Introduction:
In our previous article in this series, we discussed what Locator/ID Separation Protocol (LISP) is, the challenge(s) it solves, and the benefits it delivers. LISP leverages a number of functional components to make this all work. In this article, we discuss the key LISP architecture components and functions. This will lay the foundation for the subsequent articles in this series where we look at LISP operation and some implementation use-cases.
The concept of namespaces (Endpoint Identifiers [EIDs] and Routing Locators [RLOCs]) as used by LISP was highlighted in our previous article. There is also the concept of LISP Site which is simply a domain where EIDs are located. A non-LISP site is a network domain or infrastructure that does not implement LISP; think of this as a traditional network environment which implements conventional routing protocols like BGP(Border Gateway Protocol) and OSPF (Open Shortest Path First) to exchange routing information. Endpoint devices within a non-LISP site are assigned IP addresses based on their location within the network topology, without the separation of identity and location characteristic of LISP. It is important to understand these concepts as they will be referenced in the functional components.
LISP Functional Components:
LISP Namespace
| Component | Description / Function |
| Endpoint Identifiers (EIDs) | EIDs uniquely identify endpoints such as laptops, servers, printers etc. This is typically a single IP address that identifies an endpoint. An EID is a 32-bit (for IPv4) or 128-bit (for IPv6). The EID is assigned from the EID-Prefix. The EID-Prefix is a block or range of IP addresses within a LISP site that share a common prefix. So EIDs represent individual endpoint addresses, EID-Prefixes encompass multiple IP addresses within a subnet or address range. It is the EID-Prefixes that are associated with a set of RLOC addresses to make up the “database mapping” |
| Routing Locators (RLOCs) | RLOCs are assigned to routers. RLOC is an IPv4 or IPv6address of an Egress Tunnel Router (ETR). These are infrastructure addresses used to direct traffic to EIDs. They are globally routed in the underlying infrastructure.An EID maps to one or more RLOCs |
LISP Site Devices
| Component | Description / Function |
| Ingress Tunnel Router ( ITR ) | An ITR sits in a LISP site. It receives traffic from hosts local to that side destined for hosts outside of the site. The ITR treats the destination IP as an EID and performs EID-to-RLOC mapping lookup. If the destination is in a LISP site, the ITR encapsulates the packets and sends them to the remote LISP site. If the destination is in a non-LISP site, the ITR natively forwards the packets to the non-LISP sites. |
| Egress Tunnel Router ( ETR ) | An ETR also sits in a LISP site. It registers its EID prefixes and RLOCs with the Map-Server, and responds to map requests received by the Map-Server. It receives LISP-encapsulated IP packets from the core-facing interfaces/Internet, decapsulates the IP packets, and sends them to endpoints/EIDs in its local site. |
| xTR | An xTR performs both ITR and ETR functions. There is no requirement for a LISP site to perform both ITR and ETR functions. A device can be only an ITR or an ETR. However, it is common for a device to implement both functions. |
LISP Infrastructure Components
| Component | Description / Function |
| Map-Server (MS) | The MS contains the EID-to-RLOC mappings. The MS learns these from ETRs when ETRs send map-register messages to MS informing the MS of the EID-to-RLOC mappings they are aware of. |
| Map-Resolver (MR) | When an ITR needs to resolve EID-to-RLOC mappings, it sends a LISP-encapsulated Map Request to the MR. The Map-Resolver is typically an Anycast address which ensures the ITR can select the topologically closest MR. The MR intends forward this the Mapping System which results in the Map-Server receiving this request. The MR also sends Negative Map-Replies to ITRs in response to queries for non-LISP addresses. Both the MS and MR functions are typically implemented on a common system. |
| Alternate Logical Topology (ALT) | ALT serves as an alternative mapping system within the LISP architecture; that is an additional method for resolving EID-to-RLOC mappings. It complements the primary mapping system (typically based on Map Servers and Map Resolvers) by offering an alternative path for mapping resolution. It distributes mapping information across a network of ALT nodes strategically deployed throughout the LISP network. This enhances scalability and fault tolerance. ALT is not present in all mapping system deployments. |
| Proxy ITR (PITR) | PITR serves as an intermediary between non-LISP and LISP networks, encapsulating outgoing traffic from non-LISP networks into LISP-encapsulated packets. |
| Proxy ETR (PETR) | PETR acts as an intermediary between LISP and non-LISP networks, decapsulating incoming LISP-encapsulated packets and forwarding them to non-LISP networks. |
| PxTR | Performs both functions of the PITR and PETR. |
Topology below illustrates these functional components.
Figure 1: LISP Functional Components
References:
– [RFC 6830](https://datatracker.ietf.org/doc/html/rfc6830): “The Locator/ID Separation Protocol (LISP)”
– [RFC 9299](https://datatracker.ietf.org/doc/rfc9299): “An Architectural Introduction to the Locator/ID Separation Protocol (LISP)”