With increased complexity in the network, scalability becomes a challenge due to maintenance of RSVP-TE tunnels with complex signaling as well as high overhead on routers within the network. Backup autotunnel feature can help reduce the complexity in a Segment Routing (SR) network. Autotunnel backup feature has the following benefits:

• Backup tunnels are built automatically hence eliminating the need for users to pre-configure each backup tunnel and then assign the backup tunnel to the protected interface.

• With the backup tunnels configured, area of protection gets expanded. Fast reroute (FRR) neither protects IP traffic nor LDP labels that do not use TE tunnel.

• Backup SR-TE autotunnel allows additional means of migration to SR network without disrupting the existing traffic passing through RSVP-TE tunnels.

Backup autotunnels on a router helps to build dynamic backup tunnels whenever required. This prevents creating of static SR-TE tunnels.

To protect a label-switched path (LSP) in the absence of static SR-TE tunnels, you need to do the following:

• Preconfigure each backup tunnel.

• Assign the backup tunnels to the protected interfaces.

An LSP requests backup protection from Resource Reservation Protocol (RSVP) FRR in the following situations:

• Receipt of the first RSVP Resv message.

• Receipt of an RSVP path message with the protection attribute after the LSP has been established without protection attribute.

• Detection of changed Record Route Object (RRO).

If there is no backup tunnel protecting the interface used by the LSP, the LSP remained unprotected. Some of the reasons why a backup tunnel may not be available are:

• Static backup tunnels are not configured.

• Static backup tunnels are configured, but may not be able to protect the LSP because there is not enough bandwidth available, or the tunnel protects a different pool, or the tunnel is not available.

If a backup tunnel is not available, the following two backup tunnels are created dynamically:

• NHOP—Protects against link failure.

• NNHOP—Protects against node failure.

Note	At the penultimate hop, only an NHOP backup tunnel is created.

Link Protection

Backup tunnels that bypass only a single link of the LSP’s path provide link protection. They protect LSPs if a link along their path fails by rerouting the LSP’s traffic to the next hop (bypassing the failed link). These are referred to as NHOP backup tunnels because they terminate at the LSP’s next hop beyond the point of failure.

Node Protection

Backup tunnels that bypass next-hop nodes along LSP paths are called NNHOP backup tunnels because they terminate at the node following the next-hop node of the LSPs, thereby bypassing the next-hop node. They protect LSPs by enabling the node upstream of a link or node failure to reroute the LSPs and their traffic around the failure to the next-hop node. NNHOP backup tunnels also provide protection from link failures because they bypass the failed link and the node.

For SR-TE autotunnel backup feature to work interfaces have to be point-to-point network type.

interface Loopback0
 ip address 51.1.1.1 255.255.255.255
 ip router isis 1
end
!
interface GigabitEthernet0/2/0
 ip address 101.102.6.1 255.255.255.0
 ip router isis 1
 negotiation auto
 mpls traffic-eng tunnels
 isis network point-to-point 
 ip rsvp bandwidth
end
!
interface GigabitEthernet0/2/4
 ip address 101.104.1.1 255.255.255.0
 ip router isis 1
 negotiation auto
 mpls traffic-eng tunnels
 isis network point-to-point 
 ip rsvp bandwidth
end

1. Configure explicit path from R1/PE1 to R6/PE2 that traverses through the routers R2 and R3.

   
   ip explicit-path name path1 enable
    index 1 next-address 209.165.202.128
    index 2 next-address 209.165.201.0
    index 3 next-address 192.168.0.0
    index 4 next-address 209.165.200.224

2. Configure explicit RSVP-TE tunnel.

   
   interface Tunnel1
    ip unnumbered Loopback0
    tunnel mode mpls traffic-eng
    tunnel destination 209.165.200.224
    tunnel mpls traffic-eng autoroute announce
    tunnel mpls traffic-eng path-option 10 explicit name path1
    tunnel mpls traffic-eng record-route
   end

3. Configure the primary RSVP-TE Tunnel 1 with FRR to activate the protection process.

   
   interface tunnel 1
    tunnel mpls traffic-eng fast-reroute

4. Configure the global command to enable link protection using SR-TE autotunnel.

   
   mpls traffic-eng auto-tunnel backup segment-routing nhop-only

   Note	This command needs to be available in all the nodes that require link protection.

The Primary RSVP-TE tunnel need to be protected that gets initialized from headend R1/PE1 to destination R6/PE2 and traversing through next node R2 and so on. In this case, R1/PE1 is the Point of Local Repair (PLR) and R2 is the Mid-Point (MP). With link protection, the SR-TE Backup AutoTunnel provides protection to the link from R1/PE1 to R2 by traversing through the path R1/PE1 -> R4 and R4 -> R2, hence converging back to the MP.