MPLS TE FA query

[ulatif]

MPLS TE FA requires bidirectional LSPs but I havent been able to get sufficient reasoning as to why it needs bidir LSPs

(I read that CSPF does some bidir checks before installing it but I need some more insight into this)

Anyone has more detailed knowledge on why FA requires bidir LSPs ?

[ulatif]

Thanks for replying Rich.

For back-to-back connected routers:

[A] ---- [B]

If we have one-hop TE from A -> B, but nothing in the reverse direction, you mentioned that the TWCC is satisfied because of the physical link.

But whats confusing me is that the physical link is a seperate link than the TE FA link so is the TWCC (as per RFC) a requirement per link ? or as long as the tail-end router [B] has any direct path back to the headend [A], it fulfills the RFC 2328 requirement ??

I would have thought every link adjacency needs to have its own corresponding TWCC ?

Also the "Link Data" value that I see for TE FA related p2p link adjacency in the OSPF LSDB seems to have a value 0.0.0.xx where "xx" seems to be common for a TE shared between tunnel end-points - instead of MIB-II value as suggested by RFC-2328 ?

(pls see my previous posts that show this info)

[Richard Gallagher]

I don't see the same behavior as you, l have the same P1--P3 setup, here is the OSPF info:

Router P1:

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 192.168.100.4
     (Link Data) Router Interface address: 0.0.0.13
      Number of MTID metrics: 0
       TOS 0 Metrics: 2

P1#sh snmp mib ifmib ifindex lo0
Interface = Loopback0, Ifindex = 11

Router 3:

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 192.168.100.2
     (Link Data) Router Interface address: 0.0.0.12
      Number of MTID metrics: 0
       TOS 0 Metrics: 2

P3#show snmp mib ifmib ifindex lo0
Interface = Loopback0, Ifindex = 11

As you can see the link data is different in each case and not common as you see, but the ifindex is the same for both loopbacks.

[IVAN PEPELNJAK]

The value in the link data field should be the ifindex of the tunnel interface (the point-to-point link between the routers) not the ifindex of a loopback (MPLS TE router-id).

[ulatif]

To clear out confusion around one-hop TE and whether or not a reverse identical adjacency is required or not, I

have found the answer in the RFC 2328 (as Ivan rightly previously pointed me to this):

Section:

16.1.  Calculating the shortest-path tree for an area

Page-162:
2(b): 
Otherwise, W is a transit vertex (router or transit
                network).  Look up the vertex W's LSA (router-LSA or
                network-LSA) in Area A's link state database.  If the
                LSA does not exist, or its LS age is equal to MaxAge, or
                it does not have a link back to vertex V, examine the
                next link in V's LSA.[23]

    [23]Note that the presence of any link back to V is sufficient; it
    need not be the matching half of the link under consideration from V
    to W. This is enough to ensure that, before data traffic flows
    between a pair of neighboring routers, their link state databases
    will be synchronized.


So atleast this clears out the confusion.

[ulatif]

And in regards to the "Link Data" contained in the LSA, I still donot see it to be the same as the SNMP MIB IFINDEX, see below:

===============================

R3-PE#sh snmp mib ifmib ifindex Tun35          
Interface = Tunnel35, Ifindex = 8

R3-PE#sh ip ospf database router self-originate

            OSPF Router with ID (10.3.3.3) (Process ID 100)

                Router Link States (Area 33)

  LS age: 1482
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 10.3.3.3
  Advertising Router: 10.3.3.3
  LS Seq Number: 80000005
  Checksum: 0x186E
  Length: 72
  Number of Links: 4

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 10.5.5.5
     (Link Data) Router Interface address: 0.0.0.10
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

=======================================

Unless if MIB-II value is a different value or represented differently ? than the one shown by the "show snmp mib ifindex..." command ??