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)

I hope you like this lab. Please visit our YouTube channel https://www.youtube.com/@internetworkss

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Thursday, 14 August 2025

What is IP prefix-list? How to configure IP prefix-list?

 IP Prefix List

An IP prefix list is a network filtering tool used primarily in routing protocols to control which routes are accepted or rejected based on their network prefixes.

Key Characteristics

·        Prefix-based filtering: Matches routes based on their network address and prefix length.

·        Ordering processing: Entries are processed top-down until a match is found.

·        Implicit deny: if no match is found, the prefix is denied by default



 


R1(config)#ip prefix-list NAME seq 10 deny 192.168.1.1/24?

  ge  Minimum prefix length to be matched

  le  Maximum prefix length to be matched

  <cr>

Parameters

·        Network/len: The network address and prefix length to match.

·        Ge: “greater than or equal to” – minimum prefix length to match.

·        Le: “less than or equal to” – maximum prefix length to match.

·        Seq: optional sequence number for ordering entries.

Prefix lists are widely used in routing protocols like BGP, OSPF, and EIGRP for precise control over route advertisements and redistributions. 


Topology:-


Goal:.Configure the topology as per the diagram

  1. Configure the IP addresses as per the topology
  2. Configure IGP protocol EIGRP 100
  3. Advertise the directly connected routes 
  4. configure IP Prefix list name INTERNET
  5. Deny 192.168.31.0/24 - 192.168.32.0/24
  6. rest permit
  7. deny 192.168.11.0/24 adjust the SEQ number.
  8. make sure router 2 deny192.168.31.1/24 & 192.168.32.1/24 and 192.168.11.1/24 
  9. change the SEQ
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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...