OSPF Fast Convergence FRR

1. Experiment

1. OSPF IP FRR (Fast ReRoute, fast rerouting)

OSPF IP FRR is a dynamic IP FRR. It uses the LFA (Loop-Free Alternates) algorithm to pre-calculate the backup path and save it in the forwarding table. In case of a failure, the traffic can be quickly switched to the backup link to ensure that the traffic is not interrupted. To achieve the purpose of traffic protection, this function can reduce the fault recovery time to less than 50 ms.

The basic idea of ​​LFA to calculate the backup link is: take the neighbor that can provide the backup link as the root node, and use the SPF algorithm to calculate the shortest distance to the destination node. Then, according to the inequality, the backup link with the least overhead and no loop is calculated.

1. link protection

As shown in the figure, the traffic is forwarded from device S to D, and the network overhead value satisfies the link protection formula, which can ensure that when the primary link fails, device S can continue to forward downstream after switching the traffic to the backup link S to N. Make sure that the flow interruption is less than 50 ms.

Link protection formula: Distance_opt(N,D)<Distance_opt(N,S)+Distance_opt(S,D)

Ensure that the traffic from node N->D will not pass through node S again, that is, ensure that there is no loop. (for example)

1. Node link double protection

      As shown in the figure, traffic is forwarded from device S to D, and the network overhead value satisfies the link protection formula and node protection formula. When node E fails, it can be switched to node N.

      Link protection formula: Distance_opt(N,D)<Distance_opt(N,S)+Distance_opt(S,D)

Node protection formula: Distance_opt(N,D)<Distance_opt(N,E)+Distance_opt(E,D)

      Ensure that the traffic from node N->D will not pass through nodes S and E, that is, ensure that there is no loop.

2. Experiment

14.1 Experiment 1: FRR

1. Purpose

1. Master the fundamentals of FRR
2. Master the configuration method of FRR

1. experimental topology

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1. Experimental procedure

1. Configure IP address

1. Configuration of AR1

<Huawei>system-view

Enter system view, return user view with Ctrl+Z.

[Huawei]undo info-center enable

[Huawei]sysname AR1

[AR1]interface g0/0/0

[AR1-GigabitEthernet0/0/0]ip address 10.0.12.1 24

[AR1-GigabitEthernet0/0/0]quit

[AR1]interface g0/0/1

[AR1-GigabitEthernet0/0/1]ip address 10.0.13.1 24

[AR1-GigabitEthernet0/0/1]quit

[AR1]interface LoopBack 0

[AR1-LoopBack0]ip address 1.1.1.1 32

[AR1-LoopBack0]quit

1. Configuration of AR2

<Huawei>system-view

Enter system view, return user view with Ctrl+Z.

[Huawei]undo info-center enable

Info: Information center is disabled.

[Huawei]sysname AR2

[AR2]interface g0/0/0

[AR2-GigabitEthernet0/0/0]ip address 10.0.23.2 24

[AR2-GigabitEthernet0/0/0]quit

[AR2]interface g0/0/1

[AR2-GigabitEthernet0/0/1]ip address 10.0.12.2 24

[AR2-GigabitEthernet0/0/1]quit

[AR2]interface LoopBack 0

[AR2-LoopBack0]ip address 2.2.2.2 32

[AR2-LoopBack0]quit

1. Configuration of AR3

<Huawei>system-view

Enter system view, return user view with Ctrl+Z.

[Huawei]undo info-center enable

Info: Information center is disabled.

[Huawei]sysname AR3

[AR3]interface g0/0/0

[AR3-GigabitEthernet0/0/0]ip address 10.0.34.3 24

[AR3-GigabitEthernet0/0/0]quit

[AR3]interface g0/0/1

[AR3-GigabitEthernet0/0/1]ip address 10.0.23.3 24

[AR3-GigabitEthernet0/0/1]quit

[AR3]interface g0/0/2

[AR3-GigabitEthernet0/0/2]ip address 10.0.13.3 24

[AR3-GigabitEthernet0/0/2]quit

1. Configuration of AR4

<Huawei>system-view

Enter system view, return user view with Ctrl+Z.

[Huawei]undo info-center enable

Info: Information center is disabled.

[Huawei]sysname AR4

[AR4]interface g0/0/1

[AR4-GigabitEthernet0/0/1]ip address 10.0.34.4 24

[AR4-GigabitEthernet0/0/1]quit

[AR4]interface LoopBack 0

[AR4-LoopBack0]ip address 4.4.4.4 32

[AR4-LoopBack0]quit

1. Run OSPF routing protocol

1. Configuration of AR1

[AR1]ospf router-id 1.1.1.1

[AR1-ospf-1]area 0

[AR1-ospf-1-area-0.0.0.0]network 10.0.12.0 0.0.0.255

[AR1-ospf-1-area-0.0.0.0]network 10.0.13.0 0.0.0.255

[AR1-ospf-1-area-0.0.0.0]network 1.1.1.1 0.0.0.0

[AR1-ospf-1-area-0.0.0.0]quit

1. Configuration of AR2

[AR2]ospf router-id 2.2.2.2

[AR2-ospf-1]area 0

[AR2-ospf-1-area-0.0.0.0]network 10.0.12.0 0.0.0.255

[AR2-ospf-1-area-0.0.0.0]network 10.0.23.0 0.0.0.255

[AR2-ospf-1-area-0.0.0.0]network 2.2.2.2 0.0.0.0

[AR2-ospf-1-area-0.0.0.0]quit

1. Configuration of AR3

[AR3]ospf router-id 3.3.3.3

[AR3-ospf-1]area 0

[AR3-ospf-1-area-0.0.0.0]network 10.0.13.0 0.0.0.255

[AR3-ospf-1-area-0.0.0.0]network 10.0.23.0 0.0.0.255

[AR3-ospf-1-area-0.0.0.0]network 10.0.34.0 0.0.0.255

[AR3-ospf-1-area-0.0.0.0]network 3.3.3.3 0.0.0.0

[AR3-ospf-1-area-0.0.0.0]quit

1. Configuration of AR4

[AR4]ospf router-id 4.4.4.4

[AR4-ospf-1]area 0

[AR4-ospf-1-area-0.0.0.0]network 10.0.34.0 0.0.0.255

[AR4-ospf-1-area-0.0.0.0]network 4.4.4.4 0.0.0.0

[AR4-ospf-1-area-0.0.0.0]quit

1. Experimental debugging

1. Check the OSPF route of 4.4.4.4 on R1

<AR1>display ospf routing 4.4.4.4

         OSPF Process 1 with Router ID 1.1.1.1

 Destination : 4.4.4.4/32        

 AdverRouter : 4.4.4.4                  Area      : 0.0.0.0 

 Cost        : 2                        Type      : Stub

 NextHop     : 10.0.13.3                Interface : GigabitEthernet0/0/1 

 Priority    : Medium                   Age       : 00h01m16s 

From the above output, you can see that there is only one path from AR1 to AR4: AR1-AR3-AR4. When the link between AR1 and AR3 fails, OSPF will run the SPF algorithm and switch to AR1-AR2-AR3-AR4. The switching time of this fault is relatively long.

1. Configure FRR on AR1

[AR1-ospf-1]frr // Enable FRR function

[AR1-ospf-1-frr]loop-free-alternate // Use the LFA algorithm

[AR1-ospf-1-frr]quit

【Technical Description】

Link protection formula: Distance_opt(AR2,AR4)<Distance_opt(AR2,AR1)+Distance_opt(AR1,AR4)

In this graph 2<1+2 satisfies the condition.

1. Check the OSPF route of 4.4.4.4 on R1 again

[AR1]display ospf routing 4.4.4.4

         OSPF Process 1 with Router ID 1.1.1.1

 Destination : 4.4.4.4/32        

 AdverRouter : 4.4.4.4                  Area      : 0.0.0.0 

 Cost        : 2                        Type      : Stub

 NextHop     : 10.0.13.3                Interface : GigabitEthernet0/0/1 

 Priority    : Medium                   Age       : 00h00m09s 

 Backup Nexthop : 10.0.12.2             Backup Interface: GigabitEthernet0/0/0

 Backup Type : LFA LINK        

From the above output, you can see that OSPF has generated a backup link. When the link between AR1 and AR3 fails, switch to AR1-AR2-AR3-AR4 immediately. The time is within 50ms.