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New Member

OER with two 1811 and NAT

We are planning to install two 1811 to replace three old routers in our current CEF/PBR based Internet load balance. We want to use OER with the two border router setup. Here is a simple diagram:

SBC/T1---2610 CableModem---TimeWarner

| |

| | (NAT to TW)

| |

(Mstr/brdr)1811a------1811b (brdr)

\ /

\(NAT2SBC) /





Core L3 Switch

I read most of the OER documentations but still cannot fully understand two issues:

1) How does OER ensure symmetric routing for each session when it tries to adjust the traffic load? Obviously assymetric routing will result in broken sessions due to NAT for two different public IP ranges. I have no problem with my current setup because the CEF per destination load balancing agorithm "remembers" the route for each src-dst pair (my current setup is an 3-router setup with two border routers and a 3rd serving as a loadbalancer in between them and a PIX)

2)We use purely static routing and no BGP at this point (too small for it). For the new OER network to work, I guess I need to turn on OSPF between the 1811s and the ASA device. During out-of-policy adjustment, does the master/border router just inject temporary static routers into its own routing table and that of the other border router, and the border routers then redistribute these changes to each other and the ASA through ospf? Can I just leave the ASA out of the ospf stuff and just put two equal-cost default routes on it, and let the two border routers work it out between themselves?

I appreciate it if someone can shed some light on these.


New Member

Re: OER with two 1811 and NAT

sorry for the diagram mess-up.

1) 1811a goes to the 2610 which has a T1 to SBC.

2) 1811b goes to the TimeWarner cable modem. It will also NAT to TimeWarner's IP space.

3) 1811a and 1811b will be linked with a trunk link. They have a built-in 8 port switch.

4) The ASA will also connect to the 8-port switch of a 1811. I left out on purpose a redundant ASA which will connect to another 1811. The ASA will NAT to a class C space assigned by SBC, a.k.a., AT&T.


Re: OER with two 1811 and NAT

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