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MPLS label distribution in broadcast network

Hello Guys

I have a very basic question regarding label distribution in broadcast network,. ie how ldp/tdp prevent the possibility of duplicate label while creating label and annoucing the bindings to neighbors. (say R1,R2,R3 connected to a ethernet switch how R2 and R3 keep a track of its of bindings) with each other

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: MPLS label distribution in broadcast network

Hello Harish,

LDP sessions are actually TCP sessions.

the router that receives the LDP bindings has all the info to perform the right choices

let's suppose R1,R2,R3,R4 are all in the same LAN. net 10.40.40.0/24

Let's suppose we use OSPF as IGP

They build OSPF adjacencies.

After that the LDP sessions sourced and destinated to loopback addresses advertised in OSPF can be setup.

let's suppose a remote network 10.10.20.0/24 is reachable by R3 and R4 by two OSPF equal cost paths via R1 and R2.

R3 receives for prefix 10.10.20.0/24 three label bindings from each of its LDP neighbors on the LAN interface.

R4 binding is not used because R4 is not a next hop for the prefix.

R3 receives label 50 from LDP neighbor R1 10.250.10.1/32 and label 1070 from R1 LDP neighbor 10.250.10.2/32

the sh mpls forw 10.10.20.0 on R3 should show

local label outgoing label

80 50 via 10.250.10.1

80 1070 via 10.250.10.2

so the router can perform correct mpls forwarding decisions by using all the info it has:

LDP neighbor 10.250.10.1 is reachable via ip address 10.40.40.1

LDP neighbor 10.250.10.2 is reachable via ip address 10.40.40.2

this comes from OSPF

so if the router decides to send a packet to 10.10.20.50 it can:

build a frame destinated to 10.40.40.1's MAC address containing an MPLS frame with label 50

or

build a frame destinated to 10.40.40.2's MAC address containing an MPLS frame with label 1070

Hope to help

Giuseppe

2 REPLIES
Hall of Fame Super Silver

Re: MPLS label distribution in broadcast network

Hello Harish,

LDP sessions are actually TCP sessions.

the router that receives the LDP bindings has all the info to perform the right choices

let's suppose R1,R2,R3,R4 are all in the same LAN. net 10.40.40.0/24

Let's suppose we use OSPF as IGP

They build OSPF adjacencies.

After that the LDP sessions sourced and destinated to loopback addresses advertised in OSPF can be setup.

let's suppose a remote network 10.10.20.0/24 is reachable by R3 and R4 by two OSPF equal cost paths via R1 and R2.

R3 receives for prefix 10.10.20.0/24 three label bindings from each of its LDP neighbors on the LAN interface.

R4 binding is not used because R4 is not a next hop for the prefix.

R3 receives label 50 from LDP neighbor R1 10.250.10.1/32 and label 1070 from R1 LDP neighbor 10.250.10.2/32

the sh mpls forw 10.10.20.0 on R3 should show

local label outgoing label

80 50 via 10.250.10.1

80 1070 via 10.250.10.2

so the router can perform correct mpls forwarding decisions by using all the info it has:

LDP neighbor 10.250.10.1 is reachable via ip address 10.40.40.1

LDP neighbor 10.250.10.2 is reachable via ip address 10.40.40.2

this comes from OSPF

so if the router decides to send a packet to 10.10.20.50 it can:

build a frame destinated to 10.40.40.1's MAC address containing an MPLS frame with label 50

or

build a frame destinated to 10.40.40.2's MAC address containing an MPLS frame with label 1070

Hope to help

Giuseppe

Community Member

Re: MPLS label distribution in broadcast network

Hello Giuseppe

Got it.. Thanks very much !

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