MPLS: LDP and TDP
·
I
often see people getting confused between LDP and TDP.
·
In
simple terms, both do the same thing, i.e. distributing labels
·
But,
TDP is Cisco proprietary and LDP is industry standard.
·
TDP:
o Uses UDP broadcast on port
711 to discover the neighbors.
o Once the neighbor is
discovered, it brings up TCP connection on port 711
·
LDP:
o Uses UDP multicast to
224.0.0.2 (AllRouters) on port 646 to discover the neighbors.
o Once the neighbor is
discovered, it brings up TCP connection on port 646
·
One
main difference is Router-ID reachability.
OSPF Router-ID:
- Router-ID simply
means the name of this router in the OSPF domain.
- Before an OSPF
router can send any OSPF messages, it must choose a unique 32-bit
identifier called the router identifier.
- Cisco routers
choose the following sequence to choose their Router-ID.
1. If you
configure Router-ID manually, it will be the Router-ID
2. If the
router-ID is not configured manually and the loopback interface exists, highest
active Loopback address will be the Router-ID
3. If Loopback
interface doesn’t exists, then the highest active physical interface address.
Some
important points to be noted about Router ID:
- The Cisco OSPF will continue to use a RID learned
from a physical interface even if the interface subsequently fails or
deleted.
- The RID does not have to reachable
- The interface from which the RID is taken does
not have to be matched by an OSPF network command
- Routers consider changing the OSPF RID when the
OSPF process is restarted or when the RID changed via the configuration.
- If a RID changes, the rest of the routers in the
same area need to perform a new SPF calculation.
- If the RID is
configured via Router-id command and the command remains unchanged, RID
will never change for that router.
BGP Router-ID:
·
Router-ID
selection is same as in OSPF
·
When
BGP sources its connection, it will always source from
the outgoing interface address; hence update-source command is needed
when we are sourcing from other than the outgoing interface.
LDP or TDP
Router-ID:
·
Router-ID
selection is same as in OSPF
·
But,
when LDP or TDP initiates its connection, it is sourced from the Router-ID.
·
That
means Router-ID must be reachable
·
That
means, IGP must have been completed before enabling LDP or TDP.
·
If
you want to change the sourcing address to outgoing interface, you could use “mpls ldp discovery transport-address
interface”
·
Let
me take a small topology to try out this:
R1----Ser1/1--------------Ser1/1-----R2
R1#show run int ser1/1
Building
configuration...
Current
configuration : 121 bytes
!
interface
Serial1/0
ip address 10.1.12.1 255.255.255.0
serial
restart-delay 0
end
R1#show run int loop0
Building
configuration...
Current
configuration : 63 bytes
!
interface
Loopback0
ip address 1.1.1.1 255.255.255.255
end
R1#
R2#show run int ser1/1
Building
configuration...
Current
configuration : 121 bytes
!
interface
Serial1/1
ip address 10.1.12.2 255.255.255.0
serial
restart-delay 0
end
R2#show run int loop0
Building
configuration...
Current
configuration : 63 bytes
!
interface
Loopback0
ip address 2.2.2.2 255.255.255.255
end
R2#
R1(config-if)#do ping 10.1.12.2
Type
escape sequence to abort.
Sending
5, 100-byte ICMP Echos to 10.1.12.2, timeout is 2 seconds:
!!!!!
Success
rate is 100 percent (5/5), round-trip min/avg/max = 4/18/44 ms
R1(config-if)#
R2(config-if)#do ping 10.1.12.1
Type
escape sequence to abort.
Sending
5, 100-byte ICMP Echos to 10.1.12.1, timeout is 2 seconds:
!!!!!
Success
rate is 100 percent (5/5), round-trip min/avg/max = 4/28/92 ms
R2(config-if)#
R2(config-if)#do ping 1.1.1.1
Type
escape sequence to abort.
Sending
5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
.....
Success
rate is 0 percent (0/5)
R2(config-if)#
R1(config-if)#do ping 2.2.2.2
Type
escape sequence to abort.
Sending
5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
.....
Success
rate is 0 percent (0/5)
R1(config-if)#
Observation:
·
Ser1/1
is up and running between R1 and R2
·
No
IGP is configured between R1 and R2, hence they cannot reach other’s loopback.
·
Now,
let me configure MPLS and see what happens:
R2(config)#int ser1/1
R2(config-if)#mpls
ip
R2(config-if)#
R1(config)#int ser1/1
R1(config-if)#mpls
ip
R1(config-if)#
R1#show
mpls ldp neighbor
R1#
R2#show
mpls ldp neighbor
R2#
Observation:
·
LDP
still didn’t come upL
·
Let’s
enable some debugs and see
R1#debug mpls
ldp transport events
LDP
transport events debugging is on for
routing table default
R1#
*Mar
13 12:02:41.623: ldp: Rcvd ldp hello; Serial1/1, from 10.1.12.2 (2.2.2.2:0),
intf_id 0, opt 0xC
*Mar
13 12:02:41.627: ldp: ldp Hello from 10.1.12.2 (2.2.2.2:0) to 224.0.0.2, opt
0xC
*Mar
13 12:02:41.631: ldp: local idb = Serial1/1, holdtime = 15000, peer 10.1.12.2
holdtime = 15000
*Mar
13 12:02:41.631: ldp: Link intvl min cnt = 2, intvl = 5000, idb = Serial1/1
R1#
*Mar
13 12:02:45.779: ldp: Send ldp hello; Serial1/1, src/dst 10.1.12.1/224.0.0.2,
inst_id 0
*Mar
13 12:02:46.327: ldp: Rcvd ldp hello; Serial1/1, from 10.1.12.2 (2.2.2.2:0),
intf_id 0, opt 0xC
*Mar
13 12:02:46.331: ldp: ldp Hello from 10.1.12.2 (2.2.2.2:0) to 224.0.0.2, opt
0xC
*Mar
13 12:02:46.331: ldp: local idb = Serial1/1, holdtime = 15000, peer 10.1.12.2
holdtime = 15000
*Mar
13 12:02:46.331: ldp: Link intvl min cnt = 2, intvl = 5000, idb = Serial1/1
R1#
*Mar
13 12:02:50.099: ldp: Rcvd ldp hello; Serial1/1, from 10.1.12.2 (2.2.2.2:0),
intf_id 0, opt 0xC
*Mar
13 12:02:50.103: ldp: ldp Hello from 10.1.12.2 (2.2.2.2:0) to 224.0.0.2, opt
0xC
*Mar
13 12:02:50.103: ldp: local idb = Serial1/1, holdtime = 15000, peer 10.1.12.2
holdtime = 15000
*Mar
13 12:02:50.103: ldp: Link intvl min cnt = 2, intvl = 5000, idb = Serial1/1
*Mar
13 12:02:50.703: ldp: Send ldp hello; Serial1/1, src/dst 10.1.12.1/224.0.0.2,
inst_id 0
R1#
*Mar
13 12:02:53.987: ldp: Rcvd ldp hello; Serial1/1, from 10.1.12.2 (2.2.2.2:0),
intf_id 0, opt 0xC
*Mar
13 12:02:53.991: ldp: ldp Hello from 10.1.12.2 (2.2.2.2:0) to 224.0.0.2, opt
0xC
*Mar
13 12:02:53.995: ldp: local idb = Serial1/1, holdtime = 15000, peer 10.1.12.2
holdtime = 15000
*Mar
13 12:02:53.995: ldp: Link intvl min cnt = 2, intvl = 5000, idb = Serial1/1
*Mar
13 12:02:54.899: ldp: Send ldp hello; Serial1/1, src/dst 10.1.12.1/224.0.0.2,
inst_id 0
*Mar 13 12:02:55.423: ldp: No route to peer 2.2.2.2; set
LDP_CTX_HANDLE_ROUTEUP
R1#u
all
All
possible debugging has been turned off
R1#
R2#debug
mpls ldp transport events
LDP
transport events debugging is on for
routing table default
R2#
*Mar
13 12:02:10.411: ldp: Rcvd ldp hello; Serial1/1, from 10.1.12.1 (1.1.1.1:0),
intf_id 0, o pt 0xC
*Mar
13 12:02:10.415: ldp: ldp Hello from 10.1.12.1 (1.1.1.1:0) to 224.0.0.2, opt
0xC
*Mar
13 12:02:10.419: ldp: local idb = Serial1/1, holdtime = 15000, peer 10.1.12.1
holdtime = 15000
*Mar
13 12:02:10.419: ldp: Link intvl min cnt = 2, intvl = 5000, idb = Serial1/1
*Mar
13 12:02:10.419: ldp: Opening ldp conn; adj 0x659D58CC, 2.2.2.2 <->
1.1.1.1; with normal
priority
*Mar
13 12:02:10.423: ldp: No MD5 password protection for peer 1.1.1.1:0
*Mar 13 12:02:10.423: ldp: No route to peer 1.1.1.1; set
LDP_CTX_HANDLE_ROUTEUP
R2#
Observations:
·
Loopback0
is elected as Router-ID and the TCP connection is sourced from the Router-ID.
·
Since
the router-ID is not reachable, LDP session didn’t come up
·
Now,
let’s change the source address to interface address:
R1(config)#int
ser1/1
R1(config-if)#mpls ldp discovery transport-address interface
R1(config-if)#
*Mar
13 12:10:58.947: %LDP-5-NBRCHG: LDP Neighbor 2.2.2.2:0 (1) is UP
R1(config-if)#
R2(config)#int
ser1/1
R2(config-if)#mpls ldp discovery transport-address interface
R2(config-if)#
*Mar
13 12:10:59.175: %LDP-5-NBRCHG: LDP Neighbor 1.1.1.1:0 (1) is UP
R2(config-if)#
R1#show mpls ldp neighbor
Peer LDP Ident: 2.2.2.2:0; Local LDP Ident 1.1.1.1:0
TCP connection: 10.1.12.2.19313 - 10.1.12.1.646
State: Oper; Msgs sent/rcvd: 13/12;
Downstream
Up time: 00:06:19
LDP discovery sources:
Serial1/1, Src IP addr: 10.1.12.2
Addresses bound to peer LDP Ident:
10.1.12.2 2.2.2.2
R1#
R2#show mpls ldp neighbor
Peer LDP Ident: 1.1.1.1:0; Local LDP Ident 2.2.2.2:0
TCP connection: 10.1.12.1.646 - 10.1.12.2.19313
State: Oper; Msgs sent/rcvd: 13/14;
Downstream
Up time: 00:07:04
LDP discovery sources:
Serial1/1, Src IP addr: 10.1.12.1
Addresses bound to peer LDP Ident:
10.1.13.1 10.1.12.1 1.1.1.1
R2#
Observation:
·
LDP
session is UP nowJ
·
Router-ID
is still Loopback0
·
TCP
connection is sourced using the interface address
·
Instead
of changing the sourcing address, we can simply advertise Loopback in the IGPJ
R1(config)#int
ser1/1
R1(config-if)#no mpls ldp discovery transport-address interface
R1(config-if)#
*Mar
13 12:25:51.959: %LDP-5-NBRCHG: LDP Neighbor 2.2.2.2:0 (1) is DOWN (Discovery
Hello Hold Timer expired)
R1(config-if)#router ospf 1
R1(config-router)#network
1.1.1.1 0.0.0.0 area 0
R1(config-router)#network
10.1.12.1 0.0.0.0 area 0
R1(config-router)#
*Mar
13 12:26:38.003: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial1/1 from
LOADING to FULL, Loading Done
R1(config-router)#
*Mar
13 12:26:47.463: %LDP-5-NBRCHG: LDP Neighbor 2.2.2.2:0 (1) is UP
R1(config-router)#
R2(config)#int
ser1/1
R2(config-if)#no mpls ldp discovery transport-address interface
R2(config-if)#
*Mar
13 12:25:52.259: %LDP-5-NBRCHG: LDP Neighbor 1.1.1.1:0 (1) is DOWN (Received
error notification from peer: Holddown time expired)
R2(config-if)#router ospf 1
R2(config-router)#network
2.2.2.2 0.0.0.0 area 0
R2(config-router)#network
10.1.12.2 0.0.0.0 area 0
R2(config-router)#
*Mar
13 12:26:38.323: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial1/1 from
LOADING to FULL, Loading Done
R2(config-router)#
*Mar
13 12:26:47.707: %LDP-5-NBRCHG: LDP Neighbor 1.1.1.1:0 (1) is UP
R2(config-router)#
R1#show mpls ldp neighbor
Peer LDP Ident: 2.2.2.2:0; Local LDP Ident 1.1.1.1:0
TCP connection: 2.2.2.2.27550 - 1.1.1.1.646
State: Oper; Msgs sent/rcvd: 9/8;
Downstream
Up time: 00:02:14
LDP discovery sources:
Serial1/1, Src IP addr: 10.1.12.2
Addresses bound to peer LDP Ident:
10.1.12.2 2.2.2.2
R1#
R2#show mpls ldp neighbor
Peer LDP Ident: 1.1.1.1:0; Local LDP Ident 2.2.2.2:0
TCP connection: 1.1.1.1.646 - 2.2.2.2.27550
State: Oper; Msgs sent/rcvd: 9/9;
Downstream
Up time: 00:02:35
LDP discovery sources:
Serial1/1, Src IP addr: 10.1.12.1
Addresses bound to peer LDP Ident:
10.1.13.1 10.1.12.1 1.1.1.1
R2#
Observation:
·
LDP
session is UP nowJ
·
It
is sourced from the Router-ID
