EIGRP - Unequal-Cost LoadBalancing

From: Nitro Drops (nitrodrops@hotmail.com)
Date: Fri Apr 11 2008 - 22:52:48 ART


Guys,

was practising this scenario.

R4 S2/1 (155.1.45.4/24) >> 64K Serial 64K>> (155.1.45.5/24)S2/1 R5
R4 S2/0 (155.1.0.4/24) >> 256 K Frame Relay 256K >> (155.1.0.5/24) S2/0 R5
R4 Loopback 0 - 150.1.4.4
R5 Loopback 0 - 150.1.5.5

Variance calculated is 4.

R5:
router eigrp 100
 variance 4

R5#sh ip eigrp neighbors
IP-EIGRP neighbors for process 100
H Address Interface Hold Uptime SRTT
RTO Q Seq
                                            (sec) (ms) Cnt Num
1 155.1.0.4 Se2/0 150 00:42:28 189 1134 0 29
0 155.1.45.4 Se2/1 12 00:44:48 191 2280 0 30

R5#sh ip
route
Gateway of last resort is not set
     155.1.0.0/24 is
subnetted, 2 subnets
C 155.1.0.0 is
directly connected, Serial2/0
C 155.1.45.0 is
directly connected, Serial2/1
     150.1.0.0/24 is
subnetted, 2 subnets
C 150.1.5.0 is
directly connected, Loopback0
D 150.1.4.0 [90/40640000] via 155.1.45.4,
00:00:46, Serial2/1
                  [90/10639872] via 155.1.0.4,
00:00:46, Serial2/0

R5#trace 150.1.4.4
Type escape sequence to abort.
Tracing the route to 150.1.4.4
  1 155.1.45.4 156
msec
    155.1.0.4 148 msec
    155.1.45.4 28 msec

R5#trace 150.1.4.4
Type escape sequence to abort.
Tracing the route to 150.1.4.4
  1 155.1.0.4 176 msec
    155.1.45.4 128
msec
    155.1.0.4 152 msec

R5#trace 150.1.4.4
Type escape sequence to abort.
Tracing the route to 150.1.4.4
  1 155.1.0.4 168 msec
180 msec *

R5#trace 150.1.4.4
Type escape sequence to abort.
Tracing the route to 150.1.4.4
  1 155.1.0.4 152 msec
40 msec *

  I can see the packets being load-balanced when tracing from R4 to R5's loopback. Did 4 traces continuously and noticed the follows

Q1.) For the 1st 2 traces, how come the packets reaches R4's S2/1 interface, it will go to R4's S2/0 interface again before hitting R4's loopback? The same goes for the 2nd trace, it will go to R4's S2/0 interface 1st, followed by R4's S2/1 interface before reaching R4's loopback.

Q2) For the 3rd and 4th traces, it hits the R4's loopback0 immediately as next hop, without hitting R4's S2/0 or S2/1.

Appreciate any kind advice.

Cheers
Nit



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