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Re: [RRG] EXPLISP BOF at the Dublin IETF
The map request packet, which is perhaps an encapsulated initial
data packet, travels along the GRE tunnel to the nearest ALT router.
There are Data Probes or Map-Request packets. A Data Probe is a data
packet that gets LISP encapsulated where the inner destination address
is copied to the outer destination address.
A Map-Request is a UDP packet where the destination address is the
destination EID and the source address is the locator of the
There, over the GRE links to other ALT routers, each ALT router
forwards the packet according to its destination address 240.1.1.1.
The structure of this LAT network results in the packet arriving at
the correct ETR for this destination host. The map reply message
goes back via the ordinary Internet (no GRE tunnels or ALT routers)
to the ITR. Subsequent encapsulated data packets go from the ITR to
the ETR via the ordinary Internet too.
Its not clear to me how the 126.96.36.199 ITR has a tunnel to an ALT
router, unless the dotted line "Low Opex" indicates a tunnel,
without the need for a physical link to this ALT router.
The 188.8.131.52 ITR is a low-opex router and it has a configured GRE
tunnel to the ALT router.
The diagram makes it look easy, with a single GRE hop between the
first and the final ALT router. However, in reality, it would be
And in reality it is that easy. ;-)
common for the packet to have to ascend the highly aggregated ALT
hierarchy before it could descend the hierarchy towards the ALT
The ITR has a default 0.0.0.0/0 EID-prefix route so it knows to
forward all Data Probes or Map-Requests over the tunnel.
router with a direct link to the correct ETR. Also, each GRE tunnel
may involve traversing multiple physical routers. This long path
problem was discussed:
Yes, and so what? The logical topology can be relatively congruent to
the physical topology but because it is logical we want to reap the
benefits of it being logical so we can aggregate EID-prefixes as best
The ALT topology could be a topology where we use addressing based on
Geo location, but more importantly it is based on address allocation
rather the topological allocation.
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