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Force preferred route

We have a remote site (Interface S0) connected to our host site (Interface s0) via a frame relay cloud. For redundancy we also have a GRE over IPSec tunnel between these two routers F0 interfaces. We are using RIP2 for routing updates. Since each route is technically one hop away it appears routing is being done in a round robin manner. During the Christmas holiday our Internet T1 was running at max capacity which impacted the routing between these two sites. I ended up having to shutdown the tunnel interfaces which forced all traffic to run via the frame cloud.

How can I configure these so the frame cloud is always used over the GRE tunnel, with the exception of when the PVC goes down?

I thought that by specifying the bandwidth on each interface the router would use the higher of the two when the hop count was equal.

I set the bandwidth on the serial interface higher than the bandwidth on the tunnel interface but this didn't seem to help.

Any suggestions?

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Force preferred route

Hello,

RIP does not look at interface bandwidth when calculating the best path. The routing table entry will solely depend on the metric of RIP, i.e. hop count. Therefore setting the bandwidth did not influence the routing decision in your case. Whatever is in the routing table will determine where an IP packet is sent (somewhat simplified).

The interface bandwidth is taken into account by other IP routing protocolls like EIGRP or OSPF.

So one option to achieve your goal would be to change from RIP to f.e. OSPF.

In case you want to stay with RIPv2 you need to have a better metric (= lower hop count) towards a destination across the PVC compared to GRE.

This can be achieved by configuring an offset-list. For syntax and examples please have a look at

http://www.cisco.com/en/US/products/sw/iosswrel/ps1826/products_command_reference_chapter09186a0080087b06.html#xtocid522812

and

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a00800ca569.html#wp4653

Hope this helps! Please rate all posts

Martin

2 REPLIES

Re: Force preferred route

Hello,

RIP does not look at interface bandwidth when calculating the best path. The routing table entry will solely depend on the metric of RIP, i.e. hop count. Therefore setting the bandwidth did not influence the routing decision in your case. Whatever is in the routing table will determine where an IP packet is sent (somewhat simplified).

The interface bandwidth is taken into account by other IP routing protocolls like EIGRP or OSPF.

So one option to achieve your goal would be to change from RIP to f.e. OSPF.

In case you want to stay with RIPv2 you need to have a better metric (= lower hop count) towards a destination across the PVC compared to GRE.

This can be achieved by configuring an offset-list. For syntax and examples please have a look at

http://www.cisco.com/en/US/products/sw/iosswrel/ps1826/products_command_reference_chapter09186a0080087b06.html#xtocid522812

and

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a00800ca569.html#wp4653

Hope this helps! Please rate all posts

Martin

New Member

Re: Force preferred route

I see, this is very interesting information. Thank you so much.

I thought I had read somewhere that if the router had two routes to the same destination with everything else equal (same metric) that the bandwidth setting would determine which route it would take.

I guess this is not the case since I was seeing routing in a round robin fashion.

I will try the offsets.

Thank you again,

Denny

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