This article provides a discussion of how ISIS distributed information is used to populate the TED (Traffic Engineering Database) on a Juniper router. It references the industry well-known topic RFC5305 “IS-IS Extensions for Traffic Engineering”.

There is a specific part related to the Juniper implementation (starting with Junos OS 13.3) resulting in additional IP addresses possibly found in the TED and as such used for MPLS LSPs tunnel destinations. In particular, besides MPLS LSP tail-end's TE RID (ISIS LSP's TLV  type 134), other /32 IP addresses configured on it can find their way (by the means of ISIS TLV type 132) to the TED on other Juniper routers in the ISIS domain.

Note: This discussion is specific to ISIS, as OSPF does not exhibit the same behavior.

 

References

As per RFC5305 “IS-IS Extensions for Traffic Engineering”, the ISIS TLVs (Sub-TLVs) that are used to populate the MPLS TED with LSP's tail-end destination IP addresses are:

1: TLV type 134:  The Traffic Engineering router ID TLV

The router ID TLV contains the 4-octet router ID of the router originating the ISIS LSP.  For traffic engineering, it guarantees that we have a single stable address that can always be referenced in a path that will be reachable from multiple hops away, regardless of the state of the node's interfaces.

This sub-TLV contains a 4-octet IPv4 address for the interface described by the (main) TLV.

This sub-TLV contains a single IPv4 address for a neighbouring router on this link.

Although not mentioned in RFC5305, also note ISIS TLV type 132. In Junos OS 13.3, TLV type 132 started to play a role with TED “population”. A good reference for ISIS TLV type 132 is the famous JNCIS study guide: https://www.juniper.net/assets/us/en/local/pdf/study-guide/study-guide-jncis.pdf

IP Interface Address TLV

Each IPv4 address configured on an IS-IS router may be advertised in the IP interface address TLV (type code 132). While a minimum of one address must be included, an individual implementation may include all of the local router’s addresses. The JUNOS software default is to advertise just the address configured on the loopback interface within the TLV. 

Topology

  lo0: [47.47.3.3]                            lo0: [47.47.1.1/47.47.2.2]
   +----------+                                      +------------+             
   |          |-[xe-2/0/1.0​]------------[xe-3/0/3.0​]-|  GRANADA   |-[ge-4/1/3.0]--     
   |  BERGEN  |                                      |------------|               |     
   |          |-[xe-2/0/1.2]------------​[xe-3/0/3.2]-| GRANADA-LS |-[ge-4/1/4.0]--      
   +----------+                                      +------------+ 
                                                   lo0: [47.47.11.11] 

Router Roles

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Pre Junos OS 13.3 Behavior                                             

First, in this article the behavior of pre Junos OS 13.3 behavior will be demonstrated. For that, it is enough to have just the MPLS LSP tail-end router on pre 13.3 Junos OS release. In summary, the IP addresses "imported" in the TED are from ISIS TLV type 134 and sub-TLVs (6 and 8) of the TLV type 22.
 

MPLS LSP Head-end Router

MPLS LSP Tail-end Router

user@granada-re0> show version 
Hostname: granada-re0
Model: mx480
JUNOS Base OS boot [12.3R9.4]
<…>

user@granada-re0> show isis overview 
Instance: master
  Router ID: 47.47.1.1
  <…>

user@granada-re0> show isis adjacency 
Interface             System         L State        Hold (secs) SNPA
ge-4/1/3.0            granada-re0-milan 2 Up                 26
xe-3/0/3.0            bergen-re0     2  Up                   20

Auxiliary Router (Logical-System on the MPLS LSP tail-end node)

user@granada-re0> show isis overview logical-system milan 
Instance: master
  Router ID: 47.47.11.11
  <…>

user@granada-re0> show isis adjacency logical-system milan    
Interface             System         L State        Hold (secs) SNPA
ge-4/1/4.0            granada-re0    2  Up                   24
xe-3/0/3.2            bergen-re0     2  Up                   26

The focus is on having more than one IP addresses configured on the MPLS LSP tail-end's (egress) lo0 interface and attempting to establish CSPF routed MPLS LSP towards each of those, in particular to non TE RID IP address. For sake of simplicity, only two (2)  Lo0 addresses are configured.

As well, the Router ID (RID) is explicitly configured to correspond to one of those two (2) IP addresses:

MPLS LSP Tail-end Router

MPLS LSP Head-end Router

On the MPLS LSP  head-end (ingress) router two MPLS LSPs are configured, one to each of those 2 tail-end’s Lo0 IP addresses.

Note that only of the MPLS LSP’s is established using the CSFP (TED). The one which is up is towards the RID (47.47.1.1) of the MPLS LSP tail-end (egress) node.

The non-established LSP (towards the MPLS LSP tail-end node’s secondary Lo0 IP addresses) is failing with a reason:

CSPF failed: no route toward 47.47.2.2

That failure reason suggests us to take a look at the MPLS TED.

And indeed, there is not any (TED) reference to IP address 47.47.2.2; hence the informative failure reason.

Next, it is helpful to check what is in the ISIS LSP from the tail-end router, particularly ISIS TLV  type 134 and Sub-TLVs (6 and 8) of TLV type 22. 

Also, note ISIS TLV  type 132, as that plays a role starting with Junos OS 13.3.

Note that the only IP address encoded in the ISIS TLV type 132 is the RID (router-id).

Cross checking with the info (related to the IP addresses) in the TED makes a clear correlation:

Note the following section of the output as a reference with respect to the post Junos OS 13.3 release, discussed below.

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Junos OS 13.3 and above Behavior

In summary, starting with Junos OS 13.3, all /32 IP addresses configured are included on the lo0 interface in ISIS LSPs and they are encoded in TLV type 132. On the "receiving" (MPLS LSP ingress router) side, in the TED, the information found in ISIS TLV type 132 is imported as well. It is not that important but for the completeness, this change was introduced as a mechanism to support the following feature: https://www.juniper.net/documentation/en_US/junos/topics/concept/mpls-egress-protection-layer3-vpn-overview.html

In order to demonstrate this change the only difference to the previously described setup is the upgrade of GRANADA router to Junos OS 13.3R9:

Checking the state of our two MPLS LSPs shows that both of them are UP: