Cisco Expert – MPLS and BGP – Part 2

Alright, we have our network configured with OSPF in the transit links and we have a BGP peer session between R1 and R3. There’s only one little problem we must solve. R2 is not running BGP, therefore does not know about R1 and R3 LAN’s so is dropping any traffic for those destinations.

The solution will be enabling MPLS on the transit links between R1-R2-R3.

MPLS runs in 2 flavors, TDP and LDP. We can also use RSVP bandwidth reservations for MPLS Traffic Engineering Tunnels but that is for another day.
TDP is Cisco proprietary and stands for Tag Distribution Protocol which really is the old format for MPLS. Now, instead of tags it is called Labels.
LDP is a industry standard and stands for Lable Distribution protocol. Apart from this, they do pretty much similar.

Configuration wise, we are talking here about 2 or 3 commands, so no big deal but the concepts my friends are the problem. You have to understand the concept very well because later on when you get into MPLS VPN’s and QOS…well…Another day I will explain those topics

For now, we are going to enable MPLS.
In some IOS versions, TDP is the default, some other LDP, so check it!!!!
We are running LDP for sake of being the Industry Standard but again there’s no difference. Also be aware that some IOS versions take the command tag-switching ip, some other mpls ip. Also IP CEF is a must here.
Another very important aspect is that by default your MPLS adjacency router-id will be formed based on the highest loopback in the router and that loopback must be advertised into OSPF. So we have a problem here :). My LANs are Loppback 1, are not being advertised into OSPF and they are the highest ip. The solution is the optional command “mpls ldp router-id loopback 0 (force)”. If you are having problems, shut down the interfaces and use “force” keyword and bring them back up.

Rack1R1(config)#mpls label protocol ?
ldp Use LDP
tdp Use TDP (default)

R1 has TDP enabled my default, so I am going to change it to LDP.

R1(config)#ip cef
R1(config)#mpls ldp router-id loopback 0
R1(config)#mpls label protocol ldp
R1(config)#int e0/0
R1(config-if)#mpls ip

That’s it! Now I will enable it in all the transit links.

Verification
R2#sh mpls ldp neighbor
Peer LDP Ident: 3.3.3.3:0; Local LDP Ident 2.2.2.2:0
TCP connection: 3.3.3.3.63264 – 2.2.2.2.646
State: Oper; Msgs sent/rcvd: 10/11; Downstream
Up time: 00:01:21
LDP discovery sources:
FastEthernet0/1, Src IP addr: 172.16.23.3
Addresses bound to peer LDP Ident:
172.16.23.3 124.1.23.3 3.3.3.3 10.10.3.3
Peer LDP Ident: 1.1.1.1:0; Local LDP Ident 2.2.2.2:0
TCP connection: 1.1.1.1.646 – 2.2.2.2.20480
State: Oper; Msgs sent/rcvd: 10/10; Downstream
Up time: 00:01:15
LDP discovery sources:
FastEthernet0/0, Src IP addr: 172.16.12.1
Addresses bound to peer LDP Ident:
1.1.1.1 10.10.1.1 172.16.12.1

The process:
Now when R3 sends traffic from it’s LAN to R1’s LAN, it will sure work. Let’s check how do the routers route the traffic.

R3#sh ip route bgp
10.0.0.0/24 is subnetted, 2 subnets
B 10.10.1.0 [200/0] via 1.1.1.1, 00:03:43

R3 has 10.10.1.0 via 1.1.1.1(R1) so it will make a lookup in the MPLS table for network 1.1.1.1.

R3#sh mpls forwarding-table 1.1.1.1
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 32 1.1.1.1/32 0 Et0/1 172.16.23.2

It says to get to 1.1.1.1 PUSH a label 32 and send the traffic to the next hop address towards R2

R2 receives the traffic and checks the topmost Label of the packet which will be 32

R2#sh mpls forwarding-table 1.1.1.1
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
32 Pop tag 1.1.1.1/32 3296 Fa0/0 172.16.12.1

R2 now will POP the Label 32 and send the traffic towards R1.

R1 receives the traffic for 10.10.1.1 and knows that is directly connected.

Because this is a small topology we did not see one important step in MPLS. Imagine that between R2 and R3 we would have another router. That router would just SWAP the topmost label with another label and send the traffic to R2. Basically, labels are formed like this 16-17 17-18 18-19 19-20 and so on, so when a router sends a label, the other router is expecting that label.

In conclusion, there are 3 processes for MPLS, PUSH – SWAP – POP.

Final test, ping from R3’s LAN to R1’s LAN

R3#ping 10.10.1.1 source lo1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.1.1, timeout is 2 seconds:
Packet sent with a source address of 10.10.3.3
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

Now we have reachability!!

The MPLS routers on the core of the network they will not run BGP and have not routes for BGP networks, they will route the traffic based on LABELS only.  P->PE, Provider to Provider Edge, will always POP the label. Likewise, PE->P will always PUSH a Label. P->P will only SWAP a Label

This makes a lot of sense, If a ISP has 400 P routers and 100 PE routers, they only need to run BGP in 100 routers, COOL HA?