Routing

configuring IGP shortcut in Junos copies the IGP learnt routes from inet.0 to inet.3.

however, inet.3 is used by BGP only by default.

if we want IGP to be able to use the LSP, 'set mpls traffic-engineering bgp-igp' command can already achieve it.

so what is the purpose of IGP shortcut in Junos?

Not answering to your question, but I understood some years ago that actually «set mpls traffic-engineering mpls-forwarding» was the way to go (LSP are used for forwarding, but the active route in inet.0, used by policies, stays OSPF or ISIS).

indeed that's one of the options for IGP to use the LSP.

in fact, all 3 options below can, just in different ways.

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/traffic-engineering-edit-protocols-mpls.html

bgp-igp | bgp-igp-both-ribs | mpls-forwarding

what i find is that RFC 3906 does not mandate IGP must be able to use the IGP shortcut.

https://datatracker.ietf.org/doc/html/rfc3906

in Junos, since only BGP will use inet.3 if you don't configure any of the above options, it is conceptually like 'split tunnel' where only VPN traffic goes into the LSP.

whereas by comparison in IOS XE and XR, IGP shortcut is enabled by configuring 'autoroute announce' command under tunnel interface, which in effect changes the egress interface of the LSP destination from physical to tunnel, which per se is conceptually like 'full tunnel' where all traffic goes into the LSP.

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DANIEL NG
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