2.3.a LISP

What this objective tests

This is a describe objective. The exam wants you to explain the Locator/ID Separation Protocol (LISP) and walk a packet through it. You do not need to deploy a fabric.

In traditional IP routing, one address does two jobs. It identifies the endpoint, and it tells the network where to route. LISP splits those two jobs:

• EID: Endpoint Identifier. The address of the endpoint or endpoint subnet.
• RLOC: Routing Locator. The underlay address of the LISP router that serves the endpoint.

Why the split matters

When identity and location are tied together, moving endpoints is painful. The network must keep relearning where prefixes live.

LISP uses a mapping system instead. A router asks the mapping system where an EID lives. Then it encapsulates traffic toward the correct RLOC.

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

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

Core roles

The PITR and PETR connect the LISP world to the non-LISP world. A PITR brings outside traffic into the mapping system. A PETR delivers LISP traffic to destinations that have no mapping.

Example: one packet from Site A to Site B

Scenario: Host-A is 10.10.10.10 at Site A. Host-B is 10.20.20.20 at Site B. The map server and map resolver share 192.0.2.100.

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 server / map resolver: 192.0.2.100

The packet walk:

1. Host-A sends a packet to 10.20.20.20 through its default gateway, the Site A xTR.
2. The Site A xTR checks its map cache for 10.20.20.0/24.
3. The entry is missing, so the xTR sends a Map-Request to the map resolver.
4. The mapping system answers: 10.20.20.0/24 is behind RLOC 192.0.2.20.
5. The Site A xTR caches the mapping and encapsulates the packet toward 192.0.2.20.
6. The underlay routes the outer packet by RLOC.
7. The Site B xTR decapsulates the packet.
8. The Site B xTR forwards the original packet to Host-B.

The underlay never saw the 10.20.20.0/24 prefix. It only routed a packet to 192.0.2.20.

Reading config fragments

Platform syntax varies. For ENCOR, recognize the concepts in a fragment like this:

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 line by line:

• locator-set: which underlay address represents this site.
• database-mapping: which local EID prefix this router serves.
• map-server: where this router registers its local EIDs.
• map-resolver: where this router asks about remote EIDs.
• itr and etr: this router can encapsulate and decapsulate.

LISP in Cisco SD-Access

In Cisco SD-Access, LISP is the control plane for endpoint location. The fabric registers where endpoints are. Traffic to an endpoint goes toward the correct fabric edge node.

Endpoint appears behind fabric edge A
-> control plane registers the location
-> traffic to that endpoint is sent toward edge A

Think mapping database, not magic.

Lab: LISP packet walk table

This is a paper lab. No devices are necessary.

Use the Site A and Site B values from the example above. Do these steps:

1. Name the EID of the destination host. (Answer: 10.20.20.20.)
2. Name the RLOC the source xTR must learn. (Answer: 192.0.2.20.)
3. Name the device that answers the Map-Request. (Answer: the map resolver at 192.0.2.100.)
4. Name the device that encapsulates the packet. (Answer: the Site A xTR, acting as ITR.)
5. Name the device that decapsulates the packet. (Answer: the Site B xTR, acting as ETR.)
6. Name what the underlay uses to forward the outer packet. (Answer: the RLOC route to 192.0.2.20.)
7. A non-LISP site sends traffic to 10.20.20.20. Name the device that brings it into LISP. (Answer: a PITR.)

Verification-style commands to recognize:

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

Look for registered local EIDs, cached remote mappings, and RLOC reachability.

LISP vs normal routing

Normal routing asks: what is the best next hop for this destination prefix?

LISP asks: which RLOC currently represents this EID, and how do I encapsulate toward it?

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, not synonyms.
• The underlay must route between RLOCs.
• The mapping system is the control point that lets xTRs find remote EIDs.
• The ITR encapsulates. The ETR decapsulates. The xTR does both.
• The PITR and PETR connect LISP sites to non-LISP sites.
• LISP is describe-level in ENCOR. Focus on concepts and the packet walk.

Pass check

You are ready for this objective when you can do these things:

• Define EID, RLOC, xTR, map server, and map resolver in one sentence each.
• Explain the role of a PITR and a PETR.
• Walk a packet from source EID to destination EID in eight steps.
• Read a LISP config fragment and name what each line controls.
• Explain why the underlay still needs normal routing between RLOCs.