[c-nsp] Purely Academic: Router swap and EIGRP doesn't work

[Rick Kunkel]

Ran in to an issue yesterday and today and got it resolved, but I'm 
wondering the why...

We had a 3662 configured with EIGRP and everything working fine.  We 
wanted to put in a 3620 for a few days to use the 3662 in our lab, so we 
identically configured it (with a few interface references changed), and 
mounted it in the rack.  When ready, we quickly moved the WAN and LAN 
cables to the "newer" 3620.

But stuff didn't work.  The router could access anywhere through it's 
default route.  It's upstream EIGRP neighbor was listed.  But equipment on 
the LAN side couldn't access the WAN side. I logged into the upstream 
router, and saw the same neighbor relationship.  However, the topology was 
missing.  The upstream wasn't hearing the routes from the 3620.

We'd spent quite a bit of time getting to this point, but now that I knew 
that, I went into the 3620, removed a network statement and put it back, 
and -- viola! -- that block routed now.  I did the same for the rest, and 
all was fixed.

My question is, what causes this?  The neighbor relationship was working, 
but the upstream didn't see the routes from the 3620.  Did it have to do 
with changing the cables quickly?  I'd chalk it up to a one-time fluke, 
but we did it over and over yesterday, and it never worked.  Furthermore, 
whenever we switched it BACK to the 3662 that we were trying to pull out, 
it worked instantly....

Purely academic at this point...

Thanks,

Rick