LDPoRSVP

82> If we use a hierarchial ospf ie ospf with different areas it increases the scalability of the network however the downside is TE database could not be spread over multiple areas.
83> Opaque LSA type 10 used for TE purposes cannot cross a particular area as they are blocked by the ABR.
Hence CSPF based LSP's cannot cross multiple areas. If we create such an LSP we get an error message saying No CSPF route to destination.
84>To get rid of such solutions we implement what is known as LDPoRSVP.
PE--------------ABR1---------------------->ABR2------------------>PE2
Area 1 Area 0 Area 2
85> So we need an CSPF based LSP from PE to ABR1 we needs another LSP from ABR1 to ABR2 and the third from ABR2 to PE2
These 3 independent LSP's should be stitched to form an end to end tunnel.
86> The stitching of these independent CSPF based LSP is performed by T-LDP and is performed by the ABR. So a T-LDP session should exist between PE---ABR1 ABR1---ABR2 and ABR2---PE2.
87> So in the case of LDPoRSVP resiliency features can be done inside an Area.
88> In this case 3 label MPLS stack is used
Top label is the RSVP label (outer most)-----T-LDP label----Service label
89> The RSVP label or the tunnel label is the label which is exchanged between the PE device and the intermediate node
PE----R1-----R2(ABR)-------R3-----R4(ABR)------R5-----PE
As seen above the Tunnel label is exchanged between PE and R1.

90> Second label is the LDP transport label which is exchanged between PE and R2 ie the T-LDP label.
91>The third label is service label which is only exchanged between the PE routers.
92 > So R1 swaps the top label however it does not touch the TLDP label
R2 Pops the RSVP label and also pops the T-LDP label, and inserts a new T-LDP label and a new RSVP label.
PE pops all the 3 labels.
93 > T-LDP is usually used between PE and R2 as in the case of T-LDP the peers need not be directly connected.
94> Config router ldp
Targeted session
Peer x.x.x.x
Tunneling
Tunneling needs to be enabled under the peer.
This needs to be configured on the PE and the ABR routers and not the intermediate routers in the Area.
95 > If link ldp also exists and the T-LDP also exists between the two nodes then by default link ldp gets a preference to change that we can configure
Config router ldp
Prefer-tunnel-in-tunnel
96> Then LDP over RSVP should also be enabled inside the IGP
Config router ospf or isis ldp-over-rsvp
97> We can verify this by using the command
Show router ldp bindings active
The LSP ID is displayed as the egress interface as the ldp tunnels is encapsulated inside the RSVP.
The LSP ID in ldp bindings active and show router rsvp session tunnel id should be identical.
*A:R1# show router ldp bindings active
==========================================================================
Prefix Op IngLbl EgrLbl EgrIntf/LspId EgrNextHop
--------------------------------------------------------------------------
. . . . . . .
10.10.10.6/32 Push -- 131068 LspId 147 10.10.10.2
--------------------------------------------------------------------------
　
*A:R1# show router rsvp session
==========================================================================
From To Tunnel LSP Name
State
ID ID
--------------------------------------------------------------------------
10.10.10.1 10.10.10.2 147 18944 LSP1::fully_loose
　
98> Generally in a scalable network an area at least have two ABR for reasons that one ABR can fail in such scenarios PE needs to form a TLDP session with both the ABR's.
99> PE wil send the traffic to the closest ABR.
100> Full mesh of LSP and T-LDP sessions are required between the ABR's