On Thu, 25 Feb 1999, Eric van Dijken wrote:
> Hi,
>
> Recently we ran into the following network problem:
>
> Network topology:
>
>
> Production backbone
> --------+----------------------------+-------------
> | ---> 1 |
> | <-- 2 |
> mac1 | ---> 3 mac2 |
> ip1 | ip2 |
> .------------. .-------------.
> | | | |
> | FW | | MHH |
> | | `-------------'
> | | mac4 | ip3
> | | ip4 | mac3
> | |----------------------+--------
> | | <--- 4 Maintenance backbone
> `------------' ---> 5
> | <--- 6
> |
> |
> |
> .------------.
> | |
> | PC |
> | |
> `------------'
>
> The firewall FW routes 'forward' traffic from PC to the ip2 via the
> production backbone. The Multi-Homed-Host MHH routes the return traffic
> via the maintenance backbone.
>
> The FW reports changes in MAC address for the production interface of the
> MHH, ip2. Running tcpdump on FW reveals the following scenario:
>
> 1. PC tries to connect to ip2 (HTTP request
> 2. FW issues an arp-request (asking for mac2) on the production
> backbone: packet 1
> 3. MHH replies with an arp-reply: packet 2
> 4. FW sends first IP packet to MHH: packet 3
> 5. MHH issues an arp-request (asking for mac4) on the maintenance
> backbone: packet 4
> 6. FW replies with an arp-reply: packet 5
> 7. MHH sends first IP return packet to FW: packet 6.
>
> This should work but the arp-request from MHH, packet 4, contains unexpected
> information:
>
> source hardware addres = mac3
> source protocol address = ip2 (I expected ip3)
> destination hardawre addres = NULL
> destination protocal addres = ip4
>
> This packet forces the FW to change its arp-cache: the mac addres for ip2
> is set to mac3. This effectively blocks all traffic between PC end MHH
>
> How to solve this ?? Regrettable we cannot change the routing configuration.
>
> Greetings, Eric.
>
This looks complex, but probably becomes simple once you see that
there are duplicate routes created. You need to force a preferred
route which is usually done using the keyword "metric". The lower
the number, the most preferred.
Cheers,
Dick Johnson
***** FILE SYSTEM WAS MODIFIED *****
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