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S-Series tunnel encapsulation, Interface MTU and jumbo capability

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TitleS-Series tunnel encapsulation, Interface MTU and jumbo capability
Symptoms
  • OSPF interoperability - adjacency fails due to MTU mismatch in a multi-vendor environment
  • Jumbo frames fail through the S-series switch or router
  • Tunnel or encapsulated ethernet frames (L2 GRE, pseudo-wire style encapsulation) sourced by the router fail 
Environment
  • S-Series firmware 7.x or greater
  • K-Series firmware 7.x or greater
  • OSPF 
  • L2 GRE 
    • L3 tunnel payload includes encapsulation of the original IP header
    • L2 tunnel payload (pseudowire and L2GRE) encapsulates the entire original frame including L2.  There may be no L3 portion in the payload of an L2 tunnel.
Cause
  • MAX IP interface MTU for the S-series router is 1518.  
  • Encapsulating ethernet packets with a secondary header (i.e. VPLS, GRE, L2 GRE, MPLS, IP in IP etc) requires the router to source frames that may be larger than the router host's interface MTU.  
Resolution
Additional header is configured and applied at the reframer level.  
Router as a device (vs router host) will now be able to anchor tunnels and source frames larger than the 1518 byte maximum size host capability (this frame is not generated by the host)

Caveat for switching or routing any frame larger than max ethernet frame size is as follows: physical port used by the packet to ingress/egress the router needs to be enabled for jumbo frames. 

When anchoring L2 GRE remote port mirror, or virtual private port services, enable jumbo on the ports involved to allow switching and routing of the larger frames.
The reframer operation will handle the additional header addressing and calculations.  L2 encap does not assume IP so ICMP treatment is not appropriate for sizing.
The path must support large packets from end to end.   
Additional notes
When interop is a requirement where higher max MTU may be necessary for the routed interface, use the ignore MTU option in the S-Series OSPF configuration.  While the S-series will not source a jumbo frame from the host IP stack, the S-series will accept large LSAs or other jumbo sized host-destined packets by using soft path fragmentation within the host stack.

Additional considerations:
  • S-series host can read large routing protocol packets but will not source jumbo packets from the host IP stack
  • Encapsulation headers are added by the reframer rather than at host
  • Port setting necessary to enable jumbo support is the only configuration required to allow the S-Series to switch and route jumbo frames  
  • Examples of host-sourced packets include SNMP, OSPF and other routing protocol packets, radius, syslog and so on
  • S-Series Firmware support for various encapsulations  
    • S-Series 8.31.x firmware track  includes support for MPLS over IPv6
    • S-Series 8.21.x firmware includes support for L3 VPN over MPLS (ipv4)
    • S-Series 8.11.x firmware includes support for virtual private port service (informally pseusdowire). 
    • S-Series 8.02.x firmware includes support for L3 VPN over GRE
    • S-Series 8.02.x firmware includes support for remote port mirror (L2 GRE tunnel) and sampled port mirror (n-packet mirror)
    • S-Series 7.x firmware  support includes IP-IP, IP-IP(4 in 6), IPv6IP (6 in 4), IPv6IP (6 in 6) MPLS in IP (v4)
 

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