Re: The use of .0/.255 addresses.

[Jon Lewis <jlewis () lewis org>]

On Sun, 27 Jun 2004, Stephen J. Wilcox wrote:

> Hi Jon,
>  I currently have a few .255/32s with Cisco and Foundry products and have
> various windows/linux/OSX machines that access them without problems..

I'm pretty confident this is a classful/classless bug in 12.1T.  I just
got into the customer's router that was sending what looked like replies
to a broadcast ping, and that's just what it was.  Here's the output from
debug ip packet on the CPE that was replying when I pinged 209.208.6.255
from the 7206.

IP: s=209.208.6.xyz (Serial0.2), d=255.255.255.255, len 100, rcvd 2
IP: s=209.208.6.xyz (Serial0.2), d=255.255.255.255, len 100, rcvd 2
IP: s=209.208.6.xyz (Serial0.2), d=255.255.255.255, len 100, rcvd 2

Checked with an ACL on the input side of the CPE's serial subint,
*Mar  3 08:40:19: %SEC-6-IPACCESSLOGDP: list test permitted icmp
209.208.6.xyZ (Serial0.2 DLCI 100) -> 255.255.255.255 (0/0), 1 packet

where 209.208.6.xyz is the customer's serial IP, and 209.208.6.xyZ is the
7206's serial IP.

Both ends to have no ip directed-broadcast.

> > I found some time ago that my home DSL connected network could not reach
> > (telnet, ping, etc.) that router's loopback.  Our monitoring system could,
> > and several iBGP peers could, so I didn't notice the issue until one night
> > when trying to do some work from home.
>
> I could see the problem with DSL's where the provider may be interfering..
> surprised about your monitoring tho...

No...I said the monitoring system didn't have a problem with it.  It
fortunately doesn't have to transit the affected router which only handles
T1 & DSL aggregation (including my home DSL).

> > #sh ip ro 209.208.6.255
> > Routing entry for 209.208.6.255/32
> >   Known via "ospf 1", distance 110, metric 20, type extern 2, forward
> > metric 4
> >   Last update from 209.208.16.29 on FastEthernet0/0.1, 00:46:47 ago
> >   Routing Descriptor Blocks:
> >   * 209.208.16.29, from 209.208.6.255, 00:46:47 ago, via FastEthernet0/0.1
> >       Route metric is 20, traffic share count is 1

#sh ip cef 209.208.6.255
209.208.6.255/32, version 12215105, cached adjacency 209.208.16.29
0 packets, 0 bytes
  tag information set, shared
    local tag: 398
    fast tag rewrite with Fa0/0.1, 209.208.16.29, tags imposed: {114}
  via 209.208.16.29, FastEthernet0/0.1, 0 dependencies
    next hop 209.208.16.29, FastEthernet0/0.1
    valid cached adjacency
    tag rewrite with Fa0/0.1, 209.208.16.29, tags imposed: {114}

#sh tag-switching forwarding-table 209.208.6.255 detail
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id      switched   interface
398    114         209.208.6.255/32  0          Fa0/0.1    209.208.16.29
        MAC/Encaps=18/22, MTU=1520, Tag Stack{114}
        0001638B90000005DC493400810000018847 00072000
        No output feature configured
    Per-packet load-sharing

It knows the next hop is another 7206 with connections to the rest of our
network.  Why is it sending this out as a broadcast ping instead of
routing (tag switching) it?  I know...wrong list.  I'll ask on cisco-nsp,
but the operational lesson here is that it's not just the junk from
Redmond that may have classful/classless IP routing issues.  Even your
core routers might, depending on IOS versions.

----------------------------------------------------------------------
 Jon Lewis                   |  I route
 Senior Network Engineer     |  therefore you are
 Atlantic Net                |
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