rolande
Certifiable
Premium,Mod
join:2002-05-24
Powell, OH
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Host:
Linksys
AT&T Midwest
| reply to pdk
Re: [HELP] Prepending AS path in Multihomed setup
Most routers will use route caching for existing traffic flows, especially provider's large Internet aggregation routers. If you have a reconverge event with multiple paths for the same route, even though BGP updates properly, existing traffic flows may continue to flow down the backup path until they age out, unless the route cache is cleared. That is completely out of your control. ISP's do not do this as any BGP route flap event may cause a major route cache instability which usually triggers high cpu utilization on routers.
If your AS path prepend works under normal circumstances, then it will work once you reconverge. But, it will take time for you to start seeing traffic fall back over to the primary path. Fast failover and fallback can be deadly to your network. When you get trapped in a circuit flapping situation that you can't get recovered from, both yours and the ISP router's pay the price. ISP's frown severely on this when one customer's issue can bring down many other customers.
Remember that BGP is still a Distance Vector protocol underneath it all just with advanced metrics. It still takes time for reconvergence to fully occur throughout the network.
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