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

[Abhijit G. <gab...@ee...>]

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 ;-)

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.

no-idea

>
>thanx in advance
>bye
>anish

hth

-abhijit