The route a path takes can be recorded.
Recording the path allows the ingress LER to know, on a hop-by-hop basis, which LSRs the path traverses. Knowing the actual path of an LSP can be especially useful for diagnosing various network issues.
Network path recording is configurable per LSP. This feature is configured by enabling route recording for a specific RSVP-TE profile using the command configure mpls rsvp-te lsp profile lsp_profile_name record enabled and associating the profile to an LSP. The ExtremeXOS software sets the label recording desired flag in the path message if route recording has been enabled for the LSP.
If route recording is enabled, the record route object (RRO) is inserted into the path message using a single RRO subobject, representing the ingress LER. When a path message that contains an RRO is received by an Extreme LSR, an RRO IPv4 subobject representing the /32 address of the outgoing interface of the path message is pushed onto the top of the first RRO. The updated RRO is returned in the reserve message.
The label recording flag is supported by the ExtremeXOS software and is set automatically when route recording is enabled. The route-only option can be used when enabling route recording in the profile to prevent the label recording flag from being set. If an Extreme LSR receives a path message with the label recording flag set in the RRO, the LSR encodes the LSP label into a label subobject and pushes it onto the RRO.
If a path message is received that contains an RRO, the Extreme LSR uses the RRO to perform loop detection. The RRO is scanned to verify that the path message has not already traversed this LSR. If the RRO contains an IPv4 subobject that represents a local LSR interface, the path message is dropped and a Routing Problem error message is sent to the originating LER with an error value of Loop detected.