MPLS-TE with OSPF
In the MPLS-TE Basics series, we used ISIS as our IGP. In this article we will swap ISIS for OSPF and briefly explore the OSPF LSAs which enable MPLS-TE.
Lab
On each router I’ll simply remove ISIS and add OSPF. All links will be in area 0.
Let’s stop here and examine the OSPF database so far:
We see a new Type-10 Opaque LSA. This is used to disseminate TE information such as bandwidth availability, link affinity, TE metric, etc.
The LSA has no links, as R1 has not discovered any neighbors yet. All we see here is the MPLS TE RID. Let’s enable OSPF on R2 and then check the LSA again.
We now see the TE information for R1 Gi1 and R2 Gi1:
The Link ID 1.0.0.0 describes the router itself - its own MPLS TE RID. The Link ID 1.0.0.X describes each link, where X is the link index.
We’ll configure the rest of the routers in the topology. I’ll only show XR3, as the rest of the configuration will be similar on every other router.
Examine the OSPF LSDB on R1 again:
You can see every router has originated a 1.0.0.0 LSA. Additionally routers have advertised their own 1.0.0.X LSAs, one for each interface. These can overlap without an issue. For example, there are multiple 1.0.0.1 LSAs, but each from a different advertising router.
You may wonder why this LSA is called Opaque area. There are three LSA types which are Opaque LSAs:
Type 9 Link-local - this is used for OSPFv3 to learn the IPv6 link-local address of a directly connected neighbor
Type 10 Area-local - this is used by MPLS-TE, and only flooded within the area
Type 11 AS scope - this is used when opaque information needs to be flooded for external prefixes to the entire OSPF domain. I have seen this used with SR when a type 5 LSA has an associated prefix SID which is advertised with a type 11 Opaque so that it is flooded “alongside” the type 5 LSA.
The first eight bits (the first 1 in the LSA IDs above) is used for the Opaque Type. Type 1 is Traffic Engineering LSA. Here is a full list of Opaque Types:
https://www.iana.org/assignments/ospf-opaque-types/ospf-opaque-types.xhtml
The last 24 bits of the LSA ID is used for the Opaque ID, which you can see in the furthest-right hand column in the output above. In all our examples so far, this is a number below 255 so this fits neatly into the last octect of the LSA ID.
The word “Opaque” refers to the fact that this information is not necessary to the core functionality of OSPF, which is choosing best paths. Instead, OSPF is carrying this “extra” information for other purposes, such as MPLS-TE. OSPF simply assists with populating the TED. It does not need the information contained in the Opaque LSAs to preform SPF.
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