicks | e: my.name at poggs.co.uk | g: 0xE7C839F4 | w: www.poggs.com

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