With technologies like OTV, we need a method to optimize traffic destined for our mobile virtualized hosts, by tracking the location and updating our underlying routing system. One possible solution is known as LISP.

LISP stands for Locator ID Separation Protocol, and its function is to allow you to separate the location component of an IP address, from the identity portion of the address. Or as the Cisco LISP configuration guide states, LISP “implements the use of two namespaces instead of a single IP address.” What does this mean?

With LISP you are introduced to two new concepts:

• Endpoint Identifiers (EID)
• Routing Locators (RLOC)

The endpoint identifier (EID) is the address used to identify a specific host — this is the same as the IP addresses you use today, and it is said to be in the LISP namespace. The Routing Locator (RLOC) is the address of a router that is part of the normal routing domain but is connected to the LISP namespace and the non-LISP namespace. The RLOC is said to be part of the non-LISP namespace.

One significant difference with LISP is that you no longer have to advertise the EID address space into the normal routing domain. Instead, you rely on LISP to provide mappings between EIDs and RLOCs, and you route based on the RLOC address.

I like to think of LISP as a DNS-like system for routing, because an important part of LISP is a mapping system that maintains EID-to-RLOC mappings. Just like you use DNS to query for name-to-IP mappings, a LISP router performs a query against a LISP mapping server to find out the RLOC that should be used to reach the desired EID.

Components of a LISP system

An complete LISP infrastructure will consist of many parts:

• Ingress Tunnel Routers (ITR)
• Egress Tunnel Routers (ETR)
• XTR
• Map Server (MS)
• Map Resolver (MR)
• Proxy Ingress Tunnel Routers (PITR)
• Proxy Egress Tunnel Routers (PETR)
• PXTR

ITR

The ingress tunnel router receives unencapsulated IP packets from the EID namespace, and is responsible for performing lookups to identify EID-to-RLOC mappings for destination addresses. If the packet is destined for an EID in another LISP namespace, the ITR will encapsulate each packet with a LISP header and route the packet towards the identified RLOC. If the packet is destined for a non-LISP address, the packet is routed without any LISP modifications.

ETR

The egress tunnel router receives LISP encapsulated packets from the non-LISP portion of the network, removes the LISP header, and delivers the unencapsulated packets to the EID. The ETR is also responsible for keeping the Mapping system up to date with EID mappings and responding to Mapping system requests.

LISP XTR

An XTR is a router that performs both ETR and ITR functions.

Map Server

The map server receives EID registrations from ETRs, and responds to map request messages that are forwarded from map resolvers.

Map Resolver

The map resolver receives encapsulated map request messages from ITRs and forwards them to Map Servers that are authoritative for the EID namespace being queried.

PITR/PETR/PXTR

The Proxy ITR/Proxy ETR allows non-LISP sites to communicate with LISP sites, and vice-a-versa, by performing ITR and ETR functionality. They can be deployed separately, or together. If deployed together the device is referred to as a PXTR.

Conclusion

It took me a little while to wrap my head around the LISP concept, and one big help was working with it in a lab environment.  In the next post, I’ll walk through a lab scenario to demonstrate LISP in action.