LISP (Locator/ID Separation Protocol)

LISP (Locator/ID Separation Protocol) - Complete Guide for CCNP ENCOR

Why LISP is Important

LISP addresses fundamental challenges in modern networking by separating the identity of devices (Endpoint IDentifiers) from their location information (Routing Locators). This separation is critical because:

Scalability: Traditional routing relies on IP addresses serving dual purposes, which creates routing table bloat. LISP reduces the need for constantly updating routing tables during network changes.

Mobility: Devices can move between networks without changing their identity. Users can seamlessly move across data centers or locations while maintaining their original IP identity.

Multi-homing: Endpoints can connect through multiple providers or paths simultaneously, improving redundancy and load balancing.

Traffic Engineering: Network administrators gain finer control over how traffic is routed, enabling better optimization and policy enforcement.

Future-proofing: LISP provides a path toward addressing IPv4 address exhaustion and simplifying IPv6 migration.

What is LISP?

LISP is a network architecture that fundamentally changes how devices are identified and located on networks. It introduces a clean separation between two namespaces:

EID (Endpoint IDentifier): This is the identity of the actual host or device. It remains constant regardless of where the device is located. Think of it as the device's permanent name or identity.

RLOC (Routing Locator): This is the topological address that indicates where the device currently is located in the network. This can change as devices move or connect through different access points.

In traditional networks, a single IP address serves both purposes, creating confusion and limiting flexibility. LISP separates these concerns, much like how DNS separates human-readable names from IP addresses.

Key LISP Components:
• Map Server (MS): Maintains a database of EID-to-RLOC mappings
• Map Resolver (MR): Responds to mapping queries from ITRs
• Ingress Tunnel Router (ITR): Receives packets destined for EIDs and encapsulates them based on RLOC mappings
• Egress Tunnel Router (ETR): Receives encapsulated packets and removes the LISP encapsulation
• xTR: A device that functions as both ITR and ETR

How LISP Works

Step 1: Initial Setup
ETRs register their EID-to-RLOC mappings with the Map Server. This creates a database of who (EID) is where (RLOC).

Step 2: Packet Arrival at ITR
When an ITR receives a packet destined for an EID it doesn't already know about, it needs to find the mapping.

Step 3: Map Request
The ITR sends a Map-Request message to the Map Resolver asking: "Where is this EID?" The Map Resolver queries the Map Server's database.

Step 4: Map Reply
The Map Server (or a device that knows the mapping) responds with a Map-Reply containing the RLOC addresses associated with that EID.

Step 5: Caching and Encapsulation
The ITR caches this mapping and encapsulates the original packet with a LISP header. The outer header contains the RLOC destination address (where to send it), while the inner header contains the EID destination address (what the packet is ultimately for).

Step 6: Transit Across Network
The encapsulated packet travels through the core network using RLOC routing, which appears normal to traditional routers.

Step 7: Decapsulation at ETR
When the packet reaches the ETR at the destination, it removes the LISP encapsulation and forwards the original packet to the destination EID.

Step 8: Return Traffic
Return traffic from the EID follows the same process in reverse, potentially using a different path.

Caching and Optimization: ITRs cache mappings to avoid repeated Map-Requests. This cache has a TTL and reduces latency for ongoing flows.

LISP Encapsulation Details

LISP adds an encapsulation header between the IP header and the original packet. The structure includes:

Outer IP Header: Source = ITR's RLOC, Destination = ETR's RLOC
LISP Header: Contains flags, nonce, and Locator-Status Bits
Inner IP Header: Source = Actual sender's EID, Destination = Intended recipient's EID
Original Payload: The actual data being transmitted

This dual-header approach allows the packet to be routed based on location (RLOC) while maintaining the true identity (EID) of communicating endpoints.

LISP Deployment Scenarios

Campus Networks: LISP enables seamless mobility and simplified traffic engineering within large enterprise networks.

Data Center Interconnect: Multiple data centers can use LISP to provide transparent failover and load balancing across geographic locations.

Multi-cloud Environments: Organizations can connect resources across multiple cloud providers while treating them as a unified network.

Hybrid Cloud: On-premises data centers can connect to cloud resources with LISP providing consistent addressing and routing policies.

Common LISP Terminology

EID Space: The address space used for endpoint identifiers, typically in a private range

RLOC Space: The globally routable address space used for locating devices in the core network

LISP Site: A location or network segment that uses LISP

Core Network: The intermediate network between LISP sites, which doesn't need to understand LISP

Ingress/Egress: ITR handles ingress (packets entering LISP domain), ETR handles egress (packets leaving LISP domain)

Map Cache: The ITR's local database of EID-to-RLOC mappings, used for rapid lookups

Solicit-Map-Request (SMR): A mechanism for ETRs to notify ITRs of mapping changes, allowing cache updates without waiting for TTL expiration

LISP vs Traditional Routing

Traditional Routing:
• Single IP address space
• Routing tables contain all reachable destinations
• Device moves = routing changes needed
• Scaling limited by core routing table size

LISP Routing:
• Dual namespace (EID and RLOC)
• Core routers only know about RLOCs
• Device moves = only mapping changes needed
• Core routing tables remain stable and smaller
• Endpoints can change location transparently

Exam Tips: Answering Questions on LISP

Tip 1: Focus on the Core Concept
Remember that LISP's fundamental principle is separating identity (EID) from location (RLOC). If you're unsure about a question, ask yourself: "Does this relate to identity or location?" This mental model will guide you to the correct answer.

Tip 2: Understand Component Roles
Know what each component does:
• ITR = Encapsulates outgoing packets (sender side)
• ETR = Decapsulates incoming packets (receiver side)
• Map Server = Database keeper (stores mappings)
• Map Resolver = Query responder (finds mappings)

Test questions often ask which component performs a specific function.

Tip 3: Distinguish Between Addresses
When a question mentions an address, immediately classify it:
• Is it discussing the source/destination of the actual communication? = EID
• Is it discussing the tunnel endpoint or intermediate routing? = RLOC

This distinction is crucial for questions about encapsulation and routing behavior.

Tip 4: Master the Request/Reply Process
Expect questions about the mapping discovery process. Remember the sequence:
1. ITR sends Map-Request to Map Resolver
2. Map Resolver queries Map Server
3. Map Server (or authoritative source) sends Map-Reply
4. ITR caches the mapping
5. ITR encapsulates and sends packet to RLOC

Questions might ask what happens at each step or what component is involved.

Tip 5: Remember Caching Behavior
LISP includes intelligent caching:
• Map caches reduce repeated Map-Requests
• TTL controls cache validity
• SMR (Solicit-Map-Request) allows proactive cache updates
• Negative caching can cache "no route" information

Questions about optimization or reducing latency often relate to caching mechanisms.

Tip 6: Understand Encapsulation Overhead
LISP adds overhead through encapsulation. Be ready to discuss:
• Additional header bytes reduce effective MTU
• This can require path MTU adjustments
• Some deployments use hardware-accelerated encapsulation for performance

Questions might ask about performance implications or configuration considerations.

Tip 7: Know Common Deployment Benefits
LISP is often presented as a solution to specific problems:
• Mobility = Devices move without routing changes
• Load Balancing = Multiple RLOCs for single EID
• Multi-homing = Redundant connections
• Scalability = Reduced core routing table size

If a question asks "What problem does LISP solve?", match the scenario to these benefits.

Tip 8: Recognize LISP Limitations
Exam questions might test knowledge of LISP constraints:
• Requires compatible endpoints or tunneling infrastructure
• Not universally deployed (limited to specific network segments)
• Adds complexity and requires operational knowledge
• Encapsulation adds slight latency and overhead

Tip 9: Compare with Alternatives
Be prepared to explain how LISP differs from:
• VPN: LISP is at network layer; VPNs can be at various layers
• Traditional Routing: LISP separates identity from location; traditional routing conflates them
• NAT: LISP is purposeful address mapping; NAT is transparent rewriting

Tip 10: Practice Scenario Analysis
LISP questions often present scenarios. Develop a habit of:
1. Identifying what traffic needs to move (source EID to destination EID)
2. Determining which ITR and ETR are involved
3. Tracing the encapsulation process
4. Predicting the path through the network

Walk through the 8-step process described earlier.

Tip 11: Watch for Terminology Traps
Exam questions sometimes test precise terminology:
• Encapsulation (ITR) vs Decapsulation (ETR)
• Map-Request (query) vs Map-Reply (response)
• Registration (ETR to MS) vs Resolution (ITR querying)

Read questions carefully to ensure you're using the right term.

Tip 12: Study Configuration Elements
Be familiar with:
• EID prefixes and how they're assigned to sites
• RLOC addresses and their role in core routing
• Map Server and Map Resolver IP addresses
• TTL values and their impact on convergence
• Priority and weight values for load balancing across multiple RLOCs

Tip 13: Understand Advanced Scenarios
Be prepared for complex questions involving:
• LISP to Non-LISP Traffic: How LISP sites communicate with traditional networks (proxy ITR/ETR)
• Nested Encapsulation: LISP over other tunneling protocols
• Map Versioning: Tracking changes in mappings to optimize convergence
• Negative Caching: Handling unmapped or unreachable EIDs

Tip 14: Remember Convergence Implications
When an ETR's RLOC changes (device mobility):
• Old mappings in ITR caches become stale
• SMR notifications speed up cache invalidation
• New Map-Requests populate updated mappings
• Traffic seamlessly follows the device to its new location

Questions about mobility scenarios often test this understanding.

Tip 15: Use Process of Elimination
For multiple-choice questions:
• Eliminate answers that confuse EID and RLOC
• Eliminate answers about components not involved in the scenario
• Eliminate answers that suggest LISP requires changes to core network routers (it doesn't)
• Select answers that reflect LISP's key benefit: separation of identity from location

Sample Question Analysis

Question: "A company has implemented LISP between their data centers. A server at the New York site (EID 10.1.1.10) communicates with a database at the San Francisco site (EID 10.2.2.20). The New York ETR has an RLOC of 203.0.113.5, and the San Francisco ETR has an RLOC of 198.51.100.8. Which of the following best describes the path of traffic from New York to San Francisco?"

Analysis Approach:
1. Identify EIDs: 10.1.1.10 (NY server) and 10.2.2.20 (SF database)
2. Identify RLOCs: 203.0.113.5 (NY) and 198.51.100.8 (SF)
3. Recognize NY is sender, so its router acts as ITR
4. Recognize SF is receiver, so its router acts as ETR
5. The ITR will encapsulate packets with outer IP header: source 203.0.113.5, destination 198.51.100.8
6. Inner IP header remains: source 10.1.1.10, destination 10.2.2.20
7. Core routers route based on outer header (RLOC routing)
8. ETR removes encapsulation and delivers to 10.2.2.20

The answer would describe this encapsulation and the separate routing of inner vs. outer headers.

Key Takeaways for Exam Success

• LISP separates identity (EID) from location (RLOC)
• ITRs encapsulate outgoing traffic, ETRs decapsulate incoming traffic
• Map Servers store mappings, Map Resolvers query them
• The mapping process involves Map-Request and Map-Reply messages
• Caching optimizes performance by storing known mappings
• LISP enables mobility, load balancing, and scalability
• Encapsulation allows LISP sites to work with traditional core networks
• Always trace traffic from EID to EID while understanding RLOC routing
• Study scenario-based questions by walking through the step-by-step process
• Focus on LISP's practical benefits rather than just technical details