BGP attribute ext-next-hop

[Ondrej Zajicek]

On Wed, Apr 11, 2018 at 07:55:28AM +0000, Arvin Gan wrote:
> Hi ,

>     Thanks for your response.  Could you help me for another question.
> If ext-next-hop is enabled and AFI is BGP_AF_IPV4, next- hop is a IPV4
> address, when update message created, the function bgp_create_mp_reach is
> called, what is the format of UPDATE message ? Firstly, bgp_encode_attrs 
> function encode all attributes, is the attribute BA_NEXT_HOP encoded in
> this step ? 

Hi

In traditional BGP (encoded by bgp_create_ip_reach() function), NEXT_HOP
is encoded like other attributes by bgp_encode_next_hop() function from
attrs.c file as part of bgp_encode_attrs().

In MP-BGP (encoded by bgp_create_mp_reach() function), NEXT_HOP attribute
should not be encoded independently as part of bgp_encode_attrs(),
but is encoded by bgp_encode_next_hop_ip() function (or its variant
based on SAFI), called through dispatch function directly from
bgp_create_mp_reach()).

But now when i checked it, it seem like the condition in
bgp_encode_next_hop() does not take in to account ext-next-hop option.
So it is possible there is a bug here.


> If yes, as comment said, IPV4 NLRI is not used; If no,
> bgp_encode_next_hop assert next-hop is 16 bytes or 32 bytes, but IPV4
> next-hop is 4 bytes....

The assert in bgp_encode_next_hop_ip() function checks the length of the
internal representation of the next hop, which is always 16 bytes (type
ip_addr), even if IPv4 next hop is used (in such case it is encoded as
IPv4-mapped IPv6 address). Function ipa_to_ip4() gets 4B ip4_addr from
16B ip_addr.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."