I have been asked to with the following network scenario, which
gives routing problems:

We have 2 Digital UNIX boxes on separate LANs (A and B), with a similar
box acting as a gateway (G) on both LANs (with routing enabled between
the LANs); all boxes running "routed".
Both A and B have the appropriate side of G defined as default router.
There is a printer on each LAN (P1 and P2), P1 is an old Phaser 840
printer (formerly Tektronix, now Xerox), P2 is a new Phaser 8500
printer.

(view diagram in fixed-width font)
A P1
___|___|____
|
G
B P2 |
___|___|___|

Machines A and B have been able to print on both P1 for some time.
P2 is a new printer, and although B can print on P2, A cannot (cannot
ping).
(Swopping P1 and P2 did not , i.e. problem was with P2).

I looked at the situation, and said "we need to define a route to P2
back to A", so we setup a default gateway on P2 via G (the default
gateway was previously 0.0.0.0), and hey presto, A can now print on P2.

My questions are two (the second is the most urgent):
1) I am assured that P1 does not have a default gateway setup. How
then could B print to P1? (maybe P1 can recognise that packets come
from G, and reply there?)

2) The complication to the above picture is that in fact there are two
gateways (G1 and G2), acting as Master/Standby. I think they just rely
on RIP to route between the 2 LANs, i.e. if G1 is down, traffic routes
via G2. How do I get P2 to route to A if I cannot have a fixed Default
Gateway?

TIA
Mark

In article <1141752369.152458.147800@u72g2000cwu.googlegroups .com>,
mark.bergman@thales-is.com wrote:

> I have been asked to with the following network scenario, which
> gives routing problems:
>
> We have 2 Digital UNIX boxes on separate LANs (A and B), with a similar
> box acting as a gateway (G) on both LANs (with routing enabled between
> the LANs); all boxes running "routed".
> Both A and B have the appropriate side of G defined as default router.
> There is a printer on each LAN (P1 and P2), P1 is an old Phaser 840
> printer (formerly Tektronix, now Xerox), P2 is a new Phaser 8500
> printer.
>
> (view diagram in fixed-width font)
> A P1
> ___|___|____
> |
> G
> B P2 |
> ___|___|___|
>
> Machines A and B have been able to print on both P1 for some time.
> P2 is a new printer, and although B can print on P2, A cannot (cannot
> ping).
> (Swopping P1 and P2 did not , i.e. problem was with P2).
>
> I looked at the situation, and said "we need to define a route to P2
> back to A", so we setup a default gateway on P2 via G (the default
> gateway was previously 0.0.0.0), and hey presto, A can now print on P2.
>
> My questions are two (the second is the most urgent):
> 1) I am assured that P1 does not have a default gateway setup. How
> then could B print to P1? (maybe P1 can recognise that packets come
> from G, and reply there?)

Maybe G performs proxy ARP? If you run tcpdump, do you see P1 sending
ARP queries for B's address, and G responding to them?

> 2) The complication to the above picture is that in fact there are two
> gateways (G1 and G2), acting as Master/Standby. I think they just rely
> on RIP to route between the 2 LANs, i.e. if G1 is down, traffic routes
> via G2. How do I get P2 to route to A if I cannot have a fixed Default
> Gateway?

If G1 and G2 are Cisco, enable HSRP. If not, see if there's a VRRP
implementation for them.

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***