Eigrp unequal cost load balancing doubt

[singhankit685]

please explain the traceroute output as show in the image....what are the numbers 1 and 2...and also why the highlighted output is differing from the ones above and below it??

[Richard Burts]

Let us start with a brief review of how traceroute is implemented on IOS devices. Traceroute sends probe packets toward the destination and manipulates the Time To Live. Traceroute sends 3 copies of the probe packets for each TTL. The first probe has TTL of 1. It goes one hop and TTL expires. When TTL expires then that device sends back an ICMP message that TTL expired. And that is how you know what is the first hop. Then traceroute sends the second set of probe packets with TTL of 2 and so forth until either the probe packet gets to the destination on until a max number of hops has been used (30 by default).

So in your output the numbers 1 and 2 are used to identify the first and the second set of probe packets.

When IOS is sending probe packets and it has multiple routing table entries for the destination then the default load sharing behavior will send probe packets alternating which next hop is used. That is why the top instance and the bottom instance have three lines of output in the "1" set of probe packets. The first line of output shows using one path, the second line of output shows using the alternate path to the destination, and the third line of output shows using the first path. The middle instance does not use load sharing for the first set of probes. The "1" set of probes all use the same next hop so there is only one line of output (and it has 3 response times indicated on that line for the 3 probe packets). The fact that this instance does have load sharing on the first set of probe packets suggests that there was some change in the routing table and that when this traceroute was done that there was only a single entry in the routing table for the destination.

HTH

Rick

[AbdoRefky]

wow , nice explaination . 3 responses .. great . 

i can see that his R1 is connected to the next router via 4.4.4.1 & 1.1.1.2 

and the next router is connected the destination via 3.3.3.1 & 2.2.2.2 . 

quite interesting though that the algorithm always choose the same ip to start with . 

[Prashant Sheshasayee]

UNEQUAL COST LOAD BALANCING

• EIGRP SUPPORTS UNEQUAL LOAD COST BALANCING
• IT IS DONE USING VARIANCE COMMAND
• VARIANCE IS A MULTIPLIER VALUES RANGES IN BETWEEN(1-128) and default is 1
• The routes which can go for load balancing should satisfy the condition

The command is

router eigrp 100

variance <multiplier>

[singhankit685]

thanks richard , but i understood partially....but what is meant by a probe ?? and the second instance shows that the packet was dropped at the first router ?? as three probes have ended there?? but i did not understand what is that three response times for the second output ??

[Richard Burts]

In general terms a probe is something that you use when you are attempting to find something that is not where you are. So in discussing traceroute I (and some other people) describe the traceroute packets as probes because the router is sending the traceroute packets to attempt to discover the path to the remote destination. If the term is confusing to you then read my explanation again and everywhere that I say "probe" packet they you think "traceroute" packet.

I do not understand your second question. In every instance three probe packets end at the first hop and three probe packets end at the second hop. Traceroute reports the response time for each probe. When the first probe packet goes down one path and the second probe packet goes down a different path (load sharing being done) then there is a separate line for each probe packet and it reports the response time on each line. When all three probe packets go on the same path and the response is from a single IP then there is one line of output for the three probe packets and there are three response times reported on that line.

HTH

Rick

[Prashant Sheshasayee]

UNEQUAL COST LOAD BALANCING

• EIGRP SUPPORTS UNEQUAL LOAD COST BALANCING
• IT IS DONE USING VARIANCE COMMAND
• VARIANCE IS A MULTIPLIER VALUES RANGES IN BETWEEN(1-128) and default is 1
• The routes which can go for load balancing should satisfy the condition

The command is

router eigrp 100

variance <multiplier>