[LARTC] Problems in Dead Gateway Detection / Failover - Multiple

[Manish Kathuria <manish@tuxspace.com>]

Hello,

I have configured a load balancing router using Julian's patches and as 
described in "nano.txt" for two ISP links as shown below.



              ISP 1                   ISP 2
                .                       .
                |                       |
                |                       |
                |                       |
                | WAN              WAN  |
              +-\-+                   +-\-+
              |   |                   |   |
              |R1 | GW1           GW2 |R2 |
              |   |------.    --------|   |
              |   |      |    |       |   |
              +---+      |    |       +---+
                    EXT1 |    | EXT2
                        +\----\-+
                        |       |
                        | LINUX |
                        | ROUTER|
                        |       |
                        |       |
                        |       |
                        +---/---+
                            | INT IF
                            |
                            |
                            |
                    /----------------\
                    |      LAN       |
                    |                |
                    \----------------/


LAN NETWORK = 192.168.100.0/24
INT IF = 192.168.100.1

ISP1 NETWORK = 10.20.30.128/29
R1 - ROUTER1
GW1  = 10.20.30.129
EXT1 = 10.20.30.130

ISP2 NETWORK = 172.16.32.128/29
R2 - ROUTER2
GW2  = 172.16.32.129
EXT2 = 172.16.32.130

Both the ISPs have provided /29 subnets of Public IPs. The above 
mentioned addresses are just for example.

The gateways for both the ISPs are routers placed at the same location 
which are further connected through Radio Link and Leased Line.

Things work fine as long as both the ISP links are alive. While testing 
the dead gateway detection and failover functionality we observed that 
if we make the first hop gateway (i.e Router R1 or R2) of one of the 
ISPs dead by either disconnecting the ethernet cable between Linux 
Router and R1/R2 or by switching off the gateway (R1/R2) itself, dead 
gateway detection takes place and failover to the other ISP takes place. 
  However, if there is a problem in the ISP connectivity at any of the 
subsequent hops, there is no dead gateway detection and failover also 
does not take place. I have tested this on various linux kernels from 
2.4 as well as 2.6 series.

Somehow I have never faced a similar problem before and things have been 
working perfectly. In real life situation here, the first hop gateway is 
rarely going to be down so dead gateway detection and failover is going 
to be required whenever there is some connectivity problem at any of the 
later hops. So that's where dead gateway detection needs to work.

What could be the reason ? How can this be resolved ? I would appreciate 
any pointers or suggestions.

Thanks,

Manish Kathuria
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