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

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. 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. (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. 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.