MPLS Basic Topology
From IPFlow Netflow Collector
Basic MPLS Example
This is a simple example of MPLS basic configuration on Cisco routers (using Dynamips and GNS3).
Author: Christophe Fillot - 05-Nov-2007
| Table of contents |
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Topology
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IOS configurations
- A0 (http://www.ipflow.utc.fr/configs/MPLS_Basic/A0.cfg)
- A1 (http://www.ipflow.utc.fr/configs/MPLS_Basic/A1.cfg)
- B0 (http://www.ipflow.utc.fr/configs/MPLS_Basic/B0.cfg)
- B1 (http://www.ipflow.utc.fr/configs/MPLS_Basic/B1.cfg)
- C0 (http://www.ipflow.utc.fr/configs/MPLS_Basic/C0.cfg)
- C1 (http://www.ipflow.utc.fr/configs/MPLS_Basic/C1.cfg)
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Dynamips/Dynagen/GNS3 configuration
You can use this configuration file (http://www.ipflow.utc.fr/configs/MPLS_Basic/mpls_basic.net) directly with Dynagen or GNS3. Change the working directory and Cisco IOS image path to match your needs.
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Enabling MPLS
- Make sure you have full connectivity through your network ;
- CEF (Cisco Express Forwarding) must be enabled ("ip cef" or "ip cef distributed") ;
- Enable MPLS on your core interfaces with "mpls ip" (don't do this for interfaces facing customer equipments) ;
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Checking LDP connectivity
The LDP protocol is used to exchange labels between routers. You can verify the MPLS adjacencies with "sh mpls ldp neighbor":
A1#sh mpls ldp nei
Peer LDP Ident: 10.10.0.0:0; Local LDP Ident 10.10.0.1:0
TCP connection: 10.10.0.0.646 - 10.10.0.1.34421
State: Oper; Msgs sent/rcvd: 50/49; Downstream
Up time: 00:30:52
LDP discovery sources:
FastEthernet0/0, Src IP addr: 10.1.1.1
Addresses bound to peer LDP Ident:
10.1.1.1 10.10.0.0
Peer LDP Ident: 10.20.0.0:0; Local LDP Ident 10.10.0.1:0
TCP connection: 10.20.0.0.56148 - 10.10.0.1.646
State: Oper; Msgs sent/rcvd: 46/49; Downstream
Up time: 00:28:34
LDP discovery sources:
FastEthernet1/0, Src IP addr: 10.2.1.2
Addresses bound to peer LDP Ident:
10.2.1.2 10.20.0.0 10.2.2.1
Peer LDP Ident: 10.30.0.0:0; Local LDP Ident 10.10.0.1:0
TCP connection: 10.30.0.0.17227 - 10.10.0.1.646
State: Oper; Msgs sent/rcvd: 39/43; Downstream
Up time: 00:22:57
LDP discovery sources:
FastEthernet2/0, Src IP addr: 10.3.1.2
Addresses bound to peer LDP Ident:
10.3.1.2 10.30.0.0 10.3.2.1
Here we can see that A1 has three LDP adjacencies: A0, B0 and C0.
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Displaying the LIB (Label Information Base)
When the LDP adjacencies are established, the routers build a Label Information Base, which can be displayed with "sh mpls ldp bindings".
We can see that for a given prefix, a router receives labels from all of its LDP neighbors.
A1#sh mpls ldp binding
tib entry: 10.1.1.0/30, rev 4
local binding: tag: imp-null
remote binding: tsr: 10.10.0.0:0, tag: imp-null
remote binding: tsr: 10.20.0.0:0, tag: 18
remote binding: tsr: 10.30.0.0:0, tag: 21
tib entry: 10.2.1.0/30, rev 8
local binding: tag: imp-null
remote binding: tsr: 10.10.0.0:0, tag: 17
remote binding: tsr: 10.20.0.0:0, tag: imp-null
remote binding: tsr: 10.30.0.0:0, tag: 20
tib entry: 10.2.2.0/30, rev 14
local binding: tag: 18
remote binding: tsr: 10.20.0.0:0, tag: imp-null
remote binding: tsr: 10.10.0.0:0, tag: 20
remote binding: tsr: 10.30.0.0:0, tag: 22
tib entry: 10.3.1.0/30, rev 10
local binding: tag: imp-null
remote binding: tsr: 10.10.0.0:0, tag: 18
remote binding: tsr: 10.20.0.0:0, tag: 19
remote binding: tsr: 10.30.0.0:0, tag: imp-null
tib entry: 10.3.2.0/30, rev 20
local binding: tag: 21
remote binding: tsr: 10.30.0.0:0, tag: imp-null
remote binding: tsr: 10.10.0.0:0, tag: 23
remote binding: tsr: 10.20.0.0:0, tag: 22
tib entry: 10.10.0.0/32, rev 6
local binding: tag: 16
remote binding: tsr: 10.10.0.0:0, tag: imp-null
remote binding: tsr: 10.20.0.0:0, tag: 17
remote binding: tsr: 10.30.0.0:0, tag: 19
tib entry: 10.10.0.1/32, rev 2
local binding: tag: imp-null
remote binding: tsr: 10.10.0.0:0, tag: 16
remote binding: tsr: 10.20.0.0:0, tag: 16
remote binding: tsr: 10.30.0.0:0, tag: 18
tib entry: 10.20.0.0/32, rev 12
local binding: tag: 17
remote binding: tsr: 10.20.0.0:0, tag: imp-null
remote binding: tsr: 10.10.0.0:0, tag: 19
remote binding: tsr: 10.30.0.0:0, tag: 17
tib entry: 10.20.0.1/32, rev 16
local binding: tag: 19
remote binding: tsr: 10.20.0.0:0, tag: 20
remote binding: tsr: 10.10.0.0:0, tag: 21
remote binding: tsr: 10.30.0.0:0, tag: 16
tib entry: 10.30.0.0/32, rev 18
local binding: tag: 20
remote binding: tsr: 10.30.0.0:0, tag: imp-null
remote binding: tsr: 10.10.0.0:0, tag: 22
remote binding: tsr: 10.20.0.0:0, tag: 21
tib entry: 10.30.0.1/32, rev 22
local binding: tag: 22
remote binding: tsr: 10.30.0.0:0, tag: 23
remote binding: tsr: 10.20.0.0:0, tag: 23
remote binding: tsr: 10.10.0.0:0, tag: 24
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Displaying the LFIB (Label Forwarding Information Base)
We have seen previously that a router receives labels from its LDP neighbors for each given prefix. Then, it chooses the label to use with the help of the IGP (in this example, OSPF), so the shortest path can be selected.
The LFIB can be displayed with "sh mpls forwarding-table":
A1#sh mpls for Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 16 Pop tag 10.10.0.0/32 1914 Fa0/0 10.1.1.1 17 Pop tag 10.20.0.0/32 0 Fa1/0 10.2.1.2 18 Pop tag 10.2.2.0/30 1802 Fa1/0 10.2.1.2 19 20 10.20.0.1/32 360 Fa1/0 10.2.1.2 20 Pop tag 10.30.0.0/32 0 Fa2/0 10.3.1.2 21 Pop tag 10.3.2.0/30 1232 Fa2/0 10.3.1.2 22 23 10.30.0.1/32 360 Fa2/0 10.3.1.2
Let's take an example:
Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 22 23 10.30.0.1/32 360 Fa2/0 10.3.1.2
- 22 is the local label. The router announces to its LDP neighbors that it will use label 22 as input label to reach prefix 10.30.0.1/32 ;
- 23 is the outgoing label. The router will use output label 23 to reach prefix 10.30.0.1/32 ;
- Fa2/0 is the output interface, and 10.3.1.2 (corresponding to C1) is the next hop (see output of "show ip route 10.30.0.1") ;
- Here, when the router receives a packet with label 22, it swaps the label and replace it with label 23 ;
- When the Outgoing tag is "Pop tag", the router removes the first label in the stack.
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Useful commands for troubleshooting
IP route information:
A1#sh ip ro 10.30.0.1
Routing entry for 10.30.0.1/32
Known via "ospf 100", distance 110, metric 66, type intra area
Last update from 10.3.1.2 on FastEthernet2/0, 00:47:48 ago
Routing Descriptor Blocks:
* 10.3.1.2, from 10.30.0.1, 00:47:48 ago, via FastEthernet2/0
Route metric is 66, traffic share count is 1
CEF information:
A1#sh ip cef 10.30.0.1
10.30.0.1/32, version 29, epoch 0, cached adjacency 10.3.1.2
0 packets, 0 bytes
tag information set
local tag: 22
fast tag rewrite with Fa2/0, 10.3.1.2, tags imposed: {23}
via 10.3.1.2, FastEthernet2/0, 0 dependencies
next hop 10.3.1.2, FastEthernet2/0
valid cached adjacency
tag rewrite with Fa2/0, 10.3.1.2, tags imposed: {23}
Detailed information for the label entry:
A1#sh mpls forwarding-table label 22
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
22 23 10.30.0.1/32 360 Fa2/0 10.3.1.2
A1#sh mpls forwarding-table label 22 detail
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
22 23 10.30.0.1/32 360 Fa2/0 10.3.1.2
MAC/Encaps=14/18, MRU=1500, Tag Stack{23}
CA040A5B0000CA020A5A00388847 00017000
No output feature configured
Per-packet load-sharing
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Traceroute with MPLS labels
The traceroute commands can display the labels used between routers:
A0#traceroute 10.30.0.1 Type escape sequence to abort. Tracing the route to 10.30.0.1 1 10.1.1.2 [MPLS: Label 22 Exp 0] 76 msec 80 msec 48 msec 2 10.3.1.2 [MPLS: Label 23 Exp 0] 44 msec 52 msec 44 msec 3 10.3.2.2 48 msec * 72 msec
