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implementing load balancing through IS-IS:

Networking Requirements

As shown in Figure 1:
  • CX- A, CX- B, CX- C, and CX- D run IS-IS to implement interconnection in the IP network.
  • CX- A, CX- B, CX- C, and CX- D are Level-2 routers in area 10.
  • Load balancing is required to transmit the traffic of CX- A to CX- D through CX- B and CX- C.
Figure 1 Networking diagram of configuring IS-IS load balancing

Device
Interface
IP Address
Device
Interface
IP Address
CX-A
GE 3/0/0
172.16.1.1/24
CX-C
POS 1/0/0
10.1.2.2/24
 
POS 1/0/0
10.1.1.1/24
  
POS 2/0/0
192.168.1.1/24
 
POS 2/0/0
10.1.2.1/24
      
CX-B
POS 1/0/0
10.1.1.2/24
CX-D
GE 3/0/0
172.17.1.1/24
 
POS 2/0/0
192.168.0.1/24
  
POS 1/0/0
192.168.0.2/24
       
POS 2/0/0
192.168.1.2/24

Configuration Roadmap

The configuration roadmap is as follows:
  1. Enable basic IS-IS functions on each CX device to implement interconnection.
  2. Cancel load balancing and check the routing table.
  3. Configure load balancing on CX- A and check the routing table of it.
  4. (Optional) Configure the preference for equal-cost routes on CX- A.

Data Preparation

To complete the configuration, you need the following data:
  • Levels and the area addresses of the four CX devices.
  • Number of load balancing paths on CX- A is 1.
  • Preference value of equal-cost routes on CX- C is 1.

Procedure

  1. Assign an IP address for each CX device.
    The configuration details are not mentioned here.
  2. Configure basic IS-IS functions.
  3. Cancel load balancing on CX- A. 
    [CX-A] isis 1
    [CX-A-isis-1] maximum load-balancing 1
    [CX-A-isis-1] quit
    # Check the routing table of CX- A.
    [CX-A] display isis route
    

                             Route information for ISIS(1)
    

                             -----------------------------
    

    

                            ISIS(1) Level-2 Forwarding Table
    

                            --------------------------------
    

    

     IPV4 Destination     IntCost    ExtCost ExitInterface   NextHop         Flags
    

    -------------------------------------------------------------------------
    

     192.168.1.0/24       20         NULL    P2/0/0          10.1.2.2        A/-/L/-
    

     10.1.1.0/24          10         NULL    P1/0/0          Direct          D/-/L/-
    

     172.16.1.0/24        10         NULL    GE3/0/0         Direct          D/-/L/-
    

     172.17.1.0/24        30         NULL    P1/0/0          10.1.1.2        A/-/L/-
    

     10.1.2.0/24          10         NULL    P2/0/0          Direct          D/-/L/-
    

     192.168.0.0/24       20         NULL    P1/0/0          10.1.1.2        A/-/L/-
    

    

         Flags: D-Direct, A-Added to URT, L-Advertised in LSPs, S-IGP Shortcut,
    

                                   U-Up/Down Bit Set
    

    
As shown in the routing table, when the maximum number of equal-cost routes for load balancing is set to 1, the next hop to network segment 172.17.1.0 is 10.1.1.2. This is because the system ID of CX- B is small. IS-IS chooses the route with the next hop being 10.1.1.2 as the unique optimal route.
    

    

    

    4. 

  4. Restore the default number of load balancing paths on CX- A.

    

    

    [CX-A] isis 1
    

    [CX-A-isis-1] undo maximum load-balancing
    

    [CX-A-isis-1] quit
    

    
# Check the routing table of CX-A.
    

    [CX-A] display isis route
    

    

                             Route information for ISIS(1)
    

                             -----------------------------
    

    

                            ISIS(1) Level-2 Forwarding Table
    

                            --------------------------------
    

    

     IPV4 Destination     IntCost    ExtCost ExitInterface   NextHop         Flags
    

    -------------------------------------------------------------------------
    

     192.168.1.0/24       20         NULL    P2/0/0          10.1.2.2         A/-/L/-
    

     10.1.1.0/24          10         NULL    P1/0/0          Direct           D/-/L/-
    

     172.16.1.0/24        10         NULL    GE3/0/0         Direct           D/-/L/-
    

     172.17.1.0/24        30         NULL    P1/0/0          10.1.1.2         A/-/L/-
    

                                             P2/0/0          10.1.2.2
    

     10.1.2.0/24          10         NULL    P2/0/0          Direct           D/-/L/-
    

     192.168.0.0/24       20         NULL    P1/0/0          10.1.1.2         A/-/L/-
    

    

         Flags: D-Direct, A-Added to URT, L-Advertised in LSPs, S-IGP Shortcut,
    

                                   U-Up/Down Bit Set
    

    
As shown in the routing table, the default value is used when load balancing is canceled. The two next hops of CX-A, that is, 10.1.1.2 (that is, CX- B) and 10.1.1.2 (that is, CX- C), are valid routes. This is because the default value of the maximum equal-cost routes is 3.
    

    

    

    

    

    5. 

  5. (Optional) Configure the preference of equal-cost routes on CX-A.

    

    

    
If you do not perform load balancing through CX- B and CX- C, configure the preference of the equal-cost routes and specify the next hop.
    

    [CX-A] isis
    

    [CX-A-isis-1] nexthop 10.1.2.2 weight 1
    

    [CX-A-isis-1] quit
    

    

    

    6. 

  6. Verify the configuration.

    

    

    
# Check the routing table of CX- A.
    

    [CX-A] display isis route
    

                             Route information for ISIS(1)
    

                             -----------------------------
    

    

                            ISIS(1) Level-2 Forwarding Table
    

                            --------------------------------
    

    

     IPV4 Destination     IntCost    ExtCost ExitInterface   NextHop         Flags
    

    --------------------------------------------------------------------------------
    

     192.168.1.0/24       20         NULL    P2/0/0          10.1.2.2        A/-/L/-
    

     10.1.1.0/24          10         NULL    P1/0/0          Direct          D/-/L/-
    

     172.16.1.0/24        10         NULL    GE3/0/0         Direct          D/-/L/-
    

     172.17.1.0/24        30         NULL    P1/0/0          10.1.2.2        A/-/L/-
    

     10.1.2.0/24          10         NULL    P2/0/0          Direct          D/-/L/-
    

     192.168.0.0/24       20         NULL    P1/0/0          10.1.1.2        A/-/L/-
    

    

         Flags: D-Direct, A-Added to URT, L-Advertised in LSPs, S-IGP Shortcut,
    

                                   U-Up/Down Bit Set
    

    
As shown in the routing table, because the preference (metric is 1) of next hop 10.1.2.2 (that is, CX- C) is higher than that of next hop 10.1.1.2 (that is, CX- B), IS-IS chooses the route with the next hop being 10.1.2.2 as the optimal route.
    

    

    

    7. 








Configuration Files


    

  • 

    
Configuration file of CX- A
    

    #
    

     sysname CX-A
    

    #
    

    isis 1
    

     is-level level-2
    

     network-entity 10.0000.0000.0001.00
    

     nexthop 10.1.2.2 weight 1
    

    #
    

    interface GigabitEthernet3/0/0
    

     ip address 172.16.1.1 255.255.255.0
    

     isis enable 1
    

    #
    

    interface Pos1/0/0
    

     link-protocol ppp
    

     ip address 10.1.1.1 255.255.255.0
    

     isis enable 1
    

    #
    

    interface Pos2/0/0
    

     link-protocol ppp
    

     ip address 10.1.2.1 255.255.255.0
    

     isis enable 1
    

    #
    

    return
    

    • 

  • 

    
Configuration file of CX- B
    

    #
    

     sysname CX-B
    

    #
    

    isis 1
    

     is-level level-2
    

     network-entity 10.0000.0000.0002.00
    

    #
    

    interface Pos1/0/0
    

     link-protocol ppp
    

     ip address 10.1.1.2 255.255.255.0
    

     isis enable 1
    

    #
    

    interface Pos2/0/0
    

     link-protocol ppp
    

     ip address 192.168.0.1 255.255.255.0
    

     isis enable 1
    

    #
    

    return
    

    • 

  • 

    
Configuration file of CX- C
    

    #
    

     sysname CX-C
    

    #
    

    isis 1
    

     is-level level-2
    

     network-entity 10.0000.0000.0003.00
    

    #
    

    interface Pos1/0/0
    

     link-protocol ppp
    

     ip address 10.1.2.2 255.255.255.0
    

     isis enable 1
    

    #
    

    interface Pos2/0/0
    

     link-protocol ppp
    

     ip address 192.168.1.1 255.255.255.0
    

     isis enable 1
    

    #
    

    return
    

    • 

  • 

    
Configuration file of CX- D
    

    #
    

     sysname CX-D
    

    #
    

    isis 1
    

     is-level level-2
    

     network-entity 10.0000.0000.0004.00
    

    #
    

    interface GigabitEthernet3/0/0
    

     ip address 172.17.1.1 255.255.255.0
    

     isis enable 1
    

    #              
    

    interface Pos1/0/0
    

     link-protocol ppp
    

     ip address 192.168.0.2 255.255.255.0
    

     isis enable 1
    

    #
    

    interface Pos2/0/0
    

     link-protocol ppp
    

     ip address 192.168.1.2 255.255.255.0
    

     isis enable 1
    

    #
    

    return
    

    • 









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