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It's my first GNS3 router configuration experience. There are 4 connected routers (they can ping each other). The routing protocol is router eirgp 10.

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This is result of show ip route on R4:

D    192.100.106.0/24 [90/2273792] via 192.100.101.2, 00:04:15, Serial1/1
D    192.100.104.0/24 [90/2273792] via 192.100.102.2, 00:04:13, Serial1/0
D    192.100.105.0/24 [90/1766912] via 192.100.102.2, 00:04:13, Serial1/0
     192.100.102.0/24 is variably subnetted, 2 subnets, 2 masks
D       192.100.102.0/24 is a summary, 00:04:25, Null0
C       192.100.102.0/30 is directly connected, Serial1/0
D    192.168.0.0/24 [90/1764352] via 192.100.102.2, 00:04:13, Serial1/0
D    192.100.103.0/24 [90/2273792] via 192.100.102.2, 00:04:15, Serial1/0
                      [90/2273792] via 192.100.101.2, 00:04:16, Serial1/1
C    192.168.1.0/24 is directly connected, FastEthernet0/0
     192.100.101.0/24 is variably subnetted, 2 subnets, 2 masks
D       192.100.101.0/24 is a summary, 00:04:26, Null0
C       192.100.101.0/30 is directly connected, Serial1/1

When I trace-route R5 from R4 (Interface 192.100.105.2), the path is always R4 -> R2 -> R5. I need sometimes(randomly) it choose another path to R5, for example R4 -> R3 -> R5. How can I do it?

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You have to do two things. First, make sure both routes are in R4's route table. Second, make sure equal-cost routes are selected per packet and not per destination. –  David Schwartz Aug 17 '13 at 18:48
    
@DavidSchwartz: How can I set it to choose target per packet? –  M M. Aug 17 '13 at 19:31
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2 Answers

There are two ways to skin this cat:

1) Disable CEF on R4. No CEF means process switching which generally reverts to per packet load balancing.

 no ip cef

OR

2) Enable per-packet load sharing on each outbound interface with CEF still enabled (not recommended for real world scenarios as you will get out of order packets at your destination host)

ip cef
int fa0/0
 ip load sharing-per packet

Note: this is all generally applicable to transit (traffic passing THRU R4 as opposed to originated from R4) traffic. In your example if you're originating traceroutes from R4 my experience is the router generally sends the packets alternately down its available first hop paths. If you're not seeing that behavior, can you confirm you have two valid routes to your destination in R4's routing table?

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Please try to improve this answer by explaining what is the problem, and what do these commands do to solve it. –  dawud Aug 17 '13 at 14:07
    
Added more commentary - if you're tracerouting from R4 you should be seeing split traffic no matter what... you sure you have your routes in place? –  Jason Seemann Aug 18 '13 at 1:38
    
I updated my question and put show ip route on R4. –  M M. Aug 18 '13 at 8:51
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Now that show IP route is included, its clear that your problem is you don't have two equal cost routes in the routing table for traffic to load balance across. Your routing statement for 192.100.105.0/24 should look the same as your 192.100.103.0/24.

D    192.100.103.0/24 [90/2273792] via 192.100.102.2, 00:04:15, Serial1/0
                      [90/2273792] via 192.100.101.2, 00:04:16, Serial1/1

But you only have a single route. Check the bandwidth and delay on the links between R2 to R5 vs R3 to R5 (show interface serial n/n). They need to match. Once you get two routes in your routing table for 192.100.105.0/24 your traceroute should balance appropriately. The other answer is still valid if you want traffic THRU the router to balance packet by packet.

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The bandwidth and delay on both serial interfaces are the same (MTU 1500 bytes, BW 2048 Kbit, DLY 20000 usec) –  M M. Aug 20 '13 at 20:12
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