Re: MPLS forwarding in partial mesh frame-relay networks

Rich,

An LSR will only install a label for a prefix if the next-hop for that
prefix is an address that belongs to the router from which it learned the
label. So if Router B's next-hop for Router C is Router C's address, it will
not install the label because the label was learned from Router A.

What you can do to fix this is install a static route on router B for C's
address that points to A as the next-hop. Now the label learned from A will
be installed.

On Fri, Jan 22, 2010 at 6:00 AM, Steve Shaw <shaw38_at_gmail.com> wrote:

> Rich,
>
> Thanks for the reply. Try changing the OSPF network type on your serial
> interfaces from point-to-multipoint to broadcast (with the hub as the DR)
> and you should see the behavior I mentioned.
>
> r2#show ip route
>
> 1.0.0.0/32 is subnetted, 1 subnets
> O 1.1.1.1 [110/65] via 10.123.123.1, 00:09:33, Serial0/0.1
> 2.0.0.0/32 is subnetted, 1 subnets
> C 2.2.2.2 is directly connected, Loopback0
> 3.0.0.0/32 is subnetted, 1 subnets
> O 3.3.3.3 [110/65] via 10.123.123.3, 00:00:29, Serial0/0.1
> 10.0.0.0/24 is subnetted, 1 subnets
> C 10.123.123.0 is directly connected, Serial0/0.1
>
> r2#show mpls forwarding-table
> Local Outgoing Prefix Bytes tag Outgoing Next Hop
> tag tag or VC or Tunnel Id switched interface
> 16 Pop tag 1.1.1.1/32 0 Se0/0.1 point2point
> 17 Untagged 3.3.3.3/32 0 Se0/0.1 point2point
>
> -Steve
>
>
> On Thu, Jan 21, 2010 at 7:07 PM, Rich Collins <nilsi2002_at_gmail.com> wrote:
>
> > Hi,
> >
> > I tried to quickly reproduce this scenario but perhaps I didn't
> > configure the way you did. I see the hop at the hub however. Usually
> > it is this way though for spoke to spoke via the multipoint hub??
> > Which interfaces did you use for your targeted session?
> >
> > -Rich
> >
> > R2#sh mpls ldp neighbor
> > Peer LDP Ident: 1.1.1.1:0; Local LDP Ident 2.2.2.2:0
> > TCP connection: 1.1.1.1.646 - 2.2.2.2.42761
> > State: Oper; Msgs sent/rcvd: 10/10; Downstream
> > Up time: 00:01:02
> > LDP discovery sources:
> > Serial1/0.1, Src IP addr: 10.123.123.1
> > Addresses bound to peer LDP Ident:
> > 10.123.123.1 1.1.1.1
> > Peer LDP Ident: 3.3.3.3:0; Local LDP Ident 2.2.2.2:0
> > TCP connection: 3.3.3.3.12741 - 2.2.2.2.646
> > State: Oper; Msgs sent/rcvd: 9/9; Downstream
> > Up time: 00:00:37
> > LDP discovery sources:
> > Targeted Hello 2.2.2.2 -> 3.3.3.3, active, passive
> > Addresses bound to peer LDP Ident:
> > 10.123.123.3 3.3.3.3
> > R2#sh mp
> > R2#sh mpl
> > R2#sh mpls fo
> > R2#sh mpls forwarding-table
> > Local Outgoing Prefix Bytes tag Outgoing Next Hop
> > tag tag or VC or Tunnel Id switched interface
> > 16 Pop tag 1.1.1.1/32 0 Se1/0.1 point2point
> > 17 17 3.3.3.3/32 0 Se1/0.1 point2point
> > 18 19 10.123.123.3/32 0 Se1/0.1 point2point
> > 19 Untagged 10.123.123.1/32 0 Se1/0.1 point2point
> > R2#sh ip c
> >
> >
> > R2#sh ip cef 3.3.3.3
> > 3.3.3.3/32, version 12, epoch 0, cached adjacency to Serial1/0.1
> > 0 packets, 0 bytes
> > tag information set
> > local tag: 17
> > fast tag rewrite with Se1/0.1, point2point, tags imposed: {17}
> > via 10.123.123.1, Serial1/0.1, 0 dependencies
> > next hop 10.123.123.1, Serial1/0.1
> > valid cached adjacency
> > tag rewrite with Se1/0.1, point2point, tags imposed: {17}
> > R2#sh ip rou
> >
> > R2#sh ip route ospf
> > 1.0.0.0/32 is subnetted, 1 subnets
> > O 1.1.1.1 [110/65] via 10.123.123.1, 00:23:42, Serial1/0.1
> > 3.0.0.0/32 is subnetted, 1 subnets
> > O 3.3.3.3 [110/129] via 10.123.123.1, 00:23:42, Serial1/0.1
> > 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
> > O 10.123.123.3/32 [110/128] via 10.123.123.1, 00:23:42,
> Serial1/0.1
> > O 10.123.123.1/32 [110/64] via 10.123.123.1, 00:23:42, Serial1/0.1
> > R2#sh ip o
> > R2#sh ip ospf n
> > R2#sh ip ospf neighbor
> >
> > Neighbor ID Pri State Dead Time Address
> Interface
> > 10.123.123.1 255 FULL/ - 00:01:30 10.123.123.1
> > Serial1/0.1
> > R2#
> >
> >
> >
> >
> > On Mon, Jan 18, 2010 at 11:48 AM, Steve Shaw <shaw38_at_gmail.com> wrote:
> > > Guys (and Gals),
> > >
> > > I came across this scenario in a mock lab. Three routers sharing a
> single
> > > subnet (10.123.123.0/24) frame relay network:
> > >
> > > - A, B, C where A is the hub site with a single multi-point
> > > sub-interface
> > > - B/C as spokes with single point-to-point sub-interfaces with PVCs
> to
> > A
> > > - All are running OSPF (Non-broadcast, A is the DR) and MPLS LDP
> > > - Loopbacks are A (1.1.1.1/32), B (2.2.2.2/24) and C (3.3.3.3/32)
> and
> > > advertised in OSPF with the correct mask
> > > - Targeted LDP session built between B and C
> > >
> > > The issue I can't wrap my head around is how B and C know there is are
> > > incomplete MPLS-VPN LSPs between them. I *believe* this is a sanity
> check
> > > function of MPLS forwarding on the router but looking for a
> confirmation.
> > >
> > > From the perspective of B as the ingress LSR to C as the egress LSR:
> > >
> > > B is learning the loopback of C (3.3.3.3/32) with the next-hop
> unchanged
> > so
> > > it appears directly connected:
> > >
> > > B#show ip cef 3.3.3.3
> > > 3.3.3.3/32, version 28, epoch 0, cached adjacency to Serial0/0.1
> > > 0 packets, 0 bytes
> > > tag information set
> > > local tag: 17
> > > via 10.123.123.3, Serial0/0.1, 0 dependencies
> > > next hop 10.123.123.3, Serial0/0.1
> > > valid cached adjacency
> > > tag rewrite with Se0/0.1, point2point, tags imposed: {}
> > >
> > > In B's LIB, B is being told to pop the IGP label prior to forwarding to
> C
> > > which would be correct if they were directly connected:
> > >
> > > B#show mpls ldp bindings 3.3.3.3 32
> > > tib entry: 3.3.3.3/32, rev 6
> > > local binding: tag: 17
> > > remote binding: tsr: 1.1.1.1:0, tag: 17
> > > remote binding: tsr: 3.3.3.3:0, tag: imp-null
> > >
> > > However, in the LFIB, B is removing the label stack completely:
> > >
> > > B#show mpls forwarding-table 3.3.3.3
> > > Local Outgoing Prefix Bytes tag Outgoing Next Hop
> > > tag tag or VC or Tunnel Id switched interface
> > > 17 Untagged 3.3.3.3/32 0 Se0/0.1 point2point
> > >
> > >
> > > From a layer 3 perspective, routers B and C appear to be directly
> > connected.
> > > But from a layer 2 perspective, there is actually 2 hops from B to C
> > through
> > > the frame-relay network and hub router A.
> > >
> > > Based on the LIB, if B was to attempt to forward an MPLS-VPN labeled
> > packet
> > > to C, B would:
> > >
> > > 1. Pop the top (IGP) label
> > > 2. Forward the frame out it's serial interface toward A
> > > 3. A would receive the labeled packet, attempt to do a lookup on the
> > > MPLS-VPN label (now top label w/ BoS bit set) and drop the packet
> > >
> > > To prevent the above scenario from occurring, does B place an entry in
> > the
> > > LFIB to completely remove the stack since it knows the LSP is
> incomplete?
> > >
> > > Thanks for the help,
> > >
> > > Steve
> > >
> > >
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> > >
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-- 
Bryan Bartik
CCIE #23707 (R&S, SP), CCNP
Sr. Support Engineer - IPexpert, Inc.
URL: http://www.IPexpert.com
Blogs and organic groups at http://www.ccie.net