LISP - Learn Tech From Zero

2.3 Describe network virtualization concepts

LISP is a network protocol designed to separate device identity (who a device is) from device location (where it is in the network).

ID (Endpoint Identifier, EID): Identifies the device. Think of it like a unique identity for a host or server.
Location (Routing Locator, RLOC): Tells where the device is in the network. This is the IP address used for routing packets.

Why separate ID and location?

Traditional IP addresses do both roles: they identify a device and tell its location.
As networks grow, this dual role causes problems like:
- Difficulty moving devices between networks.
- Complex routing tables.
- Trouble with multihoming (having multiple connections to the internet).

LISP solves this by keeping identity and location separate.

EID (Endpoint Identifier)
- Assigned to hosts or devices.
- Does not change even if the host moves to a different network.
- Example: A virtual server in a data center has a unique EID.
RLOC (Routing Locator)
- Assigned to routers (or devices) that connect to the network.
- Can change depending on the network topology.
- Helps route traffic efficiently.
xTR (eXternal Tunnel Router / Ingress/Egress Tunnel Router)
- Ingress Tunnel Router (ITR): The router that receives packets from the EID network and encapsulates them for transport across the LISP network.
- Egress Tunnel Router (ETR): The router that decapsulates the packet and delivers it to the destination EID.
MS (Mapping Server)
- Stores the mapping of EIDs to RLOCs.
- Helps ITRs find out where to send packets.
MR (Map Resolver)
- Receives queries from ITRs and returns the mapping from the MS.

Let’s go step by step in simple terms:

Device wants to send a packet to another device.
- The sender knows the EID of the destination.
ITR checks its cache for EID → RLOC mapping.
- If mapping exists: encapsulate the packet and send to the RLOC.
- If mapping does not exist: query the Map Resolver/Mapping Server.
Packet is encapsulated in an outer IP header with the RLOC as the destination.
- Inner header = original EID addresses.
- Outer header = RLOC addresses (used by the network to route the packet).
ETR receives the packet, removes the outer header, and delivers it to the final EID.
Response packets follow the same path in reverse.

Mobility:
- Devices can move between networks without changing their IP identity (EID).
- Useful for cloud VMs or servers that move between data centers.
Scalability:
- Reduces the size of global routing tables because only RLOCs are in the routing table, not every EID.
Multihoming Support:
- Networks can have multiple paths to the internet without changing device IPs.
Traffic Engineering:
- Control how traffic flows through different RLOC paths.

Understand LISP roles: EID, RLOC, ITR, ETR, MS, MR.
Know encapsulation: Original packet (EID addresses) is encapsulated in outer IP header (RLOC addresses).
Use cases: Mobility, multihoming, scalable routing, traffic engineering.
Difference from traditional IP:
- Traditional IP = identity + location.
- LISP = separate identity (EID) and location (RLOC).
Mapping system: MS + MR are critical for mapping EIDs to RLOCs.

Imagine you have multiple data centers.
A server in Data Center A (EID: 10.10.10.10) moves to Data Center B.
With traditional IP: you’d need to update routes or IP addresses everywhere.
With LISP: the server keeps its EID, only the RLOC changes. Traffic automatically finds the server in the new location.

Remember EID = identity, RLOC = location.
Know the tunnel routers: ITR encapsulates, ETR decapsulates.
Understand the mapping system: MS stores, MR resolves.
Focus on benefits: mobility, scalability, multihoming, traffic engineering.
Be able to explain the encapsulation process and why it reduces routing table size.