1. LDP signaled LSPs for external prefixes
This is joint work – Juniper/ ATT
Ina Minei, Nischal Sheth - Juniper
Luyuan Fang – AT&T

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Before we start…
The discussion is only for address prefix FECs and host FECs associated with IPv4/IPv6 prefixes.
We are talking only about v4/v6 prefixes
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The problem
LDP relies on routing protocols for its forwarding decisions.
To establish LSPs, LDP needs routing table entries for all its FECs.
Not always desirable…
- As a signaling prorocol, LDP relies on routing protocols for finding routes in the network, avoiding loops, etc.
Need routing table entries for all the FECs I want to advertise into LDP.
Normailly this is not an issue, since mostly we advertise the loopbacks, which are in the IGPs anyway. However, there are scenarios where we want to inject external prefixes into LDP.
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The goal
Establish LSPs for FECs that are “external” to the routing table.
Thus, no need to inject external routes corresponding to the LDP FECs in the IGP just for the purpose of LSP establishment.
The external routes may be learnt from BGP, and we want to avoid redistirbution.
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The idea
If LSRa is the one who is injecting FECa into LDP, can use LSRa’s address in the forwarding decision making process.
Don’t need FECa’s address in the routing table, only need LSRa’s address.
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The concepts
Decouple the FEC from the routing table entry.
Specify a different address on which to apply the decision making process of whether to use the label for forwarding and whether to propagate the FEC to the neighbors.
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LDP extensions
New TLV (Resolution Nexthop TLV) specifying the address to use in the forwarding-state decision-making process.
Extensions to the message handling procedures to take advantage of the new TLV.
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8. Resolution Nexthop TLV
Carries a host address
Always shows up together with the FEC TLV
Used in label map, withdraw, release messages.
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9. Label mapping procedures
Existing algorithm
Conditions for using a label for forwarding:
the routing table contains an _exact_ entry for the address in the FEC TLV
the mapping was received from the neighbor that is the nexthop for the route found in (1).
The most significant change is to the
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10. Label mapping procedures
Extension to the algorithm
Conditions for using a label for forwarding if the label-map message contains the Resolution Nexthop TLV (RN TLV):
the routing table contains an _exact_ entry for the address in the RN TLV
the mapping was received from the neighbor that is the nexthop for the route found in (1).
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11. Label withdraw/release
Extension to the algorithm
If the label-map message contained the Resolution Nexthop TLV (RN TLV) then the withdraw/release must also contain it.
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12. Thank you!