A small question releated to Redundancy

Unanswered Question
Feb 15th, 2009

Hi NetPros

If we have two different leased line links from two different service provider and they are reaching to my remote branch network from HQ. Network looks as below.

R Router1--isp1--R1--core sw1---HQ

R Router1--isp2--R2--core sw1---HQ

My question is how R1 or R2 discover that the link of respective ISP is gone down even the next hop public ip address of ISP is PINGable or reachable to switchover to the backup link if we are using any Dynamic or Static routing redundancy method at router level?

In addition, incase of dynamic routing how the protocol use the method to find out the status of any link.

Thanks

Manish

I have this problem too.
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vincent-n Sun, 02/15/2009 - 20:19

Assuming you're using the links in primary and backup mode, by the look of things I think you can do:

---R1--isp1--R1Core--|

LAN--HSRP--| |

---R2--isp2--R2Core--|

|--Core

Run dynamic routing protocols (BGP) on all routers, tweak:

1. R2 to have lower HSRP priority

2. R2 to have lower preference path (eg inject extra BGP AS number is its advertisement)

route-map SetPath permit 10

set as-path prepend 65000 65000

router bgp 65000

neighbor 1.1.1.1 route-map SetPath out

where 1.1.1.1 is directly connected IP on isp2.

I don't think you'll need to tweak anything at Core routers since they should automatically learn the advertised routes from remote site.

LAN devices will follow HSRP device and be routed to Core automatically. If isp1 goes down, the changes will be detected automatically and switched over to isp2. You'll have to test and time the convergence. Mine is about 60-70 seconds before the site is available again.

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