Why does my network's traceroute bounce between routers?

[rn.becker]

CoreRouter#show ip int brief

Interface              IP-Address      OK? Method Status                Protocol

Dialer0                unassigned      YES unset  up                    up

Ethernet0              10.1.1.253      YES NVRAM  up                    up

Loopback0              11.255.255.1    YES NVRAM  up                    up

Serial0                192.168.1.202   YES NVRAM  up                    up

Serial1                192.168.1.226   YES NVRAM  up                    up

P1R1#show ip int brief

Interface                  IP-Address      OK? Method Status                Protocol

Ethernet0/0                192.168.11.1    YES NVRAM  up                    up

Serial0/0                  192.168.1.101   YES NVRAM  up                    up

Serial0/1                  192.168.1.201   YES NVRAM  up                    up

Ethernet1/0                unassigned      YES NVRAM  administratively down down

Loopback0                  10.200.1.1      YES NVRAM  up                    up

P1R2#show ip int brief

Interface                  IP-Address      OK? Method Status                Protocol

Ethernet0/0                192.168.11.2    YES NVRAM  up                    up

Serial0/0                  192.168.1.102   YES NVRAM  up                    up

Serial0/1                  192.168.1.225   YES NVRAM  up                    up

Ethernet1/0                unassigned      YES NVRAM  up                    down

Loopback0                  10.200.1.2      YES NVRAM  up                    up

CoreRouter#traceroute p1sw
Type escape sequence to abort.
Tracing the route to p1sw (192.168.11.10)
  1 p1r2 (192.168.1.225) 8 msec
    p1r1 (192.168.1.201) 12 msec
    p1r2 (192.168.1.225) 8 msec
  2 p1sw (192.168.11.10) 12 msec *  8 msec 

CoreRouter#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
       U - per-user static route

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, Ethernet0
     11.0.0.0/32 is subnetted, 1 subnets
C       11.255.255.1 is directly connected, Loopback0
D    192.168.11.0/24 [90/20537600] via 192.168.1.225, 1d08h, Serial1
                     [90/20537600] via 192.168.1.201, 1d08h, Serial0
     192.168.1.0/27 is subnetted, 3 subnets
D       192.168.1.96 [90/21024000] via 192.168.1.225, 5d04h, Serial1
                     [90/21024000] via 192.168.1.201, 5d04h, Serial0
C       192.168.1.224 is directly connected, Serial1
C       192.168.1.192 is directly connected, Serial0

P1R1#show ip route
Codes: 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

Gateway of last resort is not set

C    192.168.11.0/24 is directly connected, Ethernet0/0
     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D       10.1.1.0/24 [90/20537600] via 192.168.1.202, 00:11:16, Serial0/1
C       10.200.1.1/32 is directly connected, Loopback0
     192.168.1.0/27 is subnetted, 3 subnets
C       192.168.1.96 is directly connected, Serial0/0
D       192.168.1.224 [90/20537600] via 192.168.11.2, 00:11:16, Ethernet0/0
C       192.168.1.192 is directly connected, Serial0/1

P1R2#show ip route
Codes: 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

Gateway of last resort is not set

C    192.168.11.0/24 is directly connected, Ethernet0/0
     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D       10.1.1.0/24 [90/20537600] via 192.168.1.226, 00:11:31, Serial0/1
C       10.200.1.2/32 is directly connected, Loopback0
     192.168.1.0/27 is subnetted, 3 subnets
C       192.168.1.96 is directly connected, Serial0/0
C       192.168.1.224 is directly connected, Serial0/1
D       192.168.1.192 [90/20537600] via 192.168.11.1, 00:11:31, Ethernet0/0

As you can see from the traceroute, there appears to be a routing loop of some kind, but the traffic gets there consistently.

From my PC at 10.1.1.1, which is connected to CoreRouter (10.1.1.253) via the CoreSwitch (10.1.1.254), the traceroute is correct:

C:\Windows\system32>tracert 192.168.11.10

Tracing route to 192.168.11.10 over a maximum of 30 hops

  1     2 ms     2 ms     2 ms  10.1.1.253
  2    11 ms    11 ms    11 ms  192.168.1.225
  3    14 ms    14 ms    20 ms  192.168.11.10

Trace complete.

What is happening from CoreRouter's perspective when I do the traceroute from there?

[Richard Burts · Accepted solution]

The results that you are seeing reflect the effect of load balancing/load sharing for locally generated traffic. As the route table shows there are two paths toward the destination (over the two serial interfaces). When the traffic is locally generated then both paths are used. When there is traffic originated from the PC (transit traffic through the router) then only one of the paths is used.

HTH

Rick

Sent from Cisco Technical Support iPad App

[rn.becker]

Yeah, I finally figured it out when I was examining the results for the first hop from Corerouter.

It says

  1 p1r2 (192.168.1.225) 8 msec

    p1r1 (192.168.1.201) 12 msec

What confused me is a Cisco traceroute doesn't do what a Windows traceroute does (that I've ever seen).  It's actually talking to me, telling me how it's deciding which route to take.  Based on the time, it's going to take the path through P1R2, and that's what it says for the 2nd hop.

I went back and looked at the clock rates for the serial interfaces on CoreRouter.  Sure enough, the clock rate on S1 (connected CoreRouter to P1R2) is faster than for S0.

Thanks very much.

[Richard Burts]

I am glad that you found my response helpful and that you were able to find the correct explanation for this behavior. Thank you for using the rating system to mark this question as answered (and thanks for the rating). It makes the forum more useful when people can read about a problem and can read the correct explanation of the issue. Your marking has contributed to this process.

HTH

Rick