Bypassing IGP shortcut by filtering LDP advertisement

Consider the following topo :

PE1---P1----P2----P3---PE2

|----PE3

In word (in case the drawing is wrongly displayed)

PE1 to P1 , P1 to P2, P2 to P3 ,PE2 to P3 ,PE3 to P3 .

All nodes run LDP :

LDP session P2-P3 :

P3 advertise L32 for PE2 /32 prefix

P3 advertise L33 for PE3 /32 prefix

LDP session P2-P1 :

P2 advertise L22 for PE2 /32 prefix

P2 advertise L23 for PE3 /32 prefix

LDP session P1-PE1 :

P1 advertise L12 for PE2 /32 prefix

P1 advertise L13 for PE3 /32 prefix

we configure a RSVP-TE tunnel (Tunel100 ) from P1 to P3 ,

as a result of the signalling P3 advertise "implicit null"

to P2 , P2 advertise L2t to P1 .

AT P1 under tunnel100 we config autoroute-announce .

tunnel100 is considered the next-hop to P3 and any

downstream nodes (PE2 and PE3)

We initiate directed LDP session between P1 and P3 , we control P3

config such that P3 will only advertise label for PE2 over this

directed LDP session .

LDP session P1-P3 :

P3 advertise L3d for prefix PE2 /32 (L3d could be same value

as L32 ) .

For traffic from VPn1 connected on PE1 to VPN2 connected to PE2 :

PE1 will send labeled backet with depth 2

(Top Label = L12 ,bottom Label= L2vpn . L2vpn recieved through

Mp-iBGP PE1/PE2 )

P1 will swap L12 with L3d and forward the traffic over the tunnel.

The label packet forwarded to P2 now have 3 label

(Top Label = L2t ,2nd label=L3d ,last label=Lv2pn)

P2 will pop the top label and forwarded the label packet to P3

with just 2 label (Top label=L3d ,last label=Lv2pn ) .

P3 will pop the top label (L3d) and forward the packet with top

label Lv2Pn to PE2.

From the above it is working fine .

For traffic from VPn1 connected on PE1 to VPN3 connected to PE3 :

PE1 will send labeled backet with depth 2

(Top Label = L13 ,bottom Label= L3vpn . L3vpn recieved through

Mp-iBGP PE1/PE3 )

What P1 will do :

Case1 : P1 will notice that L13 is associated with action "untag"

but L13 is not the bottom of stack so it drop the packet .

Case2 : P1 next-hop to P3 and downstream nodes (PE1 and PE2)

is through tunnel100 .

P1 pop L13 and push L2t and forward to P2 ,now the

label packet with stack of depth 2 (top= L2t ,bottom=L3vpn)

P2 pop the top label and forward to P3 .

P3 will recieve a labeled packet with depth 1 and label=L3vpn.

P3 will drop the packet as l3vpn is unknown .

case3 : P1 will notice that despite tunnel100 is the next-hop to

PE3 still P1 did not recieved a label for PE3 from P3 . so

recieved packet with top label=L13 but the stack depth is > 1 can not

be forwarded over tunnel100 and it need to be forwarded over other

interface where a label for PE3 is recieved .

P1 will swap L13 with L22 ( Top label=L22, bottom label=L3vpn)

and forward to P2 .

P2 swap L22 with L32 and forward to P3 .

P3 pop L32 and forward the label packet (with single label=L3vpn)

to PE3 .

So what is the expected behaviour of P1 ?

From the above it is clear that i want to control which traffic from

P1 (to downstrean nodes of P3 ) will use the tunnel by controlling

what label(s) the tail-end (P3) will advertise through the LDP session

to the head-end (P1) ,if their is other method to achieve the same pls

advise .

Specific to which box is in use , it is cisco box but just glancing on

JNP documentation it seems (not sure) that their is way to achieve

this with JNP . JNP allow to assign distance to route learned through

LDP to be higher than route learned through IGP

(In other word , if labelled route is recieved for prefix is

cosnidered first before any un-labelled route ) .so if we ping from P1

to PE3 ,P1 will generate a label packet with single label L23 in case

of JNP while i guess cisco will generate a label packet

with top label L3t .Could You Pls comment on this point .

Brgds

The MPLS-OPS Archive

Bypassing IGP shortcut by filtering LDP advertisement