2.3.a LISP

3 min read ENCOR 350-401 v1.2

Aligned to Cisco's 350-401 ENCOR v1.2 exam topics.

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LISP stands for Locator/ID Separation Protocol.

The name tells you the point: LISP separates who the endpoint is from where the endpoint is located in the routing system.

In traditional IP routing, an IP address does two jobs:

It identifies the endpoint.
It tells the network where to route the packet.

LISP splits those jobs.

EID: Endpoint Identifier. This is the address of the endpoint or endpoint subnet.
RLOC: Routing Locator. This is the address used to reach the LISP router in the underlay.

Why this matters

If the endpoint identity and the routing location are tied together, moving endpoints can be painful. The network has to keep learning where prefixes live.

LISP solves that by using a mapping system.

EID prefix 10.10.10.0/24 lives behind RLOC 192.0.2.10
EID prefix 10.20.20.0/24 lives behind RLOC 192.0.2.20

Routers ask the mapping system where an EID lives, then encapsulate traffic toward the correct RLOC.

Core terms

Term	Meaning
EID	Endpoint Identifier; the endpoint-side address or prefix
RLOC	Routing Locator; the underlay address of a LISP router
ITR	Ingress Tunnel Router; receives traffic from a local EID and encapsulates it toward a remote RLOC
ETR	Egress Tunnel Router; receives encapsulated LISP traffic and decapsulates it toward a local EID
xTR	A router acting as both ITR and ETR
Map Server	Stores EID-to-RLOC registrations
Map Resolver	Answers or forwards mapping requests from ITRs
Map Cache	Local cache of EID-to-RLOC mappings

Packet walk

Use this packet walk until LISP feels simple.

Host-A sends packet to Host-B.
Host-A EID: 10.10.10.10
Host-B EID: 10.20.20.20
1. Host-A sends the packet to its default gateway, a LISP xTR.
2. The xTR checks whether it knows where 10.20.20.20 lives.
3. If not, it asks the mapping system.
4. The mapping system returns the RLOC for the remote EID prefix.
5. The local xTR encapsulates the packet toward the remote RLOC.
6. The underlay routes the outer packet by RLOC.
7. The remote xTR decapsulates the packet.
8. The remote xTR forwards the original packet to Host-B.

The underlay routes to RLOCs. The overlay cares about EIDs.

LISP in Cisco SD-Access

In Cisco SD-Access, LISP is part of the control-plane story. The fabric needs to know where endpoints are located, and LISP helps map endpoint identity to fabric edge locations.

Do not memorize this as magic. Think mapping database.

Endpoint appears behind Fabric Edge A.
Control plane learns/registers location.
Traffic to that endpoint can be sent toward the right edge.

Reading config fragments

Platform syntax varies. For ENCOR, focus on recognizing the concepts.

A simplified LISP-style config may reference:

router lisp
 locator-set SITE-A-RLOC
 IPv4-interface Loopback0 priority 1 weight 100
!
instance-id 101
service ipv4
eid-table default
database-mapping 10.10.10.0/24 locator-set SITE-A-RLOC
map-server 192.0.2.100 key 7 <hidden>
map-resolver 192.0.2.100
itr
etr

Read it like this:

locator-set SITE-A-RLOC -> Which underlay locator should represent this site? -> database-mapping 10.10.10.0/24 -> Which EID prefix is local? -> map-server -> Where does this router register local EIDs? -> map-resolver -> Where does this router ask about remote EIDs? -> itr / etr -> Can this router encapsulate and decapsulate?

Lab: LISP packet walk table

Goal

Explain a LISP forwarding decision without needing a full fabric lab.

Given

Site A EID: 10.10.10.0/24
Site A RLOC: 192.0.2.10
Site B EID: 10.20.20.0/24
Site B RLOC: 192.0.2.20
Map Resolver/Server: 192.0.2.100

Task

Fill in this table for traffic from 10.10.10.10 to 10.20.20.20.

Step	Device	Action
1	Host-A	Sends packet to default gateway
2	Site-A xTR	Looks for remote EID in map cache
3	Site-A xTR	Sends Map-Request if missing
4	Mapping system	Returns Site-B RLOC
5	Site-A xTR	Encapsulates toward 192.0.2.20
6	Underlay	Routes outer packet by RLOC
7	Site-B xTR	Decapsulates packet
8	Site-B xTR	Forwards to Host-B

Verification-style commands

Commands vary, but these are the kinds of outputs to know:

show lisp site
show lisp eid-table
show lisp map-cache
show lisp rloc members
show lisp session

Look for:

Local EID prefixes registered
Remote EID mappings in cache
RLOC reachability
Map server/resolver state

LISP vs normal routing

Normal routing asks:

Where is the best next hop for this destination prefix?

LISP asks:

Which RLOC currently represents this EID, and how do I encapsulate traffic there?

Normal routing still exists in the underlay. LISP does not remove the need for IP reachability between RLOCs.

Exam traps

EID and RLOC are different roles. Do not treat them as synonyms.
The underlay must route between RLOCs.
The mapping system is the control-plane piece that lets xTRs find remote EIDs.
ITR encapsulates. ETR decapsulates. xTR does both.
LISP is a describe-level topic in ENCOR 2.3, so focus on concepts and packet walk.

Quick check

What does EID mean?
What does RLOC mean?
Which device encapsulates traffic toward a remote RLOC?
Why does LISP need a mapping system?
Does the underlay still need IP reachability between RLOCs?