ToM Tunnel Selection

The AToM Tunnel Selection feature enables you to steer the AToM traffic through the MPLS network over a path that you specify. For this to work, you need to set up an MPLS TE tunnel from the PE to the PE router and then specify that the AToM traffic should take the TE tunnel instead of the default shortest labeled path. In addition, you can specify whether fallback to the default path is desirable when the TE tunnel fails.

Figure 10-21 shows an MPLS network with MPLS VPN and AToM customers. The CE routers CE1 and CE2 are part of one MPLS VPN network, whereas CE3 and CE4 are two CE routers that are connected through AToM. Normally, all labeled traffic follows the top path because it is the shortest path through the MPLS backbone. If you want to separate the AToM from the VPN traffic in the MPLS network, you can create two TE tunnels between the two PE routers (one for each direction) to carry the AToM traffic. This might be desirable if you want to ensure a certain delay in the network for each kind of traffic, without the direct use of QoS. Another driver might be the guarantee of seamless service via the use of Fast ReRouting (FRR) or bandwidth protection when using TE.

VPN One, Site 1

VPN One, Site 1

Layer 2

VPN One, Site 2

Layer 2

Default Path

TE Tunnel

VPN One, Site 2

Layer 2

If the purpose is to separate the AToM traffic from all the other MPLS traffic, do not configure autoroute announce on the TE tunnel because that steers all the traffic onto the TE tunnel. Example 10-16 shows how to configure tunnel selection. You have to configure preferred-path under the pseudowire class and create a TE tunnel between the PE routers without the autoroute announce command.

Example 10-16 Tunnel Selection Example

pseudowire-class pw1 encapsulation mpls preferred-path interface Tunnell

interface Tunnell ip unnumbered Loopback0 tunnel source Loopback0 tunnel destination 10.1.1.2 tunnel mode mpls traffic-eng tunnel mpls traffic-eng path-option 1 dynamic

interface FastEthernet2/6 no ip address xconnect 10.1.1.2 100 pw-class pw1

To verify that the tunnel selection is working, look at the pseudowire with the command show mpls l2transport vc [vcid vc-id] I [vc-id-min vc-id-max] [interface name [local-circuit-id]] [destination ip-address I name] [detail]. Example 10-17 shows this.

Example 10-17 Verifying Tunnel Selection

PE#show mpls l2transport vc 100 detail

Local interface: Fa2/6 up, line protocol up, Ethernet up

Destination address: 10.1.1.2, VC ID: 100, VC status: up

Preferred path: Tunnell, active

Default path: ready

Tunnel label: 3, next hop point2point

Output interface: Tu1, imposed label stack {12307 20}

Create time: 00:00:11, last status

change time: 00:00:11

Signaling protocol: LDP, peer 10.1

1.2:0 up

MPLS VC labels: local 21, remote

20

Group ID: local 0, remote 2

MTU: local 1500, remote 1500

Remote interface description:

Sequencing: receive disabled, send

disabled

VC statistics:

packet totals: receive 1, send 6

byte totals: receive 368, send

0

packet drops: receive 0, send 0

Following is the full syntax of the preferred path command:

preferred path [interface tunnel tunnel-number | peer {ip address | host name}] [disable-fallback]

Instead of specifying a TE tunnel, you can specify an IP address (or host name) of the remote PE router. You should choose a different IP address on the remote PE router from the one the default LSP uses. The path to that IP address must be an LSP, but you can route it differently through the MPLS network than the default path, used by the labeled traffic between the two loopback IP addresses on the PE routers. Tunnel selection uses fallback, meaning that if the preferred path (TE tunnel or the specific IP address) becomes unavailable, the AToM traffic is sent across the default path. You can, however, disable it by specifying the disable-fallback keyword on the preferred-path command, as shown in Example 10-18.

Example 10-18 Fallback Disabled Example pseudowire-class pw1

encapsulation mpls preferred-path interface Tunnell disable-fallback

PE#show mpls l2transport vc 100 detail

Local interface: Fa2/6 up, line protocol up, Ethernet up Destination address: 10.1.1.2, VC ID: 100, VC status: up

Preferred path: Tunnell, active Default path: disabled Tunnel label: 3, next hop point2point Output interface: Tu1, imposed label stack {12307 20} Create time: 00:00:13, last status change time: 00:00:13 Signaling protocol: LDP, peer 10.1.1.2:0 up MPLS VC labels: local 16, remote 20 Group ID: local 0, remote 2 MTU: local 1500, remote 1500 Remote interface description: Sequencing: receive disabled, send disabled VC statistics:

packet totals: receive 2, send 6 byte totals: receive 189, send 0 packet drops: receive 0, send 0

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