Information About TTL Propagate Disable and Site-ID Qualification
LISP site is a set of routers in an edge network that are under a single technical administration. LISP routers in the edge network are the demarcation points to separate the edge network from the core network.
Map Server (MS)
An MS implements part of the distributed LISP mapping database by accepting registration requests from its client Egress Tunnel Routers (ETRs) and aggregating the successfully registered EID prefixes of ETRs.
Routing Locator (RLOC)
An RLOC is an IPv4 or IPv6 address of an Egress Tunnel Router (ETR).
The traceroute tool is used to discover the routes that packets take when traveling to their destination.
Site ID Qualification
A site is best conceptualized as an authentication domain: A set of ETRs under the same administrative control. The map server authenticates all ETRs in a site using the same shared key. Without the concept of a site, the map server would be required to have prior knowledge of every ETR in the network along with its authentication key. Site managers will not be able to deploy new ETRs without changing the configuration of the map servers. When a site is considered as an authentication domain as opposed to a topological grouping, then it is easy to see that the benefit of site ID qualification resides in the ability of reaching an EID prefix through ETRs under different administrative control.
With Site ID Qualification, the map server can have the same prefix configuration under multiple sites. The name of the feature stems from the requirement that any two sites with at least one prefix in common must be qualified with a unique site IDs.
- A LISP ITR encapsulates a packet and copies TTL value from inner header to outer header.
- A LISP ETR decapsulates a packet and copies TTL value from outer header to inner header if the outer header TTL value is smaller than the inner header TTL.
When TTL propagation is enabled the traceroute tool can display all middle hops between an LISP ITR and ETR. However, when RLOC and EID are of different address-family the traceroute output is undesirable.
When the above cross address-family situation exists, LISP does not propagate TTL between inner and outer IPv4 or IPv6 headers. During encapsulation, ITR uses the maximum permissible TTL in the outer header instead of using the TTL value from the inner header.
It is better to make the LISP tunnel between the ITR and ETR appear as a single hop to the client of traceroute. This is done through the disable-ttl-propagate configuration CLI either for a specific eid-table or the entire router lisp tag.
The TTL propagation is turned on automatically.