EIGRP attributes

[asit1111990]

Hello team,

We known that while calculating the metric in eigrp we use two main attributes  bandwidth and delay . ie 

Metric=10^7/least bandwidth in kbps + delay /10

suppose we have 10 routers and we need the traffic to move from r1 to r10 and we have many interlinks/path to reach them

now while calculating the metric how does r1 known that x is the least bandwidth among the links . how does router known that the total  delay is y

are the values exchanges in eigrp packets or some eigrp adjacency packets ? How does router known the least bandwidth among the series of link in the path .

Eg if r1 has multilple path ie 3 paths to reach destination ..how does router known that path three consists of least bandwidth ?

Thank you!

[Julio E. Moisa]

Hi, 

You can find the BW and delay of a interface executing ie: show interface Fa x/y. There are other metrics in EIGRP but it uses BW and delay by default, the other ones are not used. EIGRP shares its routing table when a change is detected or a new neighbor is discovered, this way the routing information is shared between the EIGRP neighbors. 

The delay associated a route is the sum of the delay values in the path. The bandwidth represents the slowest bandwidth value in the interfaces going to the destination.The delay can be changed using delay <value> command. You could change the BW value configuring under the interface the bandwidth <value> command BUT it doesn´t change the real bandwidth of the interface, Bandwidth command is used to manipulate the traffic only. If you have the same BW and delay value on the 3 interface you will have a load balance. 

If you execute: sh ip eigrp topology <destination network> <subnet mask> you will see 3 paths to the destination.

Please see my example, I have 5 routers, R1 is advertising the prefix 2.2.2.2/32, R5 received the prefix and know it through 3 different paths:

R5#sh ip eigrp top 2.2.2.2 255.255.255.255
IP-EIGRP (AS 100): Topology entry for 2.2.2.2/32
State is Passive, Query origin flag is 1, 3 Successor(s), FD is 189952
Routing Descriptor Blocks:
10.25.0.2 (FastEthernet0/1), from 10.25.0.2, Send flag is 0x0
Composite metric is (189952/187392), Route is Internal
Vector metric:
Minimum bandwidth is 45045 Kbit
Total delay is 5200 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2

10.35.0.3 (FastEthernet0/0), from 10.35.0.3, Send flag is 0x0
Composite metric is (189952/187392), Route is Internal
Vector metric:
Minimum bandwidth is 45045 Kbit
Total delay is 5200 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2

10.45.0.4 (FastEthernet1/0), from 10.45.0.4, Send flag is 0x0
Composite metric is (189952/187392), Route is Internal
Vector metric:
Minimum bandwidth is 45045 Kbit
Total delay is 5200 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2

For testing purposes i have configured the following command under the interface on R5:  ip load-sharing per-packet, now i have executed a ping:

R5#ping 2.2.2.2 source l0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
Packet sent with a source address of 5.5.5.5
!!!!!

Now we can see the load balance:

R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet0/1 (next hop 10.25.0.2)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet1/0 (next hop 10.45.0.4)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet0/0 (next hop 10.35.0.3)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet0/1 (next hop 10.25.0.2)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet1/0 (next hop 10.45.0.4)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet0/0 (next hop 10.35.0.3)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet0/1 (next hop 10.25.0.2)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet1/0 (next hop 10.45.0.4)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet0/0 (next hop 10.35.0.3)
R5#sho ip cef exac 5.5.5.5 2.2.2.2
5.5.5.5 -> 2.2.2.2 : FastEthernet0/1 (next hop 10.25.0.2)

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