Here is the following topology:
All routers are configured in EIGRP AS1, R1 is advertising
subnet 192.168.11.0/24 while R4 is advertising subnet 192.168.41.0/24.
Let’s examine R1 routing table:
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R1#sh 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
1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected,
Loopback0
C 192.168.11.0/24 is directly connected,
Loopback1
D 192.168.41.0/24
[90/158720] via 10.1.13.3, 00:00:17, FastEthernet0/1
[90/158720] via 10.1.12.2, 00:00:17, FastEthernet0/0
10.0.0.0/24 is subnetted, 4 subnets
C 10.1.13.0 is directly connected,
FastEthernet0/1
C 10.1.12.0 is directly connected,
FastEthernet0/0
D 10.1.24.0 [90/30720] via 10.1.12.2,
00:00:17, FastEthernet0/0
D 10.1.34.0 [90/30720] via 10.1.13.3,
00:00:19, FastEthernet0/1
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And EIGRP topology:
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R1#sh ip eigrp topology
IP-EIGRP Topology Table for
AS(1)/ID(1.1.1.1)
Codes: P - Passive, A -
Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.1.13.0/24, 1 successors,
FD is 28160
via Connected, FastEthernet0/1
P 10.1.12.0/24, 1 successors,
FD is 28160
via Connected, FastEthernet0/0
P 10.1.24.0/24, 1 successors,
FD is 30720
via 10.1.12.2 (30720/28160),
FastEthernet0/0
P 10.1.34.0/24, 1 successors,
FD is 30720
via 10.1.13.3 (30720/28160),
FastEthernet0/1
P 192.168.41.0/24, 2 successors, FD is 158720
via 10.1.12.2
(158720/156160), FastEthernet0/0
via 10.1.13.3
(158720/156160), FastEthernet0/1
P 192.168.11.0/24, 1
successors, FD is 128256
via Connected, Loopback1
|
We can see that R1 is learning subnet 192.168.41.0/24 from
both R2 and R3 with the same metric, note that two numbers are displayed here:
435200 and 409600, the first is FD and the second is RD.
Here is an explanation taken from Cisco CCNP ROUTE 642-902
Official Certification Guide:
Feasible Distance (FD): Integer metric for the route,
from the local router’s perspective, used by the local router to choose the
best route for that prefix.
Reported Distance (RD): Integer metric for the route,
from the neighboring router’s perspective (the neighbor that told the local
router about the route). Used by the local router when converging to new
routes.
As you may recall the formula used by EIGRP to calculate the
metric is using the least bandwidth and cumulative delay of the link. But which
link exactly does it means?
Let’s try to change the delay on R3 Fa0/0 and see what
happens:
|
R3#configure terminal
Enter configuration commands,
one per line. End with CNTL/Z.
R3(config)#interface
fastEthernet 0/0
R3(config-if)#delay 5000
R3(config-if)#end
R3#
R3#clear ip eigrp neighbors
|
And now let’s look again on R1:
|
R1# 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
1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected,
Loopback0
C 192.168.11.0/24 is directly connected,
Loopback1
D 192.168.41.0/24
[90/158720] via 10.1.13.3, 00:00:29, FastEthernet0/1
[90/158720] via 10.1.12.2, 00:00:29, FastEthernet0/0
10.0.0.0/24 is subnetted, 4 subnets
C 10.1.13.0 is directly connected,
FastEthernet0/1
C 10.1.12.0 is directly connected,
FastEthernet0/0
D 10.1.24.0 [90/30720] via 10.1.12.2,
00:00:36, FastEthernet0/0
D 10.1.34.0 [90/30720] via 10.1.13.3,
00:00:36, FastEthernet0/1
R1#show ip eigrp topology
IP-EIGRP Topology Table for
AS(1)/ID(1.1.1.1)
Codes: P - Passive, A -
Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.1.13.0/24, 1 successors,
FD is 28160
via Connected, FastEthernet0/1
P 10.1.12.0/24, 1 successors,
FD is 28160
via Connected, FastEthernet0/0
P 10.1.24.0/24, 1 successors,
FD is 30720
via 10.1.12.2 (30720/28160),
FastEthernet0/0
P 10.1.34.0/24, 1 successors,
FD is 30720
via 10.1.13.3 (30720/28160),
FastEthernet0/1
P 192.168.41.0/24, 2 successors, FD is 158720
via 10.1.12.2
(158720/156160), FastEthernet0/0
via 10.1.13.3 (158720/156160),
FastEthernet0/1
P 192.168.11.0/24, 1
successors, FD is 128256
via Connected, Loopback1
|
As you can see nothing changed! R1 is still learning subnet
192.168.41.0/24 from R2 and R3 with the same metric.
That’s because R1 is calculating the metric to this subnet in
the following manner:
1.
Least bandwidth on the
link, in our topology the bandwidth is the same for all interfaces
2.
Cumulative delay of the
link
The link in this case is composed from link 13 + link 34 toward
R4 subnet (upstream), so R1 is calculating the delay on the interfaces toward
R4, using R1 Fa0/1 and R3 Fa0/1, as cumulative delay in the formula!
The blue dots indicate the interfaces which R1 used in his
formula to calculate the delay toward R4:
Even configuring delay on R4 Fa0/1 won’t change the metric
calculation for R1.
The only way to
change the metric for this link will be on R1 Fa0/1 or R3 Fa0/1 interfaces.
R1 is looking on the link toward the advertising router,
from his perspective, upstream.
Configuring bandwidth is the same beside the fact that the
least bandwidth is taken into calculation and not in cumulative way as delay.
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