Failover setup with 2 PTP links

[sahmad]

Here is the setup:

2 2600 routers on one LAN on the same subnet. The serial interfaces are connected to 2 point to point links that terminate into 2 other 2600 routers, each with a fastethernet on a separate LAN. Example:

Lan 1 - fa0/0 - 10.10.10.1/24 > s0/0 - 172.16.1.1/30 - PTP link - 172.16.1.2/30 s0/0 > 192.168.2.1/24 - fa0/0 - LAN2

Lan 1 - fa0/0 - 10.10.10.2/24 > s0/0 - 172.17.1.1/30 - PTP link - 172.17.1.2/30 s0/0 > 192.168.3.1/24 - fa0/0 - LAN3

I need to have LAN 2 and LAN 3 to talk to LAN 1 over their respective point to point links during normal operation. If one of the links goes down, I need traffic from the failed link to travel over the other point to point link. This is purely a redundant setup. There is no load balancing required. Could someone point me to a config or lend some advice using EIGRP to acheive this setup?

Thanks,

RJ

[lgijssel]

The first condition to achieve this is that LAN2 and LAN3 are located in the same site. The most preferrable option is to implement HSRP and make the two routers each others backup. Typically, you would assign a VLAN-ID to LAN2 and LAN3 and define subinterfaces on the routers. In this setup, they will both route for LAN 2 and 3. You will need IP plus SW for this.

Regards,

Leo

[sahmad]

OK. Thanks for the reply. BTW, both LAN 2 and LAN 3 are in the same site.

I have 2 more questions.

1. It seems this setup will route LAN 2 and LAN 3 over the same link at the same time. I need to make sure that LAN 2 routes over its point to point and LAN 3 routes over its point to point during normal operation (when both links are up). I only need LAN 2 to fail over to the LAN 3 point to point when the LAN 2 point to point goes down and vice versa. Will this setup still work for this purpose?

2. What about the routers on the other side (LAN 1)? Run EIGRP?

Thanks,

RJ

[tbaranski]

One issue is that the LAN2/LAN3 routers are on different networks, and hence won't be able to talk to each other directly unless there's a link between them that isn't included in your diagram. This is important: for example, if the LAN2 router is acting as a default gateway for LAN2 devices, it has to be able to send traffic destined to LAN1 to the LAN3 router when the LAN2 serial link is down. It can't do that unless it has a way of talking to the LAN3 router via one of its Ethernet interfaces (e.g., a crossover cable that connects the two routers to each other).

Once such a link exists, the failover configuration is fairly trivial via a routing protocol such as OSPF or EIGRP. But let's get this issue resolved first.

[vcjones · Accepted solution]

In order to achieve the resilience you seek, there must be a mechanism for the routers on LAN2 and LAN3 to communicate through a local connection. This could be a dedicated link (serial or LAN), or both LANs could be logically independent but physically one LAN.

With a dedicated link, all you need to do to get proper routing is to ensure that the "cost" of the cross link is higher than the "cost" of the link across LAN1. This is by default if LAN1 is Ethernet and the dedicated link is a serial link. Otherwise, use the "delay" command on the interface used between the LAN2 and LAN3 routers to ensure the cost is higher (don't use "bandwidth" to do the job).

Making the connection by sharing the LAN is trickier, as both routers must have the same LAN as their primary IP address and support the other "LAN" using a secondary address on the interface. You will then need to adjust the EIGRP advertisements from the two routers so that the "LAN2" router has the best route to LAN2 and the "LAN3" router has the best route to LAN3. There are several ways to do this, the cleanest is to use route-maps applied to the EIGRP configuration. If you take this approach, you will probably want to adjust the IP addresses of the routers so you can use HSRP to allow continued communications from both LAN2 and LAN3 back to LAN1, even if one of the routers dies (an option not available if you use a dedicated link and two physically independent LANs.)

I've used both approaches in various production environments. Which is best for you will depend upon your specific needs.

Good luck and have fun!

Vincent C Jones

www.networkingunlimited.com

[sahmad]

Thanks for all your posts. I have just decided for the interim to put both LAN2 and LAN3 on the same subnet making them both LAN2 and use floating static routes towards each router's fa0/0 for PTP failover only.

Vincent,

Your post is exactly what I was looking for with route maps applied to EIGRP configuration. I will implement that in the future when time permits.

Thanks again,

RJ