ISIS – Maximum path

Today my friend asked me why he could not see all the vlans available in IS-IS when he issued show ip route isis command. When I saw it, it turned out that there are four paths leading to the same destination and they are doing load balancing.

C7609-1#sh ip route isis | begin Gateway
Gateway of last resort is not set

      192.168.100.0/32 is subnetted, 2 subnets
i L1     192.168.100.151 [115/20] via 192.168.101.2, 00:11:54, Vlan101
                         [115/20] via 192.168.20.2, 00:11:54, Vlan20
                         [115/20] via 192.168.10.2, 00:11:54, Vlan10
                         [115/20] via 192.168.1.2, 00:11:54, Port-channel1

The default IS-IS equal paths is 4, you can change to up to 32 in Cisco 7609 software version c7600rsp72043_rp-ADVIPSERVICESK9-M.

C7609-1(config)#router isis
C7609-1(config-router)#maxim
C7609-1(config-router)#maximum-paths ?
  <1-32>  Number of paths

My friend changed the maximum-path to 8, and he could see all paths to reach the destination:

C7609-1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
C7609-1(config)#router isis
C7609-1(config-router)#maxim
C7609-1(config-router)#maximum-paths 8
C7609-1(config-router)#end
C7609-1#sh ip route isis
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, + - replicated route

Gateway of last resort is not set

      192.168.100.0/32 is subnetted, 2 subnets
i L1     192.168.100.151 [115/20] via 192.168.101.2, 00:00:03, Vlan101
                         [115/20] via 192.168.70.2, 00:00:03, Vlan70
                         [115/20] via 192.168.60.2, 00:00:03, Vlan60
                         [115/20] via 192.168.50.2, 00:00:03, Vlan50
                         [115/20] via 192.168.40.2, 00:00:03, Vlan40
                         [115/20] via 192.168.20.2, 00:00:03, Vlan20
                         [115/20] via 192.168.10.2, 00:00:03, Vlan10
                         [115/20] via 192.168.1.2, 00:00:03, Port-channel1
C7609-1#

Use traceroute to test
You use traceroute to test if the load balancing is in action or not. Cisco traceroute sends out 3 UDP packets each UDP packet has a TTL of 1 for the start, if the router sends back the reply as time out the TTL value is incremented by 1, this process continues until it reaches the destination, for this example a TTL of 1 is sufficient as it reaches the destination by one routed hop.

C7609-1#traceroute 192.168.100.151

Type escape sequence to abort.
Tracing the route to 192.168.100.151

  1 192.168.1.2 0 msec
    192.168.10.2 0 msec
    192.168.20.2 0 msec
C7609-1#traceroute 192.168.100.151

Type escape sequence to abort.
Tracing the route to 192.168.100.151

  1 192.168.40.2 0 msec
    192.168.50.2 0 msec
    192.168.60.2 0 msec
C7609-1#traceroute 192.168.100.151

Type escape sequence to abort.
Tracing the route to 192.168.100.151

  1 192.168.70.2 0 msec
    192.168.101.2 0 msec
    192.168.1.2 0 msec
C7609-1#traceroute 192.168.100.151

Type escape sequence to abort.
Tracing the route to 192.168.100.151

  1 192.168.10.2 0 msec
    192.168.20.2 0 msec
    192.168.40.2 0 msec
C7609-1#traceroute 192.168.100.151

Type escape sequence to abort.
Tracing the route to 192.168.100.151

  1 192.168.50.2 4 msec
    192.168.60.2 0 msec
    192.168.70.2 0 msec
C7609-1#traceroute 192.168.100.151

Type escape sequence to abort.
Tracing the route to 192.168.100.151

  1 192.168.101.2 0 msec
    192.168.1.2 0 msec
    192.168.10.2 0 msec
C7609-1#

Reference: Understanding the Ping and Traceroute Commands
As you can see each UDP packet with a TTL of 1 is sent to the destination and different hops can be seen.