BGP config "local-as" possible issues

Unanswered Question
Sep 24th, 2008

Greetings All,

Someone had mentioned to me before that using the config "local-as" to mask the real AS number could bring up some issues specially when dealing with multiple ISP's. Can someone enlighten on what it is? or Can someone tell me what are the possible issues that i might face when masking my "real" as number.

Thanks in Advance

I have this problem too.
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Giuseppe Larosa Thu, 09/25/2008 - 12:47

Hello Merrylle,

the local-as feature has been introduced to help in migration after two ISPs merge.

With this feature you don't need to ask to your eBGP neighbors to update their config because AS number is changed.

the feature is neighbor oriented and can be used only on a real eBGP session ( no confederation support)

the default behaviour is to add the local-as, prepend it on advertisements without hiding the real AS number.

However a number of options are possible.

the replace-as allows to hide your real AS number.


You shouldn't try to advertise the same ip prefix to different eBGP peers as it was originated in different AS numbers because this would be an inconsistency.

So you should care to be consistent with what is registered with the RIRs (ARIN, RIPE, etc) about IP prefixes and AS numbers.

Then if later the registrations are updated you can move the prefixes to the new/real AS number.

Hope to help



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