The MPLS WG Archive

[Yakov Rekhter <yakov@cisco.com>]

Janardan,
 
> RFC-2547.bis says that format for vpn-ipv4 address
> is 8 byte RD + 4 byte prefix. Suresh sent out a
> question regarding the prefix length.
> 
> In the reply received from Yakov, it said that the
> prefix length is endoded as per multiprotocol extensions to
> BGP (rfc2858).
> 
> My undertanding of the MPLS_REACH_NLRI (based on 2858 and
> carrying labels in BGP draft is as follows)
> 
> 
> NLRI Len (1 byte), Label (3bytes), NLRI variable.

Not exactly - the label part may carry one or more labels
(see the carrying labels in BGP draft).

> The draft says that the length includes the length of the label and
> the NLRI length.

yes, as with any TLV-style encoding...

> 
> 
> For the VPN-ipv4 case, the MP_REACH_NLRI will look like
> 
> NLRI Len (1byte) = 14, Label (bytes), NLRI Len=prefixlen, 8 byte NLRI + 4
> byte prefix.

No, this is not correct. From draft-ietf-mpls-bgp4-mpls-03.txt:

   The Length field indicates the length in bits of the address prefix plus
   the label(s). 
> 
> I feel that this model has deviated from the representation of IP prefix in
> traditional > ipv4 BGP.
> 
> Should the format for vpn-ipv4 address be as follows?
> 
> 8 byte RD + 1 byte prefix len + 4 bytes prefix  (with trailing zeros)

No, the address prefix that is handled by BGP in the BGP/MPLS VPN case
is *not* an IPv4 prefix, but a VPN-IPv4 prefix.

> What is the current format used both when conveying the information
> and storing the routes in the routing table?

I am not sure I understand this question. Please elaborate.

Yakov.