MPLS VPN : 12-Byte Next-Hop

[amit.bhagat]

Hi Guys,

As per RFC 4760, the MP_REACH_NLRI attribute in BGP Update message has variable length Next-HOP field. For MPLS VPN, this field is sent as 12-byte value of the form RD:IP_Address.

My question is - what is the need for RD in the Next-Hop field (and as such, this is set to 0:0)? Configured RD is carried in the NLRI anyway.

Any ideas?

Regards,

Amit.

[Harold Ritter]

Hi Amit,

Here's an excerpt from RFC4364 stating the following:

"When a PE router distributes a VPN-IPv4 route via BGP, it uses its
 own address as the "BGP next hop".  This address is encoded as a
 VPN-IPv4 address with an RD of 0.  ([BGP-MP] requires that the next
 hop address be in the same address family as the Network Layer
 Reachability Information (NLRI).)"

[BGP-MP]          Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
                     "Multiprotocol Extensions for BGP-4", RFC 2858,
                     June 2000.

Regards

[amit.bhagat]

Hi Harold,

Thanks for replying - I missed this bit in the RFC. Nonetheless, the question still remains as to why we need RD in the Next-Hop field? What purpose does it solve?

Or, to put in other words, what other applications set this RD value and use it to some effect? As far as I know, in BGP VPLS NLRI, does not include RD value as per RFC4761.

From RFC4761, section 3.3-

"The Network Layer protocol associated with the Network Address of the Next Hop for the combination SAFI> is IP; this association is required by [4], Section 5. If the value of the Length of the Next Hop field is 4, then the Next Hop contains an IPv4 address. If this value is 16, then the Next Hop contains an IPv6 address."

Regards,

Amit.

[Harold Ritter · Accepted solution]

Hi Amit,

I don't think there is any special purpose for the RD other than making the next hop the same as the address family. It seem to have been the intent stated in RFC4364.

Regards