Here's a condensed summary of our December, 2012 Facebook forum in a Q&A format.
Which IGPs are used for distributing MPLS TE topology ?
We use link state protocols (OSPF and ISIS ) to have complete knowledge of topology.
OSPF => Opaque LSA type 9 (link-local only), opaque type 10 (area wid) and opaque type 11(autonomous system wide).
ISIS => TLV Type 22 (Topology details), TLV type 134 (TE router id ).
What are the Traffic Parameter Attributes?
TE metric, Maximum bandwidth, Maximum reservable bandwidth, Unreserved bandwidth, Administrative group.
Is it Must to use Record route while defining MPLS TE tunnels?
Record route is not necessary for basic TE deployment. However, for NNHOP FRR we need record route to know the label used by the NNHOP. Also Record route is used for loop detection and to see the path TE tunnel traversed.
What is mpls traffic engineering ERO ?
RSVP-TE Explicit Route Object (ERO) is the path of an MPLS LSP which includes sequenced list of Label Switching Routers (LSRs) that the LSP must pass through between the ingress and egress LSRs. RSVP-TE uses the path described in the ERO to signal and set up the LSP . The explicit path could be administratively specified, or automatically computed using an algorithm such as constrained shortest path first (CSPF).
Whats difference between TE fRR & standby hot LSP (Protected LSP ) ?
Standby LSP is created on the headend router so that the when primary lsp goes down the standby takes over, standby LSP is in ready state to prevent delay of signalling once the primary LSP gors down . However in FRR , backup LSP can be on any hop in the path of primary lsp which is protecting the primary LSP ( link or node protection).
What is MPLS TE forwarding-adjacency ?
The MPLS TE Forwarding Adjacency feature allows traffic engineering tunnel as a link in an Interior Gateway Protocol (IGP) network based on the Shortest Path First (SPF) algorithm. MPLS TE forwarding adjacency tunnels must be configured bidirectionally.
What are Strict and Loose EROs ?
The explicit path specified in the ERO may be a strict path or a loose path. A strict path means that a network node and its preceding node in the ERO must be adjacent and directly connected.A loose hop means that a network node specified in the ERO must be in the path but is not required to be directly connected to its preceding node. If a loose hop is encountered during ERO processing, the node that processes the loose hop can update the ERO with one or more nodes along the path from itself to the next node in the ERO. The advantage of a loose path is that the entire path does not need to be specified or known when creating the ERO.
How does RSVP handles changes dynamically like change in interface bandwidth, TE re-optimization ?
Basically due to any reason when reoptimization happens , in shared explicit style the make before break LSP happens. Once the new LSP is created traffic is routed on it and after that the old LSP is broken. Another thing is on the common links where the old and new LSP passes , bandwidth is shared b.w old and new LSP so that double resources are not reserved for the same tunnel during reoptimization.
Here is some bonus information on this topic prepared by our expert in a Q&A format.
Can you give a quick overview of MPLS traffic engineering?
MPLS TE are unidirectional tunnels from a source (Headend) to the destination (TAILend) in the form of LSPs, which is then used to forward traffic. MPLS TE works by learning about the topology and resources available in a network using a link state routing protocol such as Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (IS-IS).
The Constraint-Based Shortest Path First (CSPF) algorithm that operates on the tunnel headend is used for finding the shortest path to a particular network that meets the resource requirements of traffic flow (hence called constraint based SPF) . Based on the best path calculated by the CSPF algorithm , Resource Reservation Protocol (RSVP) with TE extensions is used as the signaling protocol. This protocol reserves resources for traffic flow and establishes the LSP for traffic flow by exchanging labels.
What are main RSVP message types and fields in RSVP Signalling ?
The primary RSVP messages are PATH and RESV messages. Senders send a PATH message from the source to the receiver with label request and specification of the reservation requirements of a data flow. Receivers send a RESV message to the sender to reserve resources for the flow along with the LABEL.
The base Session construct for RSVP TE is based on the triple {Tunnel Remote Address, Tunnel ID, Extended Tunnel ID}. The PATH message contain a Label- Request object (LRO) that results in a label being assigned during the RESV, a Session-Attribute object that is used to provide additional requirements for a session, and an Explicit-Route object (ERO) that specifies the data path traversed by the PATH message, which could be independent of IP-routing. The RESV message includes a Label object that contains the MPLS label and a Record-Route object (RRO) to record the path taken followed by the RESV message.
Can MPLS TE tunnel be created without LDP ?
Yes , TE signals labels through RSVP as label signalling protocol ( in basic setup ).
What are TE setup and hold priorities .
The setup priority indicates how important the tunnel is to preempt the other tunnels, whereas the holding priority indicates how much the weight of that tunnel is to hold on to its reservations on the links. A tunnel that has a lower setup priority than the holding priority of a second tunnel can preempt that second tunnel thats why The setup priority of a tunnel should not be lower than its holding priority .
What is RSVP RRO ?
PATH and RESV messages carry this object, which stores the IP addresses of the routers that the TE tunnel traversed. You can record in the RRO the label that is associated with each hop.
RRO is mainly used to avoid loops , NNHOP fast reroute and while troubleshooting to understand which path TE tunnel is taking.
What is TE Reoptimisation ?
Reoptimization makes existing TE tunnel to be rerouted in the network onto the more optimal path / better path which can be done Manually , periodically and event driven ( Link up ).
What is TE SRLG ?
Shared risk link groups is the identifier mark links which share the same fiber network / same provider / same conduit. This is used which MPLS TE backup tunnel for FRR is created to ensure that backup tunnel doesnot use the protection link which shares the same SRLG as the protected link.
What is MPLS TE FRR and backup Tunnel ?
MPLS TE supports local repair of TE LSPs using FRR. FRR uses a local protection approach that relies on a presignaled backup TE LSP (backup tunnel) to reroute traffic in case of a failure on the link or next neighboring hop . Head end of backup tunnel is called PLR ( point of local repair ) and tail end called MP ( merge point ).
There are two types of protection.
Link protection uses a backup TE LSP destined to the PLR next hop (NHOP). When a node signals a TE LSP with link protection desired, nodes along the path attempt to associate the TE LSP with a backup TE LSP to the NHOP downstream.
Node protection uses a backup TE LSP destined to the PLR next-next hop (NNHOP). When a node signals a TE LSP with node protection desired, nodes along the path attempt to associate it with a backup TE LSP to the NNHOP downstream.The PLR learns the NNHOP label from the RECORD_ROUTE object in Resv messages .
For more information visit: http://www.cisco.com/en/US/docs/switches/datacenter/sw/5_x/nx-os/mpls/configuration/guide/mp_te_RSVP.html
------------------------------------------------------------------------------------------------------------------------------------------------
Here are the links to the forum questions on our wall and events page:
http://www.facebook.com/133380531411/posts/10151380148311412 http://www.facebook.com/events/473508526025670/permalink/475673472475842/ http://www.facebook.com/events/473508526025670/permalink/475679262475263/ http://www.facebook.com/events/473508526025670/permalink/475687072474482/
Here's a link to the forum archive on Facebook:
http://www.facebook.com/notes/cisco-online-support-community-netpro/facebook-forum-summary-configuring-and-troubleshooting-mpls-traffic-engineering/490311594340782
Here's the link to this event announcement page on Facebook
http://www.facebook.com/events/473508526025670/
