Solved: Re: EIGRP metric calculation?

David Salazar · ‎12-28-2009

EIGRP metric calculation?

I have a 6513 with EIGRP routing protocol.

When I try to calculate the EIGRP metric of a interface loopback 0, using the
bandwidth value and delay value that show interface loopback 0 command.

es-plcdatacenter2#sh int loo
es-plcdatacenter2#sh int loopback 0
Loopback0 is up, line protocol is up
Hardware is Loopback
Internet address is 172.16.3.36/32
MTU 1514 bytes, BW 8000000 Kbit, DLY 5000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
Encapsulation LOOPBACK, loopback not set
Last input 00:00:02, output never, output hang never
Last clearing of "show interface" counters 1w3d
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/0 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
L2 Switched: ucast: 0 pkt, 0 bytes - mcast: 0 pkt, 0 bytes
L3 in Switched: ucast: 0 pkt, 0 bytes - mcast: 0 pkt, 0 bytes mcast
L3 out Switched: ucast: 0 pkt, 0 bytes mcast: 0 pkt, 0 bytes
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     189839 packets output, 11390340 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out
es-plcdatacenter2#
es-plcdatacenter2#

but when I was used the "SHOW IP EIGRP TOP x.x.x.x/x" command, I see the next information "Minimum bandwidth is 10000000 Kbit"

What about that?

In this link http://ccie-en-espanol.blogspot.com/2009/05/eigrp-mtu-y-metrica.html#comment-form

I found the following information:

Some default values for Bandwidth and Delay associate to some interfaces type are listen on the next table

Media.........Bandwidth..Delay
FastEthernet...100000K...100µS
Ethernet........10000K...1000µS
T1..............1544K....20000µS
DS0..............64K....20000µS
56K..............56K....20000µS

es-plcdatacenter2#
es-plcdatacenter2#sh ip eigrp top 172.16.3.36/32
IP-EIGRP (AS 4944): Topology entry for 172.16.3.36/32
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256
Routing Descriptor Blocks:
0.0.0.0 (Loopback0), from Connected, Send flag is 0x0
      Composite metric is (128256/0), Route is Internal
      Vector metric:
        Minimum bandwidth is 10000000 Kbit
        Total delay is 5000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1514
        Hop count is 0

I will apreciate some assistance about that.

David Salazar

Giuseppe Larosa · ‎12-28-2009

Hello David,

I would say it is a resolution problem.

With effective settings the contribute of BW to metric is : (10^7 / 8000000)*256 = 1.25 *256

the metric is calculated taken the truncated version of 1.25 = 1

you see metric as 128256 = 500*256 + 256*1

using 1,25 it would be: 500*256+1,25*256 = 128320

then the sh ip eigrp topology expands the metric in the two components giving 5000 microseconds and 10000000 kbps

Hope to help

Giuseppe

View solution in original post

Jon Marshall · ‎12-28-2009

nando02 wrote:

Hello,

Giuseppe

Thank a lot, for your valuable information.

Can you give me a technical reference where it is define the use of truncated method for EIGRP metric calculation?

I was thinking about the use of this method before writing on the Cisco Support Community but I do not find technical reference. I tried to remeber that I was read about it long time ago but I don't have writing reference.

Every time that division result in numerical fractional is used truncated method?

Thanks

David

From the EIGRP whitepaper section on metric calculation -

Cisco routers do not perform floating point math, so at each stage in the calculation, you need to round down to the nearest integer to properly calculate the metrics

Full link -

EIGRP metrics

Jon

View solution in original post

Giuseppe Larosa · ‎12-28-2009

Hello David,

I would say it is a resolution problem.

With effective settings the contribute of BW to metric is : (10^7 / 8000000)*256 = 1.25 *256

the metric is calculated taken the truncated version of 1.25 = 1

you see metric as 128256 = 500*256 + 256*1

using 1,25 it would be: 500*256+1,25*256 = 128320

then the sh ip eigrp topology expands the metric in the two components giving 5000 microseconds and 10000000 kbps

Hope to help

Giuseppe

David Salazar · ‎12-28-2009

Hello,

Giuseppe

Thank a lot, for your valuable information.

Can you give me a technical reference where it is define the use of truncated method for EIGRP metric calculation?

I was thinking about the use of this method before writing on the Cisco Support Community but I do not find technical reference. I tried to remeber that I was read about it long time ago but I don't have writing reference.

Every time that division result in numerical fractional is used truncated method?

Thanks

Jon Marshall · ‎12-28-2009

nando02 wrote:

Hello,

Giuseppe

Thank a lot, for your valuable information.

Can you give me a technical reference where it is define the use of truncated method for EIGRP metric calculation?

I was thinking about the use of this method before writing on the Cisco Support Community but I do not find technical reference. I tried to remeber that I was read about it long time ago but I don't have writing reference.

Every time that division result in numerical fractional is used truncated method?

Thanks

David

From the EIGRP whitepaper section on metric calculation -

Cisco routers do not perform floating point math, so at each stage in the calculation, you need to round down to the nearest integer to properly calculate the metrics

Full link -

EIGRP metrics

Jon

David Salazar · ‎12-28-2009

Thank a lot, Jon.