EIGRP offset-list

[The_guroo_2]

Hi Guys i have the concept of off-set list but getting confused abt in and out

for example

if i have an off-set :

router eigrp 1

offset list 1 out 1000 interface ethernet 1/0

Access-list 1 permit 10.0.0.0/24

so what will it do and if i type

offset-list 1 in 1000 interface ethernet 1/0

can someone please explain

Thanks

[Jon Marshall · Accepted solution]

offset-list out - will apply the metric offset to the 10.0.0.0/24 route being advertised out of the e1/0 interface.

offset-list in - will apply the metric offset to the 10.0.0.0/24 route when it is received from another router on the e1/0 interface.

Both increase the metric of the 10.0.0.0/24 route by the amount you have specified ie. 1000.

Jon

[Richard Burts]

This is a good response from Jon. It is difficult to do better but I offer this as a supplement to his answer.

First let us be clear about the function of offset list - an offset list increases the metric of routes being advertised.

Then let us deal with the in or out aspect of offset list. Here is how I think about it and keep it straight in my mind: offset list is similar to access group. They both deal with things in terms of things coming into the router or things going out of the router. So offset-list in will increase the metric of routes being advertised into the router. And offset-list out will increase the metric of routes being advertised from the router.

HTH

Rick

Sent from Cisco Technical Support iPhone App

[CiscoIsInYou · Accepted solution]

EIGRP uses Bandwidth,Delay,Load,Reliance & MTU for calculating the metric.

However it also uses HOP COUNT as a parameter in Message exchanges (though not used while calculating metric).

This HOP-COUNT works similar to DV protocols like RIP stating the number of HOPS required to reach the SUBNET.

1000 will set the HOP-COUNT to 1000 for the subnet 10.0.0.0/24 when advertised OUT.

HTH.

Mark Answered if this resolves & rate if helpful.

Thanks

Shaz

[Jon Marshall]

Shaz

This is not right. EIGRP has a maximum hop count of 255. The 1000 used in the example above is nothing to do with hop count, it is to do the composite metric that EIGRP calcluates for each route.

Jon

[dslice]

Jon is correct. Hop count is/was used to avoid count to infinity problems (or at least bound infinity). Offset list modifies the delay value to cause the calculated metric to be changed by the offset value.

Sent from Cisco Technical Support iPad App

[CiscoIsInYou]

My Bad.

That's not the hop-count & neither modifies delay.

To be very specific the Offset-List Adds the Offset (here 1000) to the Reported Distance (Or Advertised Distance) to the metric & so the FD also offsets that number.

Thanks

Shaz

[Peter Paluch]

Hello Shaz,

That's not the hop-count & neither modifies delay.
To be very specific the Offset-List Adds the Offset (here 1000) to the Reported Distance (Or Advertised Distance) to the metric 

I am sorry but this is not true. The offset list indeed increases the Delay component of all received or advertised routes (depending on the offset-list direction) in EIGRP. Why? Because the Delay is the only cumulative component of the metric, and the metric is never advertised as the resulting number but rather as all the constituent factors.

EIGRP metrics are always carried in their individual form in EIGRP Update packets: Bandwidth, Delay, Reliability, Load, all of them are located in the Update packet not summed together. This is done because there are different operations required on individual metric components: the Bandwidth component goes a through a min() function processing while the Delay is increased.

This is the reason why the value from the offset-list cannot be simply added to the resulting metric because the resulting metric is actually never advertised. Thus, the offset-list value must be added to some factor, and because of obvious reasons, the Delay factor was chosen for this modification.

Best regards,

Peter

[Marwan ALshawi]

Peter 5+ described nicely

by the way there is a trick with eigrp to setup max hops to block routes of higher hop counts

for example if you set max hops to 10 any route with 11 or more hop counts will not be installed in the routing table