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On NX-OS 6.x releases, Nexus 3000 sends the initial SYN or SYN+ACK packet used for TCP 3-way handshake with a MSS indicated with a size of approx 500. On NX-OS 7.x versions the behavior was changed, Nexus 3000 sends the initial SYN or SYN-ACK packet used for TCP 3-way handshake with a MSS indicated of approx. the size of the configured MTU of the interface. This with no "ip tcp path-mtu-discovery" in the configuration.
BGP on Nexus 3000 or 9000.
a) Enable "ip tcp path-mtu-discovery" on the configuration. or b) Ensure the data path connecting the BGP peers is properly configured to forward Jumbo Frame size packets.
When using Jumbo Frame size MTU on the outbound interface used to establish the TCP socket, BGP can initially negotiate the TCP socket using a large size MSS value on 7.x version. This could cause fragmentation or large BGP UPDATE packets if data path connecting the BGP peers is not properly configured to forward Jumbo Frame size packets.
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