Clarification about BGP filtering vs. memory utilization

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Apr 27th, 2007
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I'm trying to recall the memory utilization inherent with filtering BGP prefixes.


I seem to recall somewhere that if I have a session between router A and router B, where router A sends 10 routes to router B and router B has a filter to accept only one of the 10 prefixes from router A, the overall memory utilization on router B will still be the same as what it would be if router B accepted all 10 prefixes.


If that is true, is it only true if soft-reconfiguration is enabled, or is it true regardless of the soft-reconfiguration?

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Harold Ritter Fri, 04/27/2007 - 07:05
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Jason,


Yes, it is true only if "neighbor soft-reconfiguration inbound" is configured.


Note that you don't need to configure the "soft-reconfiguration inbound" if the router (and its peer(s)) implements the BGP soft reset, as this feature allows you to dinamically send a route-refresh message to the peer, which causes the peer to send all of its updates all over again.


Please refer to the following URL for more information on the BGP soft reset feature.


http://www.cisco.com/en/US/products/sw/iosswrel/ps1830/products_data_sheet09186a0080087b3a.html


Hope this helps,

jlixfeld Fri, 04/27/2007 - 07:52
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So after reading about the soft-reconfiguration enhancement (route refresh): It is true that soft-reconfiguration uses more memory, and the route-refresh notes say it uses "no extra resources" but I wasn't clear in the notes I read whether or not route-refresh uses no more resources than soft-reconfiguration, or no more resources than normal BGP with no soft-reconfiguration configured.


Basically, I have a client who wants a default route from a provider partly due to memory requirements. The providers says that they will announce the default route, but will also announce the full table. I want the soft-reconfiguration features so I can change policy on received prefixes without hard clearing the session, but the memory overhead required by soft-reconfiguration may kill me. I'm hoping route-refresh will give me the gains of soft-reconfiguration withouth the cons of soft-reconfiguration.

Harold Ritter Fri, 04/27/2007 - 08:10
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BGP soft reset uses no more resources than normal BGP as the local router doesn't have to keep all of the paths received from the peer that are rejected by the inbound policy.


You are correct that BGP soft reset feature will give you the advantages of "soft-reconfiguration inbound" (soft reset) without the disadvantages (memory consumption).


Just make sure the provider supports that feature.


Hope this helps,

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