LARTC - Jan 2005 - equal cost multipath routing and incoming services...

We''re trying to figure out how to have equal cost multipath routing
using OSPF (quagga) and have come up with the following...

Has anyone done something similar? -- How do you deal with incoming services
(mail/web/etc) in such a scenario as below - And does anyone have any
suggestions?

There''s more detail below....


Diagram and overview:

Network A connects to Core1 and Core2 via 100mbit ethernet, and uses
a virtual default gateway IP address provided by UCARP. Core1 and
Core2 are each connected to Border1 and Border2 via 100mbit
ethernet links. Border1 is connected to ISP1 via a T1, same thing
with Border2 and ISP2. Remote1 is on a T1, Remote2 is on a DSL line.
Each remote router connects via IPSec to both Border1 and Border2.
We run GRE tunnels over these IPSec connections and route traffic
over the GRE tunnels using OSPF with Quagga. This allows us to do
failover and load balancing.

The core router tier does not exist right now. We want to put it in
place so that the OSPF costs will be the same for either path to
each remote site.

We desire to continue to have incoming services hosted by servers on
Network A. While packets will come in from, say, ISP1, be NAT''d by
border1, and arrive at the server on Network A without a problem,
we need to ensure that core1 and core2 route those return packets
to border1 instead of border2. Likewise for the services we want to
host using ISP2.


        [Network A]
         |       |
      [core1]  [core2]
         | \   / |
         |  \ /  |
         |   X   |
         |  / \  |
         | /   \ |
    [border1] [border2]
         |       |
        ISP1    ISP2
         |       |
          \     /
      ( The Internet )
          /     \
         |       |
        ISP3    ISP4         (Networks D through P
         |       |            not shown for clarity)
    [remote1]  [remote2]
         |       |
  [Network B]  [Network C]

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