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IPv4 + Label


I just thought about something. When you transmit MPLS Labels over L2 technology (ATM/FR/Eth) L2 header identifies that encapsulated traffic is Label and it's done via EtherType or SNAP headers.

I assume that when LDP gets enabled on the interface, it allows the interface to transmit and receive labeled packet which otherwise would have been dropped.

In case of IPv4+Label in either CsC or Inter-AS VPN scenarios, LDP is not enabled on the interface, so how do PE routers know to accept labeled packets and not drop them?

Maybe if there was a way to enable labeled packets processing without configuring LDP on the interface, cause with IPv4+Label feature you still want to process labeled packets, but you're not interested in activating LDP on the interface and exachanging labels this way.




Re: IPv4 + Label

Hello David,

you´re right, LDP is not activated on the Link. However you have eBGP on the link and thus labeled traffic is allowed on the link (only with learned labels for security resons afaik).

IPv4+Label was invented because ISPs do not want any other interAS protocol, but BGP.

Once a router learns a label through IPv4+Label it will insert it into the LFIB and forwarding can start. On the data plane there is no difference on how you learn the labels.

Hope this helps! Please rate all posts.

Regards, Martin

New Member

Re: IPv4 + Label


I think that on the data forwarding plane, you still have to make sure that labeled packets are accepted and it's not done by default. Activating LDP on the interface takes care of that and I assume that IOS code treats interfaces through which the IPv4+Label advertisemets were received also as interfaces enabled for label packet forwarding.

I guess we would need an "internal" Cisco info to confirm if that's so :-)


Re: IPv4 + Label


as I wrote, IPv4+Label activates labels on the link, where the eBGP is started.

Hope this helps!

Regards, Martin

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