Saturday, 30 August 2025

What is an Offset-list? How to configure EIGRP Offset-list?

 What is an Offset-list?

An Offset-list is an EIGRP and RIPv2 feature that allows you to add a fixed value (an “offset”) to the composite metric of a route that matches an access control list (ACL). You can apply this to inbound (IN) and outbound (OUT) routing updates.

By increasing the metric of a route, you make it less preferable. EIGRP will always choose the path with the lower composite metric. It’s a simple yet powerful way to de-prioritize a path without using a route filter to deny it entirely. Offset value to add to the matric (1-2147483647)





Why would you use one?

Common use cases include:

  • ·        Basic load balance: making a primary link preferred while keeping a backup link active by making its metric slightly worse.
  • ·        Policy-based routing: influencing outbound traffic to take a specific path based on network policy.
  • ·        Cost engineering: simulating a longer path for a route learned from one neighbor compared to another.

 let's see the configuration:-

Topology:-



  • Configure the topology as per the diagram
  • Configure the IP addresses as per the topology 
  • Configure EIGRP 100 to advertise directly connected interfaces
  • Configure access-list standard permit 30.1.1.1 network
  • Make sure router 1 will use the serial 5/0 to reach the 30.1.1.1/8 network
  • Ensure no load balancing is used with the offset list; add 5000 metrics to the serial 5/3 route.

R1(config)#interface serial 5/0
R1(config-if)#ip address 1.1.1.1 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface serial 5/3
R1(config-if)#ip address 4.4.4.2 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface FastEthernet 0/0
R1(config-if)#ip address 10.1.1.1 255.0.0.0
R1(config-if)#no keepalive
R1(config-if)#no shutdown
R1(config-if)#exit

R2(config)#interface serial 5/0
R2(config-if)#ip address 1.1.1.2 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface serial 5/1
R2(config-if)#ip address 2.2.2.1 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface FastEthernet 0/0
R2(config-if)#ip address 20.1.1.1 255.0.0.0
R2(config-if)#no keepalive
R2(config-if)#no shutdown
R2(config-if)#exit

R3(config)#interface serial 5/2
R3(config-if)#ip address 3.3.3.1 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface serial 5/1
R3(config-if)#ip address 2.2.2.2 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface FastEthernet 0/0
R3(config-if)#ip address 30.1.1.1 255.0.0.0
R3(config-if)#no keepalive
R3(config-if)#no shutdown
R3(config-if)#exit

R4(config)#interface serial 5/2
R4(config-if)#ip address 3.3.3.2 255.0.0.0
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface serial 5/3
R4(config-if)#ip address 4.4.4.1 255.0.0.0
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface FastEthernet 0/0
R4(config-if)#ip address 40.1.1.1 255.0.0.0
R4(config-if)#no keepalive
R4(config-if)#no shutdown
R4(config-if)#exit

R1(config)#router eigrp 100
R1(config-router)#network 10.0.0.0
R1(config-router)#network 1.0.0.0
R1(config-router)#network 4.0.0.0
R1(config-router)#no auto-summary
R1(config-router)#exit


R2(config)#router eigrp 100
R2(config-router)#network 20.0.0.0
R2(config-router)#network 1.0.0.0
R2(config-router)#network 2.0.0.0
R2(config-router)#no auto-summary
R2(config-router)#exit


R3(config)#router eigrp 100
R3(config-router)#network 30.0.0.0
R3(config-router)#network 3.0.0.0
R3(config-router)#network 2.0.0.0
R3(config-router)#no auto-summary
R3(config-router)#exit

R4(config)#router eigrp 100
R4(config-router)#network 40.0.0.0
R4(config-router)#network 3.0.0.0
R4(config-router)#network 4.0.0.0
R4(config-router)#no auto-summary
R4(config-router)#exit

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

C    1.0.0.0/8 is directly connected, Serial5/0
D    2.0.0.0/8 [90/2681856] via 1.1.1.2, 00:26:52, Serial5/0
D    3.0.0.0/8 [90/2681856] via 4.4.4.1, 00:26:52, Serial5/3
C    4.0.0.0/8 is directly connected, Serial5/3
D    20.0.0.0/8 [90/2172416] via 1.1.1.2, 00:26:52, Serial5/0
D    40.0.0.0/8 [90/2172416] via 4.4.4.1, 00:26:52, Serial5/3
C    10.0.0.0/8 is directly connected, FastEthernet0/0
D    30.0.0.0/8 [90/2684416] via 4.4.4.1, 00:24:15, Serial5/3
                [90/2684416] via 1.1.1.2, 00:24:15, Serial5/0

R1#traceroute 30.1.1.1

Type escape sequence to abort.
Tracing the route to 30.1.1.1

  1 4.4.4.1 48 msec
    1.1.1.2 28 msec
    4.4.4.1 28 msec
  2 2.2.2.2 80 msec
    3.3.3.1 80 msec
    2.2.2.2 80 msec
R1#show ip route 30.1.1.1 255.0.0.0
Routing entry for 30.0.0.0/8
  Known via "eigrp 100", distance 90, metric 2684416, type internal
  Redistributing via eigrp 100
  Last update from 4.4.4.1 on Serial5/3, 00:01:28 ago
  Routing Descriptor Blocks:
    4.4.4.1, from 4.4.4.1, 00:01:28 ago, via Serial5/3
      Route metric is 2684416, traffic share count is 1
      Total delay is 40100 microseconds, minimum bandwidth is 1544 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2
  * 1.1.1.2, from 1.1.1.2, 00:01:28 ago, via Serial5/0
      Route metric is 2684416, traffic share count is 1
      Total delay is 40100 microseconds, minimum bandwidth is 1544 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2

(as you can see, both route metrics are the same. So now we are going to add 5000 metric to the serial 5/3 route.


R1(config)#ip access-list standard Offset
R1(config-std-nacl)#permit 30.0.0.0 0.255.255.255
R1(config-std-nacl)#exit

R1(config)#router eigrp 100
R1(config-router)#offset-list Offset in  5000 serial 5/3
R1(config-router)#end

*Aug 30 16:14:11.471: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 4.4.4.1 (Serial5/3) is resync: route configuration changed

R1#clear ip route *

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

C    1.0.0.0/8 is directly connected, Serial5/0
D    2.0.0.0/8 [90/2681856] via 1.1.1.2, 00:00:39, Serial5/0
D    3.0.0.0/8 [90/2681856] via 4.4.4.1, 00:00:39, Serial5/3
C    4.0.0.0/8 is directly connected, Serial5/3
D    20.0.0.0/8 [90/2172416] via 1.1.1.2, 00:00:39, Serial5/0
D    40.0.0.0/8 [90/2172416] via 4.4.4.1, 00:00:39, Serial5/3
C    10.0.0.0/8 is directly connected, FastEthernet0/0
D    30.0.0.0/8 [90/2684416] via 1.1.1.2, 00:00:39, Serial5/0


R1#traceroute 30.1.1.1

Type escape sequence to abort.
Tracing the route to 30.1.1.1

  1 1.1.1.2 40 msec 32 msec 32 msec
  2 2.2.2.2 76 msec 56 msec 72 msec


R1#show ip route 30.1.1.1 255.0.0.0
Routing entry for 30.0.0.0/8
  Known via "eigrp 100", distance 90, metric 2684416, type internal
  Redistributing via eigrp 100
  Last update from 1.1.1.2 on Serial5/0, 00:01:57 ago
  Routing Descriptor Blocks:
  * 1.1.1.2, from 1.1.1.2, 00:01:57 ago, via Serial5/0
      Route metric is 2684416, traffic share count is 1
      Total delay is 40100 microseconds, minimum bandwidth is 1544 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2

R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(10.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status
P 1.0.0.0/8, 1 successors, FD is 2169856
        via Connected, Serial5/0
P 2.0.0.0/8, 1 successors, FD is 2681856
        via 1.1.1.2 (2681856/2169856), Serial5/0
P 3.0.0.0/8, 1 successors, FD is 2681856
        via 4.4.4.1 (2681856/2169856), Serial5/3
P 4.0.0.0/8, 1 successors, FD is 2169856
        via Connected, Serial5/3
P 10.0.0.0/8, 1 successors, FD is 28160
        via Connected, FastEthernet0/0
P 20.0.0.0/8, 1 successors, FD is 2172416
        via 1.1.1.2 (2172416/28160), Serial5/0
P 30.0.0.0/8, 1 successors, FD is 2684416
        via 1.1.1.2 (2684416/2172416), Serial5/0
        via 4.4.4.1 (2689416/2177416), Serial5/3
P 40.0.0.0/8, 1 successors, FD is 2172416
        via 4.4.4.1 (2172416/28160), Serial5/3

 ( as you can see, the metric has been increased to 2177416, and previously this metric was 2172416)

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What is an Offset-list? How to configure EIGRP Offset-list?

 What is an Offset-list ? An Offset-list is an EIGRP and RIPv2 feature that allows you to add a fixed value (an “offset”) to the composi...