[j-nsp] question regarding l3vpn

[Pedro Roque Marques]

Nabarun Baks wrote:
>  Hi Robert,
> 
> Thanks very much !
> 
> Also can you say whether I need to order the BGP Attributes. Currently
> I am sending Extended Community attribute placed after MP_REACH_NLRI
> attribute (as per BGP rule), the router accepts the route but ethereal
> fails to show the extended community attribute and gives a hex dump.
> 
> Also when examining packets from the router I saw that extended
> community attribute is placed before the MP_REACH_NLRI attribute.
> 
> Is there any requirement to place Extended Community attribute before
> MP_REACH_NLRI?
> 

While the base BGP spec says that attributes should be ordered in 
ascending type value, in this case the preferred way would be to place 
extended communities before MP_REACH.

The reason being that MP_REACH contains a list of prefixes for which the 
path attributes (such as extended communities) apply. i.e. MP_REACH is 
essentially the NLRI list w/ a new encoding not really an attribute.

Thus you will find that most implementations format an UPDATE w/ [path 
attributes] + nlri... where NLRI is encoded as an MP_REACH.

The receiver should be able to parse this in any order... but it is much 
friendlier to generate an advertisement w/ all attributes before the NLRI.

Current versions of tcpdump do support decoding of extended communities.

   Pedro.