The MPLS-OPS Archive

[Christopher Lewis <chrlewis@cisco.com>]

My comments refer to the Cisco implementation on the 7600. Keep in mind 
there are two labels applied, just as in the L3 vpn case. The first is the 
outer or tunnel label that is created via the usual L3 means (between PE 
loopbacks), the second inner label is via a directed LDP session between 
the ingress PE and egress PE, which are remote label distribution peers, 
this is used to exchange VC labels. All VC label bindings exchanged over 
this LDP session use the Virtual Circuit FEC element type 128 as defined in 
the martini l2circuit draft via the LDP "downstream unsolicited mode" 
described in RFC 3036.

In L2 ethernet over MPLS, the VC Virtual Circuit is a VC is configured 
between two different interfaces the LERs. The VC is used to define a point 
to point circuit over which Layer 2 PDUs are transported. Since a 
uni-directional LSP is used to transport PDUs in each direction, a VC 
requires 2 LSPs to carry layer 2 traffic back and forth across the Virtual 
Circuit.

The VC label Refers to the label provided from the egress LER. The ingress 
LER prepends this label so that the egress router knows which output 
interface and VC to route a packet to. When the packet travels over a MPLS 
backbone, the tunnel label is popped by the neighboring LSR prior to 
passing it to the egress LER by implicit null.

A full description is given at 
http://www.cisco.com/warp/public/cc/pd/rt/7600osr/prodlit/emp76_tc.htm

Other vendors do different things, one might use some means to connect 
traffic on an interface to a traffic engineered tunnel for instance. 
However, it seems that as far as encapsulation types for this effort, the 
Martini draft is gaining more support and is now specified in the Kompella 
ppvpn draft.

Chris

At 10:47 PM 8/17/2001, vinod ab wrote:
>Hi,
>Would anyone kindly let me know how the
>FEC to label mapping is done in L2_VPN at the
>Ingress and Egress Node.
>
>The creation of FEC at Ingress can be assumed to take
>place mapping the various L2 protocols to label.
>
>BUT HOW IT WOULD BE DONE AT EGRESS?
>
>No Routing protocol exists (unlike L3_vpn case) to
>create the L2 mappings for the creation of FEC Table.
>
>Has LDP any role to play in the creation of FEC at
>distant end.
>Regards,
>Vinod
>
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