Subject: Question on constructing nexthop in MP_REACH_NLRI attribute

From: "Reddy, Sudhakar" <[email-masked]>
Date: Wed, 29 Jan 2003 05:04:56 -0500

Hi All,
I have a question on constructing the next hop field in MP_REACH_NLRI
attributes. Section 3 
of the RFC 2545 says that:

        "A BGP speaker shall advertise to its peer in the Network Address of
NextHop field 
         the global IPv6 address of the next hop, potentially followed by
the link-local IPv6
         address of the next hop"

Now my question is:
In a scenario where two IPv6 clouds are connected by a IPv4 network and the
edge routers of 
both clouds Router-A, and Router-B are dual stack routers:

If Router A wants to send a MP-BGP update to Router-B, what nexthop it
should encode while 
sending update to its peer?. If it has to be an IPv6 global address, how the
intermediate routers 
in the IPv4 network will resolve this nexthop?.

Thanks,
Sudhakar
----------------------------------------------------------------------------
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Sudhakar Reddy
mailto:[email-masked]
http://www.netplane.com

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From: Pedro Roque Marques <[email-masked]>
Date: Wed, 29 Jan 2003 11:20:01 -0800 (PST)

Reddy, Sudhakar writes:

> Hi All, I have a question on constructing the next hop field in
> MP_REACH_NLRI attributes. Section 3 of the RFC 2545 says that:

>         "A BGP speaker shall advertise to its peer in the Network
> Address of NextHop field the global IPv6 address of the next hop,
> potentially followed by the link-local IPv6 address of the next hop"

> Now my question is: In a scenario where two IPv6 clouds are
> connected by a IPv4 network and the edge routers of both clouds
> Router-A, and Router-B are dual stack routers:

> If Router A wants to send a MP-BGP update to Router-B, what nexthop
> it should encode while sending update to its peer?. If it has to be
> an IPv6 global address, how the intermediate routers in the IPv4
> network will resolve this nexthop?.

The intermediate ipv4 network should be no different than an
intermediate atm or frame network. All these tecnologies can provide
transport for IPv6 packets. On all of them the IPv6 interfaces would
use IPv6 addresses. Addresses which would be then advertised on an
eBGP session...

On point-to-point interfaces the link-local addresses are not required
or useful.

To be more specific, the requirement is that the address used as
non-local nexthio on an eBGP peering is an address that is significant
across the iBGP mesh of the receiving AS, since by default BGP
advertises such an address into iBGP.

Somehow i've the impression i didn't answer the right question
although...

  Pedro.

---

From: Jeffrey Haas <[email-masked]>
Date: Wed, 29 Jan 2003 14:33:18 -0500

> > network will resolve this nexthop?.

Since they are disconnected IPv6 clouds where the intermediate transport
is ipv4, you need some sort of tunnel connecting the two clouds at
the border router.  The tunnel can be whatever.  If the tunnel tehchnology
presents to the ipv6 layer a normal ipv6 interface, you would send 
your global address of your side of the tunnel as expected.  
Since all routes are extremely likely to be first party routes,
you would also send the link local of your side of the tunnel.

-- 
Jeff Haas 
NextHop Technologies