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https://issues.apache.org/jira/browse/HADOOP-10181?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14316613#comment-14316613
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Chris Nauroth commented on HADOOP-10181:
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[~tlipcon], no worries. Thanks for following up.
> GangliaContext does not work with multicast ganglia setup
> ---------------------------------------------------------
>
> Key: HADOOP-10181
> URL: https://issues.apache.org/jira/browse/HADOOP-10181
> Project: Hadoop Common
> Issue Type: Bug
> Components: metrics
> Affects Versions: 2.6.0
> Reporter: Andrew Otto
> Assignee: Andrew Johnson
> Priority: Minor
> Labels: ganglia, hadoop, metrics, multicast
> Fix For: 2.7.0
>
> Attachments: HADOOP-10181.001.patch, HADOOP-10181.002.patch,
> HADOOP-10181.003.patch
>
>
> The GangliaContext class which is used to send Hadoop metrics to Ganglia uses
> a DatagramSocket to send these metrics. This works fine for Ganglia
> multicast setups that are all on the same VLAN. However, when working with
> multiple VLANs, a packet sent via DatagramSocket to a multicast address will
> end up with a TTL of 1. Multicast TTL indicates the number of network hops
> for which a particular multicast packet is valid. The packets sent by
> GangliaContext do not make it to ganglia aggregrators on the same multicast
> group, but in different VLANs.
> To fix, we'd need a configuration property that specifies that multicast is
> to be used, and another that allows setting of the multicast packet TTL.
> With these set, we could then use MulticastSocket setTimeToLive() instead of
> just plain ol' DatagramSocket.
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