CCNP ROUTE Study – EIGRP Offset Lists

In certain situations an engineer may need to manipulate EIGRP metrics, and one method is to use Offset Lists to increase both the AD (Advertised Distance) and FD (Feasible Distance) of a route by a certain value – the offset.

Offset List configurations define the following:

• route(s) that we want to amend the metric for (matching an ACL)
• direction of the updates being sent or received
• interface on which updates are sent or received
• offset integer value

Network diagram:

Objective One:

when router WEST advertises prefix 10.1.1.0 out of interface Serial0/0, it should add 100 to the metric

Lets first check the FD and AD values that HQ has in its topology table for prefix 10.1.1.0/24:

HQ#sh ip eigrp topology 10.1.1.0/24
IP-EIGRP (AS 1): Topology entry for 10.1.1.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2297856
  Routing Descriptor Blocks:
  192.168.1.2 (Serial0/0), from 192.168.1.2, Send flag is 0x0
      Composite metric is (2297856/128256), Route is Internal

Next, we’ll configure the offset list on router WEST:

WEST(config)#access-list 10 permit 10.1.1.0
WEST(config)#router eigrp 1
WEST(config-router)#offset-list 10 out 100 serial 0/0

If we check the FD and AD values on the HQ router again, we’ll see they have both increased by 100:

HQ#sh ip eigrp topology 10.1.1.0/24
IP-EIGRP (AS 1): Topology entry for 10.1.1.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2297856
  Routing Descriptor Blocks:
  192.168.1.2 (Serial0/0), from 192.168.1.2, Send flag is 0x0
      Composite metric is (2297956/128356), Route is Internal

Objective Two:

when HQ router receives an advertisement for prefix 10.1.2.0 in via interface Serial0/0, it should add 100 to the metric

Lets first check the FD and AD values that HQ has in its topology table for prefix 10.1.2.0/24:

HQ#sh ip eigrp topology 10.1.2.0/24
IP-EIGRP (AS 1): Topology entry for 10.1.2.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2297856
  Routing Descriptor Blocks:
  192.168.1.3 (Serial0/0), from 192.168.1.3, Send flag is 0x0
      Composite metric is (2297856/128256), Route is Internal

Next, we’ll configure the offset list on router HQ:

HQ(config)#access-list 10 permit 10.1.2.0  
HQ(config)#router eigrp 1                  
HQ(config-router)#offset-list 10 in 100 serial 0/0

If we check the FD and AD values on the HQ router again, we’ll see they have both increased by 100:

HQ#sh ip eigrp topology 10.1.2.0/24
IP-EIGRP (AS 1): Topology entry for 10.1.2.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2297856
  Routing Descriptor Blocks:
  192.168.1.3 (Serial0/0), from 192.168.1.3, Send flag is 0x0
      Composite metric is (2297956/128356), Route is Internal

I hope this has been a useful explanation.  Thanks for reading, and good luck with your CCNP studies!

Rich

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