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Re: default routes question or any way to do the rebundant

  • From: Adrian Chadd
  • Date: Thu Mar 20 21:44:33 2008

On Thu, Mar 20, 2008, ann kok wrote:
> Sorry all. i don't want to make any argument

Don't worry, the argument is elsewhere. :)

> For me, i really want to get mailling list about
> networking to help. and I heard there are professional
> networking guys in nanog. they might help me.

There's certainly a lot of clue here. Its just coloured by 15+ years
of jaded network and systems support. :)

> I still have many networking questions. 
> for the mtu issue, I couldn't find out until I know
> someone changes the mtu. it really made me panic
> before.
> honestly, telecom company couldn't help me. I still
> don't know how they setup the jumbo frame in their
> side but DSL clients are only using mtu1492.

Approach it scientifically. The trouble with not having exposure to low-level
stuff as a pre-requisite for doing higher-level stuff is that you've probably
missed out on all of the boring details that you could feed into solving the
issue methodically. Path MTU discovery pops up as one of those things you'd
think about after you learn about ICMP and PMTU in an intro networking course
or book.

(Or in my case, junior sysadmin, IRC and hanging around NANOG/RIPE meetings..)

A lot of modern CPEs will actually rewrite the MSS of the TCP connection
to make sure frames aren't bigger than the ISP provided MTU, thus trying to
avoid PMTU. The trouble is that devices -other than the ISP/CPE- could be
filtering PMTU, and sometimes its unavoidable to run MTU < 1500 to the client.

(in fact, on a completely side note, sometimes you -want- to run small
client-facing MTUs.)

> Another question about private address, my router
> upstream interface can listen many private address.
> I asked the upstream ISP but they said they don't have
> any private address export. 
> we have /30 connect to them. where is the private
> addresses coming?
> have you encountered this problem?

Which private addresses? A number of ISPs will use RFC1918 addresses
on PtP links to clients (and their dial infrastructure!), assigning
real public IPs on the PPP end-points. Some others (like my 3G mobile
broadband provider) run their entire dial infrastructure and end-user
addressing on RFC1918 and do NAT elsewhere.

"Private address export" needs defining too?