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LISP Architecture

Chapter Description

In this sample chapter from LISP Network Deployment and Troubleshooting: The Complete Guide to LISP Implementation on IOS-XE, IOS-XR, and NX-OS, you will explore LISP core architecture and components, including the roles and functionality of xTRs, PxTRs, MR/MS, and ALT.

LISP Control Plane Messages

A LISP control plane message is a User Datagram Protocol (UDP) message with either a source or destination UDP port of 4342. The control plane packet is either in IPv4 or IPv6 format. Figure 2-13 shows the IPv4 control plane message format, and Figure 2-14 shows the IPv6 control plane message format. From the packet headers shown in Figures 2-13 and 2-14, notice that LISP message formats are similar to the previously discussed packet headers; however, in place of the LISP header, there is a LISP message, and there is no header for the EID.


FIGURE 2-13 LISP IPv4 Message Format


FIGURE 2-14 LISP IPv6 Message Format

These are the various LISP control plane messages, as defined in RFC 6830:

  • Type 0: Reserved

  • Type 1: LISP map request

  • Type 2: LISP map reply

  • Type 3: LISP map register

  • Type 4: LISP map notify

  • Type 8: LISP encapsulated control message

LISP Map Request

As defined in RFC 6830, a map request is initiated by the ITR and sent to the ETR of interest by the map server. A map request is initiated by an xTR when

  • It needs a mapping for an EID.

  • It wants to test an RLOC for reachability.

  • It wants to refresh a mapping before TTL expiration.

  • It receives from the ETR a solicitation for a map request (SMR) in case of a virtual host move.

As stated earlier, map requests can also be LISP encapsulated with a LISP type value set to encapsulated control message when sent from an ITR to an MR. In addition, map requests are LISP encapsulated the same way from an MS to an ETR. When a map request is sent, the source UDP port is chosen by the xTR (sender), but the destination UDP port is always 4342.

Figure 2-15 shows the map request message format. The map request contains information about the EID prefix, EID prefix length, and AFI that it belongs to, the ITR-RLOC address, and the ITR-RLOC AFI to which the ITR-RLOC address belongs. Note that at the end of the map request message, there is a map reply record. This field is useful when the M bit (the map data present bit) is set and can be used by an ETR to update its map cache entry for the EID-to-RLOC mapping for the source EID. When the M bit is set, it indicates that the map reply record segment is included in the map request.


FIGURE 2-15 LISP Map Request Message Format

LISP Map Reply

In response to a map request, the ETR sends a map reply with the EID-to-RLOC mapping information that is requested either in the destination field of the IP header of a data probe or the EID record of a map request. The RLOCs are globally routable IP addresses of all ETRs for the LISP site. In a map reply message for the map request message, the destination address is copied from the source address of the data probe message. The map reply can have an empty locator set. This kind of reply message is called a negative map reply message. The map reply is sent with the source UDP port number 4342, and the destination UDP port number is copied from the source port of either the map request or the invoking data packet.

Figure 2-16 shows the map reply message format. In the map reply, the record consists of information about the EID and the locator along with the respective AFI. It also contains a record time-to-live (TTL) field. The value of this field is the time, in minutes, the recipient of the map reply stores the mapping. If the TTL value is set to 0, the entry is removed from the cache immediately. If the value is set to 0xffffffff, the recipient decides locally how long it wants to save the entry in the map cache. Note that in Cisco’s implementation, when there is just 1 minute left for the map cache entry to expire, the xTR initiates another map request to update its map cache before the entry expires.


FIGURE 2-16 LISP Map Reply Message Format

LISP Map Register Message

As explained in RFC 6833, an xTR sends a map register message to an MS to register its associated EID prefixes and also includes one or more RLOCs to be used by the MS when forwarding map requests. An MS can send a map reply in response to a map request on behalf of the xTR upon request. It is done by setting the P bit (the proxy map replay bit) in the map register message. The map register message is sent with the destination UDP port number 4342 and a random source UDP port. Figure 2-17 shows the map register message format. The map register message format is similar to the map reply message format, except that there are a few extra bits in a map reply message, and have an additional 8 bytes of key ID, authentication data length, and authentication data.


FIGURE 2-17 LISP Map Register Message Format

LISP Map Notify Message

An MS sends a map notify message to an xTR to confirm that the map register message was received and processed. An xTR requests that a map notify be returned by setting the M bit in the map register message. The map notify uses 4342 as both the source and destination UDP port number. Figure 2-18 shows the map notify message format.


FIGURE 2-18 LISP Map Notify Message Format

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