What port does LDP use?

LDP discovers neighbours on UDP 646, then switches to TCP 646 for sessions

What is the default CSPF method of tie-breaking which equal path to build an LSP on?

Default is random. Can also have least-fill, most-fill.

Which OSPF LSA is used to share traffic engineering data?

Type 10 (opaque LSA)

How many bits is the admin-groups 'colour' space?

32 bits

How do you set a colour called green to equate to 31?

set protocols mpls admin-groups green 31

Which admin-group policy is equivalent to a logical OR?

include-any

Where do you assign an interface to an admin-group (or groups)?

Under `set protocols mpls interface x admin-group`

When would you use a bidirectional LSP association? What do you have to give up to use them?

When you want to use OAM to track statistics/performance of an LSP, you need to associate two uni-directional LSPs together. You have to give up corouted-bidirectional LSPs in order to use this.

What MPLS label is the implicit null? Who sends it?

implicit null = 3 It's sent by the egress LSR to its closest transit LSR, indicating it expects penultimate hop popping

What is the purpose of the RSVP-TE PATH message?

To discover and record the route an LSP will take, advertise traffic parameters, and trigger downstream path state installation at each hop.

What is the purpose of the RSVP-TE RESV message?

To confirm bandwidth reservation and distribute label bindings hop-by-hop from egress back to ingress, establishing the label-switched forwarding path.

What does the SESSION object identify?

The LSP, using a combination of the egress destination address, a tunnel ID, and an extended tunnel ID (usually the ingress address).

What is the SENDER_TSPEC and what does it carry?

The traffic specification object; it carries the token bucket rate (r), token bucket depth (b), peak rate (p), and min/max packet sizes that define the traffic contract for the LSP.

What is the ERO and how is it used?

The Explicit Route Object; it carries an ordered list of hops computed by CSPF at the ingress. Each transit LSR pops its own address from the ERO before forwarding the PATH message onward.

What is the RRO and how does it differ from the ERO?

The Record Route Object; it records the actual path taken by appending each router's address as the PATH message passes through. Unlike the ERO (which is pre-computed), the RRO is built dynamically hop by hop.

What is CSPF and why does the ingress use it?

Constrained Shortest Path First; a modified Dijkstra algorithm run at the ingress that computes a path satisfying constraints such as available bandwidth, TE metric, and link admin groups, using the TE topology database.

What is RSVP soft state and why does it matter?

Path and reservation state at each router expires unless periodically refreshed by PATH and RESV messages. If the ingress fails, downstream routers time out and release resources automatically without needing an explicit teardown.

What label operation does the ingress LER perform, and how does it differ from a transit LSR?

The ingress *imposes* (pushes) a label onto an unlabelled IP packet. Transit LSRs *swap* the incoming label for the outgoing label learned from the RESV message.

What is Penultimate Hop Popping (PHP) and how is it signalled?

PHP is when the second-to-last router removes the MPLS label before forwarding to the egress, reducing egress processing. The egress signals this by advertising the implicit null label (value 3) in the RESV LABEL object.

What triggers a PathTear message and what does it do?

The ingress sends PathTear when an LSP is no longer needed. It propagates downstream and causes each router to immediately release path state and reserved resources, rather than waiting for soft-state timeout.

What is admission control in RSVP-TE and where does it happen?

Each router along the path checks whether it has sufficient resources (bandwidth, buffer) to honour the FLOWSPEC in the RESV message. If not, it rejects the reservation and sends a ResvErr back toward the ingress.

What is the difference between a PathErr and a ResvErr?

A PathErr travels upstream (toward the ingress) and reports a problem with the PATH message, such as an unroutable ERO. A ResvErr also travels upstream but reports a failure in the reservation phase, such as insufficient bandwidth.

What problem does RSVP-TE Refresh Reduction (RFC 2961) solve?

At scale, periodic PATH and RESV refresh floods consume significant control-plane bandwidth and CPU. RFC 2961 introduces summary refresh messages and bundle messages so routers can acknowledge state is still valid without retransmitting full message objects.

What is the LABEL_REQUEST object and which message carries it?

An object carried in the PATH message that tells downstream routers (and the egress) that MPLS label bindings are required for this LSP. Without it, RSVP operates in IntServ mode without label distribution.