Have you ever heard what LSP is in networking? If not, then don’t worry! This article will provide what you need to know about what LSP means. We must define what this term means before continuing with the rest of the information. Let’s start by defining what LSP stands for and how it works.
What is LSP: How Does it Work?
Short for Link State Packet, LSP is an MPLS label that records the LSR’s (Label Switching Router) LSP database. The LSP contains information about links and nodes, which are used for forwarding data packets through the network.
LSP refers to how packets travel through computer networks as they switch paths from their origination point towards their destination (source-target). This method allows communications that use different routing protocols to follow the same.
LSP stands for Link State Protocol, and it’s a type of routing protocol that sends messages on networks to determine the best path. LSP can be used to measure link quality, calculate routes between hosts, and detect the failure of network links.
Types of LSP
Label Distribution Protocol (LDP)
Label Distribution Protocol LSP is the most commonly used form of LSP, and they use label distribution protocol for communication between routers. It’s a distance vector routing protocol with link-state updates similar to opening the shortest path first (OSPF).
LDP also uses hello messages and LDP LSP keep-alive messages. LDP can be used to transport both IPvX and MPLS traffic on the same router, but they cannot do so simultaneously. LDP LSP can be unidirectional or bidirectional.
Unidirectional
These LSPs are created in one direction, and they do not require any additional configuration when the network topology changes. They can be used to support traffic that is unidirectional such as voice, video streaming, etc.
Bidirectional
These LSPs are run in two directions. They can be used to support traffic that is bi-directional such as IP transport, etc.
RSVP-TE LSP
It is a resource reservation protocol that allows for end-to-end LSPs that are fast-rerouted capable. This LSP also enables the routers to communicate with each other at each point in the network. Therefore, they will know when there has been an outage and can then use this information to help create new detour paths if needed.
RSVP
Takes into account bandwidth availability on different links, the latency of specific links, etc. Any router along the path receives all traffic destined towards the endpoint through the label distribution process.
It provides mechanisms for reserving resources along an LSP as well as protection against misbehaving LSRs. LSP Ping is used to check the LSP path and LSP traceroute, which allows for troubleshooting in case of an outage or misbehaving router along the LSP path.
Benefits of LSPs
- LSPs provide high availability for MPLS traffic and improved performance in faster convergence times when there is a change in network status or topology.
- LSPs also allow you to have multiple paths through your network with load balancing between them, increasing bandwidth utilization and redundancy.
- LSRs can be configured to monitor LSP lifetime, so if an outage occurs, it will trigger rerouting before the LSP expires. RSVP allows for signaling among all routers along an LSP path and requires no configuration on any router except those participating as ingress and egress points.
- Any router having at least one interface connected utilizing label switch capable (MPLS) link installs local labels corresponding to LSPs. LSPs do not need to be advertised across the network; instead, LSRs can simply use their local label table to route traffic and establish LSP paths.
- RSVP also has two modes. Either hop-by-hop or end-to-end, proper when adding MPLS VPN service providers into your network structure since you only need to configure different interfaces with unique IP addresses to maintain separate routing domains.
Why is LSP Important?
LSP helps reduce route flapping, leading to faster failover times if a link goes down between two systems/devices; it also reduces control plane overhead since packets are forwarded along prearranged paths instead of using the shortest path first algorithm.
By following these LSP paths, traffic can be forwarded from source to destination without considering the routing tables.
Reasons Why LSP Needs to Configure
There are some reasons why we need to configure the LSP in our network. These might be the reasons:
- The optimize-timer statement is used to configure the ongoing optimization procedure.
- The present route has connection issues.
- The LSP is forced to reroute after being preempted by another LSP set with the priority statement.
- The explicit path information for a current LSP is changed, or the bandwidth of the LSP is raised.
LSP Basic Configuration
Static LSP Configuration
Please note that before you configure a static LSP, you need to perform the following task:
- Determine the static LSP’s entrance, transit, and egress nodes.
- Enable MPLS on all interfaces that are involved in MPLS forwarding. See “Configuring Basic MPLS” for additional details.
- Ascertain that the ingress node has a route to the LSP’s destination address.
Ingress Node of Static LSP Configuration
Roles of the Ingress Node
- Determines a packet’s Forwarding Equivalence Class (FEC) based on the destination address.
- Insert the FEC label into the package.
- Sends the packet to the next-hop or out the incoming interface.
To configure:
Enter the system view.
| INGRESS Static LSP Configuration |
|---|
| system-view |
To configure the ingress node of a static LSP.
| INGRESS Static LSP Configuration |
|---|
| static-lsp ingress lsp-name destination ip-address { mask | mask-length } { nexthop next-hop-ip-address | outgoing-interface interface-type interface-number } out-label out-label |
| static-lsp ingress lsp-name destination ip-address { mask | mask-length } { nexthop next-hop-ip-address | outgoing-interface interface-type interface-number } out-label out-label |
Transit Nodes of Static LSP Configuration
Role of the Transit Nodes
A transit node replaces the label carried by a received packet with an outgoing label before forwarding the packet to the next-hop or out of the outgoing interface.
To configure:
Enter the system view.
| TRANSIT STATIC LSP configuration |
|---|
| system-view |
To configure a transit node of a static LSP.
| TRANSIT STATIC LSP CONFIGURATION |
|---|
| static-LSP transit LSP-name in-label in-label { next hop next-hop-IP-address | outgoing-interface interface-type interface-number } out-label out-label |
Configuring the Egress Node of Static LSP
If PHP is not configured, the egress node will parse a received packet as follows:
- Displays the packet’s incoming label.
- Conducts label forwarding or IP forwarding based on the inner label of the packet.
You must complete this task to define the incoming label on the egress node. If PHP is configured, you do not need to do this step. To set up PHP for a static LSP, change the outgoing label to 0 or 3 on the static LSP’s penultimate hop.
To configure:
Enter to the system view.
| EGRESS STATIC LSP configuration |
|---|
| system-view |
To configure the egress node of a static LSP.
| EGRESS STATIC LSP CONFIGURATION |
|---|
| static-LSP egress LSP-name in-label in-label [ destination IP-address { mask | mask-length } ] |
Network Configuration
Static LSP Configuration Topology
In the image above, Switch A, Switch B, and Switch C all support MPLS.
Establish static LSPs between Switch A and Switch C so that subnets 11.1.1.0/24 and 21.1.1.0/24 can access each other over MPLS.
This configuration will create VLAN and configure IP addresses for all interfaces, including the loopback interfaces.
Configure a static route to the destination address of each static LSP:
| SLP Configuration (SWITCH) |
|---|
| # On Switch A, configure a static route to network 21.1.1.0/24 |
| <SwitchA> system-view |
| [SwitchA] ip route-static 21.1.1.0 24 10.1.1.2 |
| # On Switch C, configure a static route to network 11.1.1.0/24 |
| <SwitchC> system-view |
| [SwitchC] ip route-static 11.1.1.0 255.255.255.0 20.1.1.1 |
Configure basic MPLS on the switches:
| SLP CONFIGURATION ( MULTIPLE SWITCHES) |
|---|
| # Configure Switch A |
| [SwitchA] mpls lsr-id 1.1.1.9 |
| [SwitchA] interface vlan-interface 2 |
| [SwitchA-Vlan-interface2] mpls enable |
| [SwitchA-Vlan-interface2] quit |
| # Configure Switch B |
| [SwitchB] mpls lsr-id 2.2.2.9 |
| [SwitchB] interface vlan-interface 2 |
| [SwitchB-Vlan-interface2] quit |
| [SwitchB] interface vlan-interface 3 |
| [SwitchB-Vlan-interface3] mpls enable |
| [SwitchB-Vlan-interface3] quit |
| # Configure Switch C |
| [SwitchC] mpls lsr-id 3.3.3.9 |
| [SwitchC] interface vlan-interface 3 |
| [SwitchC-Vlan-interface3] mpls enable |
| [SwitchC-Vlan-interface3] quit |
Configure a static LSP from Switch A to Switch C:
| SLP CONFIGURATION (Switch A to Switch C) |
|---|
| # Configure the LSP ingress node, Switch A |
| [SwitchA] static-lsp ingress AtoC destination 21.1.1.0 24 nexthop 10.1.1.2 out-label 30 |
| # Configure the LSP transit node, Switch B |
| [SwitchB] static-lsp transit AtoC in-label 30 nexthop 20.1.1.2 out-label 50 |
| # Configure the LSP egress node, Switch C |
| # Configure Switch B |
| [SwitchC] static-LSP egress AtoC in-label 50 |
Configure a static LSP from Switch C to Switch A:
| SLP CONFIGURATION (Switch C to Switch A) |
|---|
| # Configure the LSP ingress node, Switch C |
| [SwitchC] static-lsp ingress CtoA destination 11.1.1.0 24 nexthop 20.1.1.1 out-label 40 |
| # Configure the LSP transit node, Switch B |
| [SwitchB] static-lsp transit CtoA in-label 40 nexthop 10.1.1.1 out-label 70 |
| # Configure the LSP egress node, Switch A |
| [SwitchA] static-lsp egress CtoA in-label 70 |
To verify the configuration:
| Verify SLP CONFIGURATION |
|---|
| # Display static LSP information on switches, for example, on Switch A. |
| [SwitchA] display mpls static-LSP |
| Total: 2 |
| Name FEC In/Out Label Nexthop/Out Interface State |
| AtoC 21.1.1.0/24 NULL/30 10.1.1.2 Up |
| CtoA -/- 70/NULL – Up |
| # Test the connectivity of the LSP from Switch A to Switch C |
| [SwitchA] ping mpls -a 11.1.1.1 ipv4 21.1.1.0 24 |
| MPLS ping FEC 21.1.1.0/24 with 100 bytes of data: |
| 100 bytes from 20.1.1.2: Sequence=1 time=4 ms |
| 100 bytes from 20.1.1.2: Sequence=2 time=1 ms |
| 100 bytes from 20.1.1.2: Sequence=3 time=1 ms |
| 100 bytes from 20.1.1.2: Sequence=4 time=1 ms |
| 100 bytes from 20.1.1.2: Sequence=5 time=1 ms |
— Ping statistics for FEC 21.1.1.0/24 —
5 packets transmitted, 5 packets received, 0.0% packet loss
Round-trip min/avg/max = 1/1/4 ms
| # Test the connectivity of the LSP from Switch C to Switch A |
| [SwitchC] ping mpls -a 21.1.1.1 ipv4 11.1.1.0 24 |
| MPLS ping FEC 11.1.1.0/24 with 100 bytes of data: |
| 100 bytes from 10.1.1.1: Sequence=1 time=5 ms |
| 100 bytes from 10.1.1.1: Sequence=2 time=1 ms |
| 100 bytes from 10.1.1.1: Sequence=3 time=1 ms |
| 100 bytes from 10.1.1.1: Sequence=4 time=1 ms |
| 100 bytes from 10.1.1.1: Sequence=5 time=1 ms |
— Ping statistics for FEC 11.1.1.0/24 —
5 packets transmitted, 5 packets received, 0.0% packet loss
Round-trip min/avg/max = 1/1/5 ms
FAQs About LSP
1. Are LSR and LSP the same thing?
They may be used interchangeably, but they are not the same. LSR performs label switching, whereas LSP takes the shortest path.
2. Why is LSC an abbreviation for LDP signaling channels?
This means that LSP employs label distribution protocol, which assists LSP in discovering network topology and determining shortest path routing, loop detection, traffic engineering capabilities, and a fast reroute mechanism.
3. When do I need to configure LSP?
LSP is not mandatory to have configured on a network. When LSP is required, it is during routing protocols such as Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System(IS-IS).
In this case, LSP will utilize IS-IS or OSPF protocol messages to create an LSP that will enable faster convergence times when changes within the network occur.
Conclusion
LSP is a vital part of any modern networking system because it can make networks scalable by using the shortest path first (SPF) algorithm and providing traffic engineering capabilities.
We hope this article clarifies what LSP means and what it can do for your network. Let others discover how to use it on a day-to-day basis by sharing this information with your family and friends.
