Re: [mpls-linux-general] default entry in mpls forwarding table

[James R. L. <jl...@mi...>]

See my comments within:

On Mon, Jan 28, 2002 at 09:08:41PM +0530, Abhijit Gadgil wrote:
> anish  verma wrote :
> 
> >
> >hi jim,
> >i have some queries.. please help me
> >
> >1.  is it possible to forward packets to some default entry through
> >mpls-linux as it done in ip-network..
> 
> I can answer this query on Jim's behalf ;-)

Thank you.  You did a fine job of answering it as well :-)

> Binding labels to default entry is not a good thing. The reason is as
> follows.
> 
> 1. The labels bound to default entry will have to leave the MPLS domain
> through a 'default egress' router. As the fec, which is '0.0.0.0' in this
> case has to be uniquely interpreted throughout the domain.
> 
> eg. In following figure 'd is the default egress'
> 
> a-----b------c------d
>       |
>       |
>       e------f------g
> 
> Now assume that a label is bound to default entry in the routing table so
> a packet gets forwarded on LSP a-b-c-d . However it may be possible that
> the _best_ egress router in this case is router g. And d will indeed
> conclude so by inspecting the IP packet. (through BGP next hop. As the
> packet that will be routed through the default root is most likely to be
> external to the domain or your IGP is screwed up.) So this results in
> suboptimal routing. The TTL may expire in this prematurely.
> 
> >2. as i learn through ur code that mpls_input function recieve pkt from
> >DLC layer. if i am right then what is the significance of Push opcode in
> >mpls_input() function coz when a packet traverse up in network layer
> >headers r ripped off but in mpls_input (push) it is inserted. if i am
> >wrong please also explain the case when header is pushed in mpls_input()
> >happen.


It is for flexibility.  There is nothing saying you MUST do a push as part
mpls_input().  Infact to date, I haven't come up with a senerio that can
could use it.  But none-the-less I've left it there because there are
people in the world that are crazier then me :-)

Jim
-- 
James R. Leu