Configure EIGRP to Influence Path Selection
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Contents
Introduction
This document describes the process of creating a preferred path by influencing different Enhanced Interior Gateway Routing Protocol (EIGRP) features.
Prerequisites
Requirements
Cisco recommends that you have knowledge of these topics:
- Knowledge of Basic IP Routing
- Knowledge of EIGRP Protocol
- Knowledge of Cisco IOS®XE Command Line Interface (CLI)
Components Used
This document is not restricted to specific software and hardware versions, however, the information in this document is based on these software and hardware versions:
- Router ASR 1000
- Router ISR 4000
- Cisco IOS XE 17.9.x
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
Background Information
The EIGRP path selection can be influenced by manipulating various metrics that the protocol uses to determine the best path to a destination. EIGRP calculates the best path to a destination based on different metrics, and the path selection process involves evaluating these metrics to determine the optimal route. The EIGRP metrics include bandwidth, delay, load, reliability, and Maximum Transmission Unit (MTU). Understanding these metrics and their significance helps network administrators to modify EIGRP path selection based on specific requirements or network conditions. By default, from the different metric values, EIGRP only uses the minimum bandwidth on the path to a destination network and the total delay to compute routing metrics. Furthermore, the bandwidth and delay metrics are determined from static values configured on the interfaces from devices along the path towards the destination, in other words these two parameters are not measured dynamically.
Aside from the metric manipulation, route filtering can also be used to influence the path selection in EIGRP. Route filtering involves controlling the information that is allowed or denied to enter or exit a router routing table. Filtering routes can be done for various reasons, including optimizing routing tables, or managing network traffic. Some of the key features related to route filtering in EIGRP include, distribute lists, prefix lists, route maps and leak maps. These mechanisms offer a powerful and flexible way for controlling routing information that can be used by network administrators to tailor EIGRP routing tables to meet specific criteria and improve network efficiency.
Scenarios
In the dynamic landscape of routing protocols, administrators often find themselves faced with the need to tailor routing decisions to align with specific network requirements and optimize traffic flow. This involves leveraging various techniques and configurations to influence how routers make path selection decisions. The next examples provide different alternatives where administrators can employ strategic configurations to manipulate EIGRP path selection.
1. InfluencePath Selection by modifying the Delay metric
Adjusting the delay metric on a router interface allows administrators to influence routing decisions by affecting this particular parameter on a link. This subtle manipulation can guide traffic to take preferred paths based on the altered delay values.
2. Influence Path Selection with the use an offset-list
Employing an offset-list enables the selective modification of metrics for specific prefixes, providing a targeted approach to influencing path selection over a particular interface. This mechanism is used for increasing incoming and outgoing metrics to routes learned via EIGRP and to selectively prefer some prefixes over a particular path.
3. Influence Path Selection with Summarization
Introducing summary routes allows administrators to influence the longest match preference for a prefix. Route summarization can impact the granularity of routing decisions, optimizing routing tables and improving overall network efficiency.
4. Influence Path Selection with the use of Leak Maps
Leveraging leak-maps during the advertisement of summary routes provides a mechanism for announcing more specific routes selectively. This approach ensures that summarized information is announced strategically, maintaining routing flexibility and influencing path selection.
5. Influence Path Selection by modifying the Administrative Distance (AD) of a prefix
Altering the administrative distance of a prefix is a helpful technique to control the source of routing information. This can be particularly useful in scenarios where routes from certain sources need to be excluded from the Routing Information Base (RIB).
6. Influence Path Selection with Route Filtering
Route filtering is a powerful method used to control the advertisement or acceptance of specific routes in or out of a routing protocol. It is commonly used to filter routing information based on specified criteria, preventing certain routes from being advertised or learned.
A distribute-list is one of the main tools used to filter prefixes in EIGRP and it can work in conjunction with an access-list (ACL), prefix-list or route-map.
Employing a prefix-list facilitates the granular filtering of prefixes from specific neighbors. This level of control is essential to manage routing updates to modify the path preference.
Network Diagram
EIGRP Topology
Initial Configurations
Before modifying any configuration, it is important to review the initial configuration and status of the devices (initial configuration is the same in each scenario). Based on the network diagram, R1, R2, R3 and R4 are EIGRP neighbors (each router has two adjacencies) with R4 also being part of the Intermediate System-to-Intermediate System (IS-IS) domain, and doing mutual redistribution between IS-IS and EIGRP. It is important to notice that R1 has two paths in the routing table (through interface Gi1/0/3 and Gi1/0/4) to the 10.20.x.x and 10.30.x.x subnets via EIGRP, and subnets 10.10.x.x are directly connected.
| R1 | |
| Configurations | Status |
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In the case of R2 and R3, all the prefixes 10.10.x.x, 10.20.x.x and 10.30.x.x are being learned through EIGRP.
| R2 | |
| Configurations | Status |
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| R3 | |
| Configurations | Status |
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| R4 | |
| Configurations | Status |
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Scenario 1: Influence Path Selection by modifying the Delay metric
In this example, the Delay value is used to influence EIGRP to prefer the path through R3. Before doing any change, you can confirm that EIGRP is load balancing between interfaces Gi1/0/3 and Gi1/04 as both interfaces have the same Delay value of 10 microseconds.
R1#show ip route eigrp
Codes: L - local, 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, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
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
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 12 subnets, 2 masks
D EX 10.20.40.0/24
[170/66560] via 192.168.3.2, 5d22h, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 5d22h, GigabitEthernet1/0/4
D EX 10.20.50.0/24
[170/66560] via 192.168.3.2, 5d22h, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 5d22h, GigabitEthernet1/0/4
D EX 10.20.60.0/24
[170/66560] via 192.168.3.2, 5d22h, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 5d22h, GigabitEthernet1/0/4
D 10.30.70.0/24 [90/16000] via 192.168.3.2, 5d22h, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 5d22h, GigabitEthernet1/0/4
D 10.30.80.0/24 [90/16000] via 192.168.3.2, 5d22h, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 5d22h, GigabitEthernet1/0/4
D 10.30.90.0/24 [90/16000] via 192.168.3.2, 5d22h, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 5d22h, GigabitEthernet1/0/4
172.16.0.0/30 is subnetted, 2 subnets
D 172.16.2.0 [90/15360] via 192.168.1.2, 1w5d, GigabitEthernet1/0/4
D 172.16.4.0 [90/15360] via 192.168.3.2, 1w5d, GigabitEthernet1/0/3
R1#show interface GigabitEthernet1/0/3 | i DLY
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
R1#show interface GigabitEthernet1/0/4 | i DLY
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,Now, lets modify and increase the delay to interface GigabitEthernet1/0/4. By changing the delay value to 100 (tens of microseconds) the RIB installs the path through interface Gi1/0/3 only.
By looking at the EIGRP topology table, you can confirm that interface Gi1/0/4 is still showing as a feasible successor for the all the prefixes and has a higher total delay.
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#interface GigabitEthernet1/0/4
R1(config-if)#delay 100
R1(config-if)#end
R1#show ip route eigrp
Codes: L - local, 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, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
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
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 12 subnets, 2 masks
D EX 10.20.40.0/24
[170/66560] via 192.168.3.2, 00:05:52, GigabitEthernet1/0/3
D EX 10.20.50.0/24
[170/66560] via 192.168.3.2, 00:05:52, GigabitEthernet1/0/3
D EX 10.20.60.0/24
[170/66560] via 192.168.3.2, 00:05:52, GigabitEthernet1/0/3
D 10.30.70.0/24
[90/16000] via 192.168.3.2, 00:05:52, GigabitEthernet1/0/3
D 10.30.80.0/24
[90/16000] via 192.168.3.2, 00:05:52, GigabitEthernet1/0/3
D 10.30.90.0/24
[90/16000] via 192.168.3.2, 00:05:52, GigabitEthernet1/0/3
172.16.0.0/30 is subnetted, 2 subnets
D 172.16.2.0 [90/20480] via 192.168.3.2, 00:05:52, GigabitEthernet1/0/3
D 172.16.4.0 [90/15360] via 192.168.3.2, 00:05:52, GigabitEthernet1/0/3
R1#show interface GigabitEthernet1/0/4 | i DLY
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 1000 usec,
R1#show ip eigrp topology
EIGRP-IPv4 VR(LAB) Topology Table for AS(100)/ID(192.168.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 192.168.3.0/30, 1 successors, FD is 1310720
via Connected, GigabitEthernet1/0/3
P 10.30.70.0/24, 1 successors, FD is 2048000
via 192.168.3.2 (2048000/1392640), GigabitEthernet1/0/3
via 192.168.1.2 (66928640/1392640), GigabitEthernet1/0/4
P 10.20.50.0/24, 1 successors, FD is 8519680
via 192.168.3.2 (8519680/7864320), GigabitEthernet1/0/3
via 192.168.1.2 (73400320/7864320), GigabitEthernet1/0/4
P 10.30.80.0/24, 1 successors, FD is 2048000
via 192.168.3.2 (2048000/1392640), GigabitEthernet1/0/3
via 192.168.1.2 (66928640/1392640), GigabitEthernet1/0/4
P 172.16.2.0/30, 1 successors, FD is 2621440
via 192.168.3.2 (2621440/1966080), GigabitEthernet1/0/3
via 192.168.1.2 (66846720/1310720), GigabitEthernet1/0/4
P 10.10.30.0/24, 1 successors, FD is 163840
via Connected, Loopback30
P 10.20.60.0/24, 1 successors, FD is 8519680
via 192.168.3.2 (8519680/7864320), GigabitEthernet1/0/3
via 192.168.1.2 (73400320/7864320), GigabitEthernet1/0/4
P 192.168.1.0/30, 1 successors, FD is 66191360
via Connected, GigabitEthernet1/0/4
via 192.168.3.2 (3276800/2621440), GigabitEthernet1/0/3
P 10.20.40.0/24, 1 successors, FD is 8519680
via 192.168.3.2 (8519680/7864320), GigabitEthernet1/0/3
via 192.168.1.2 (73400320/7864320), GigabitEthernet1/0/4
P 10.10.20.0/24, 1 successors, FD is 163840
via Connected, Loopback20
P 10.30.90.0/24, 1 successors, FD is 2048000
via 192.168.3.2 (2048000/1392640), GigabitEthernet1/0/3
via 192.168.1.2 (66928640/1392640), GigabitEthernet1/0/4
P 172.16.4.0/30, 1 successors, FD is 1966080
via 192.168.3.2 (1966080/1310720), GigabitEthernet1/0/3
P 10.10.10.0/24, 1 successors, FD is 163840
via Connected, Loopback10
R1#show ip eigrp topology 10.20.40.0/24
EIGRP-IPv4 VR(LAB) Topology Entry for AS(100)/ID(192.168.3.1) for 10.20.40.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 8519680, RIB is 66560
Descriptor Blocks:
192.168.3.2 (GigabitEthernet1/0/3), from 192.168.3.2, Send flag is 0x0
Composite metric is (8519680/7864320), route is External
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 120000000 picoseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 172.16.6.1
External data:
AS number of route is 0
External protocol is IS-IS, external metric is 20
Administrator tag is 0 (0x00000000)
192.168.1.2 (GigabitEthernet1/0/4), from 192.168.1.2, Send flag is 0x0
Composite metric is (73400320/7864320), route is External
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 1110000000 picoseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 172.16.6.1
External data:
AS number of route is 0
External protocol is IS-IS, external metric is 20
Administrator tag is 0 (0x00000000)
R1#traceroute 10.20.40.1 source loopback10
Type escape sequence to abort.
Tracing the route to 10.20.40.1
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.3.2 1 msec 0 msec 0 msec
2 172.16.4.2 0 msec 0 msec 1 msec
3 172.16.6.2 1 msec 1 msec *
R1#show ip cef 10.20.40.1
10.20.40.0/24
nexthop 192.168.3.2 GigabitEthernet1/0/3
Modifying the delay can be a useful tool to control the traffic flow and change the overall network behavior. Delay is a cumulative value that grows based on the delay of each segment within the path. It is also important to note that given that bandwidth can be used by other protocols calculations, changes to the interface delay parameter is a preferred method. The changes to delay, however, are useful only in scenarios where one path is preferred over another for all the routes being received.
Note: Be careful when you select the new delay value, you do not want to increase the delay to a point where EIGRP no longer views those routes as a feasible successor.
Scenario 2: Influence Path Selection with the use an Offset-List
In this scenario, the interesting traffic or prefix that needs to be manipulated is selected with the use of an ACL. An ACL is used to match these prefixes and, for this example, next configuration is added to manipulate the traffic destined to subnets 10.20.60.0/24 and 10.30.90.0/24.
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#access-list 20 permit 10.20.60.0 0.0.0.255
R1(config)#access-list 30 permit 10.30.90.0 0.0.0.255
!
R1#show access-lists 20
Standard IP access list 20
10 permit 10.20.60.0, wildcard bits 0.0.0.255
R1#show access-lists 30
Standard IP access list 30
10 permit 10.30.90.0, wildcard bits 0.0.0.255
The objective is to modify the metric of the specific prefixes but without affecting all other EIGRP traffic. This example uses an offset-list to add an offset to the metric of the selected prefixes (10.20.60.0/24 and 10.30.90.0/24) in the inbound direction of R1.
The idea is to prefer the path through R2 via interface Gi1/0/4 when reaching the subnet 10.20.60.0/24 (from R1) and prefer the path through R3 via interface Gi1/0/3 when reaching the subnet 10.30.90.0/24 (from R1).
The configuration make use of the command offset-list {ACL name|ACL number} {in|out} <offset> <interface> as shown next:
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router eigrp LAB
R1(config-router)#address-family ipv4 unicast autonomous-system 100
R1(config-router-af)#topology base
R1(config-router-af-topology)#offset-list 20 in 200 GigabitEthernet1/0/3
R1(config-router-af-topology)#endThe results of the configuration can be verified by checking the RIB, the Forwarding Information Base (FIB) and EIGRP Topology table. In the next outputs, it can be seen that the offset applied to the interface Gi1/0/3 affected the metric of this specific prefix, in other words, making this path less desirable:
R1#show ip route 10.20.60.0
Routing entry for 10.20.60.0/24
Known via "eigrp 100", distance 170, metric 66560, precedence routine (0), type external
Redistributing via eigrp 100
Last update from 192.168.1.2 on GigabitEthernet1/0/4, 00:01:31 ago
Routing Descriptor Blocks:
* 192.168.1.2, from 192.168.1.2, 00:01:31 ago, via GigabitEthernet1/0/4
Route metric is 66560, traffic share count is 1
Total delay is 120 microseconds, minimum bandwidth is 1000000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
R1#show ip cef 10.20.60.0
10.20.60.0/24
nexthop 192.168.1.2 GigabitEthernet1/0/4
R1#show ip eigrp topology 10.20.60.0/24
EIGRP-IPv4 VR(LAB) Topology Entry for AS(100)/ID(192.168.3.1) for 10.20.60.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 8519680, RIB is 66560
Descriptor Blocks:
192.168.1.2 (GigabitEthernet1/0/4), from 192.168.1.2, Send flag is 0x0
Composite metric is (8519680/7864320), route is External
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 120000000 picoseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 172.16.6.1
External data:
AS number of route is 0
External protocol is IS-IS, external metric is 20
Administrator tag is 0 (0x00000000)
192.168.3.2 (GigabitEthernet1/0/3), from 192.168.3.2, Send flag is 0x0
Composite metric is (8519880/7864520), route is External
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 120003052 picoseconds <---
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 172.16.6.1
External data:
AS number of route is 0
External protocol is IS-IS, external metric is 20
Administrator tag is 0 (0x00000000)
A similar process is completed for prefix 10.30.90.0/24, the offset-list is added now to prefer R3 path through interface Gi1/0/3 (but applying the offset to Gi1/0/4). In a similar way, by reviewing the RIB, FIB and EIGRP Topology, it can be seen that the preferred path for the selected prefix is through R3:
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router eigrp LAB
R1(config-router)#address-family ipv4 unicast autonomous-system 100
R1(config-router-af)#topology base
R1(config-router-af-topology)#offset-list 30 in 300 gigabitEthernet 1/0/4
R1(config-router-af-topology)#end
R1#show ip route 10.30.90.0
Routing entry for 10.30.90.0/24
Known via "eigrp 100", distance 90, metric 16000, precedence routine (0), type internal
Redistributing via eigrp 100
Last update from 192.168.3.2 on GigabitEthernet1/0/3, 00:00:25 ago
Routing Descriptor Blocks:
* 192.168.3.2, from 192.168.3.2, 00:00:25 ago, via GigabitEthernet1/0/3
Route metric is 16000, traffic share count is 1
Total delay is 21 microseconds, minimum bandwidth is 1000000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
R1#show ip cef 10.30.90.0
10.30.90.0/24
nexthop 192.168.3.2 GigabitEthernet1/0/3
R1#show ip eigrp topology 10.30.90.0/24
EIGRP-IPv4 VR(LAB) Topology Entry for AS(100)/ID(192.168.3.1) for 10.30.90.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2048000, RIB is 16000
Descriptor Blocks:
192.168.3.2 (GigabitEthernet1/0/3), from 192.168.3.2, Send flag is 0x0
Composite metric is (2048000/1392640), route is Internal
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 21250000 picoseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 172.16.6.1
192.168.1.2 (GigabitEthernet1/0/4), from 192.168.1.2, Send flag is 0x0
Composite metric is (2048300/1392940), route is Internal
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 21254578 picoseconds <---
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 172.16.6.1 Looking at show ip route eigrp command you can confirm that the configuration is successful and only the specific prefixes were affected and all other routes remained intact. Also running a traceroute confirms that the traffic is taking the desired path:
R1#show ip route eigrp Codes: L - local, 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, m - OMP n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA 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 H - NHRP, G - NHRP registered, g - NHRP registration summary o - ODR, P - periodic downloaded static route, l - LISP a - application route + - replicated route, % - next hop override, p - overrides from PfR & - replicated local route overrides by connected Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 12 subnets, 2 masks D EX 10.20.40.0/24 [170/66560] via 192.168.3.2, 00:22:32, GigabitEthernet1/0/3 [170/66560] via 192.168.1.2, 00:22:32, GigabitEthernet1/0/4 D EX 10.20.50.0/24 [170/66560] via 192.168.3.2, 00:22:32, GigabitEthernet1/0/3 [170/66560] via 192.168.1.2, 00:22:32, GigabitEthernet1/0/4 D EX 10.20.60.0/24 [170/66560] via 192.168.1.2, 00:16:54, GigabitEthernet1/0/4 D 10.30.70.0/24 [90/16000] via 192.168.3.2, 00:22:32, GigabitEthernet1/0/3 [90/16000] via 192.168.1.2, 00:22:32, GigabitEthernet1/0/4 D 10.30.80.0/24 [90/16000] via 192.168.3.2, 00:22:32, GigabitEthernet1/0/3 [90/16000] via 192.168.1.2, 00:22:32, GigabitEthernet1/0/4 D 10.30.90.0/24 [90/16000] via 192.168.3.2, 00:04:56, GigabitEthernet1/0/3 172.16.0.0/30 is subnetted, 2 subnets D 172.16.2.0 [90/15360] via 192.168.1.2, 00:22:32, GigabitEthernet1/0/4 D 172.16.4.0 [90/15360] via 192.168.3.2, 00:22:32, GigabitEthernet1/0/3R1#traceroute 10.20.60.1 source loop10 Type escape sequence to abort. Tracing the route to 10.20.60.1 VRF info: (vrf in name/id, vrf out name/id) 1 192.168.1.2 1 msec 1 msec 0 msec <--- R2 2 172.16.2.2 1 msec 1 msec 0 msec 3 172.16.6.2 1 msec 1 msec * R1#traceroute 10.30.90.1 source loop10 Type escape sequence to abort. Tracing the route to 10.30.90.1 VRF info: (vrf in name/id, vrf out name/id) 1 192.168.3.2 0 msec 1 msec 0 msec <--- R3 2 172.16.4.2 1 msec 1 msec *
Scenario 3: Influence Path Selection with Summarization
In this scenario route summarization is used to prefer one path over the other. EIGRP has the flexibility to configure a summary route per interface, and in this example a summary route is configured on R4 to summarize the 10.30.x.x prefixes and another one for the 10.20.x.x prefixes. The idea is that R4 announces the summary route 10.30.0.0/16 over interface GigabitEthernet1/0/1 and summary route 10.20.0.0/16 over interface GigabitEthernet1/0/2, and with this configuration traffic is influenced by the longest match preference. This causes that traffic source from R1 and destined to 10.30.x.x subnets selects the path through R3 and traffic destined to subnets 10.20.x.x selects the path through R2. The configuration is shown next:
R4#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#router eigrp LAB
R4(config-router)#address-family ipv4 unicast autonomous-system 100
R4(config-router-af)#af-interface gigabitEthernet 1/0/1
R4(config-router-af-interface)#summary-address 10.30.0.0/16
R4(config-router-af-interface)#exit
R4(config-router-af)#af-interface gigabitEthernet 1/0/2
R4(config-router-af-interface)#summary-address 10.20.0.0/16
R4(config-router-af-interface)#end
R4#Now, by checking the routing table from R1, it can be verified that there is a summary route for 10.20.0.0/16 being learned through interface GigabitEthernet1/0/3 (connected to R3) and a summary route 10.30.0.0/16 learned over GigabitEthernet1/0/4 (connected to R2). The result of this configuration is that traffic with a destination of 10.20.60.1 is routed trough R2 and traffic with destination of 10.30.90.1 is routed though R3. The reason is that R1 prefers the longest match prefixes that are still learned through the other interface, and can be confirmed via FIB and traceroute outputs:
R1#show ip route eigrp
Codes: L - local, 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, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
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
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 14 subnets, 3 masks
D 10.20.0.0/16
[90/66560] via 192.168.3.2, 00:00:16, GigabitEthernet1/0/3
D EX 10.20.40.0/24
[170/66560] via 192.168.1.2, 00:00:16, GigabitEthernet1/0/4
D EX 10.20.50.0/24
[170/66560] via 192.168.1.2, 00:00:16, GigabitEthernet1/0/4
D EX 10.20.60.0/24
[170/66560] via 192.168.1.2, 00:00:16, GigabitEthernet1/0/4
D 10.30.0.0/16
[90/16000] via 192.168.1.2, 00:00:44, GigabitEthernet1/0/4
D 10.30.70.0/24
[90/16000] via 192.168.3.2, 00:00:44, GigabitEthernet1/0/3
D 10.30.80.0/24
[90/16000] via 192.168.3.2, 00:00:44, GigabitEthernet1/0/3
D 10.30.90.0/24
[90/16000] via 192.168.3.2, 00:00:44, GigabitEthernet1/0/3
172.16.0.0/30 is subnetted, 2 subnets
D 172.16.2.0 [90/15360] via 192.168.1.2, 02:42:44, GigabitEthernet1/0/4
D 172.16.4.0 [90/15360] via 192.168.3.2, 02:42:44, GigabitEthernet1/0/3
R1#show ip route 10.20.0.0
Routing entry for 10.20.0.0/16
Known via "eigrp 100", distance 90, metric 66560, precedence routine (0), type internal
Redistributing via eigrp 100
Last update from 192.168.3.2 on GigabitEthernet1/0/3, 00:12:07 ago
Routing Descriptor Blocks:
* 192.168.3.2, from 192.168.3.2, 00:12:07 ago, via GigabitEthernet1/0/3
Route metric is 66560, traffic share count is 1
Total delay is 120 microseconds, minimum bandwidth is 1000000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
R1#show ip route 10.30.0.0
Routing entry for 10.30.0.0/16
Known via "eigrp 100", distance 90, metric 16000, precedence routine (0), type internal
Redistributing via eigrp 100
Last update from 192.168.1.2 on GigabitEthernet1/0/4, 00:12:50 ago
Routing Descriptor Blocks:
* 192.168.1.2, from 192.168.1.2, 00:12:50 ago, via GigabitEthernet1/0/4
Route metric is 16000, traffic share count is 1
Total delay is 21 microseconds, minimum bandwidth is 1000000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
R1#show ip cef exact-route 10.10.10.1 10.20.60.1
10.10.10.1 -> 10.20.60.1 =>IP adj out of GigabitEthernet1/0/4, addr 192.168.1.2
R1#traceroute 10.20.60.1 source loop10
Type escape sequence to abort.
Tracing the route to 10.20.60.1
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.1.2 1 msec 1 msec 0 msec <--- R2
2 172.16.2.2 1 msec 1 msec 0 msec
3 172.16.6.2 1 msec 1 msec *
R1#show ip cef exact-route 10.10.10.1 10.30.90.1
10.10.10.1 -> 10.30.90.1 =>IP adj out of GigabitEthernet1/0/3, addr 192.168.3.2
R1#traceroute 10.30.90.1 source loop10
Type escape sequence to abort.
Tracing the route to 10.30.90.1
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.3.2 1 msec 0 msec 1 msec <--- R3
2 172.16.4.2 0 msec 1 msec *
Scenario 4: Influence Path Selection with the use of Leak Maps
Using leak-maps during the advertisement of summary routes provides a flexible mechanism to announce more specific routes selectively and then take advantage from the longest match to prefer a desired path.
In this example, a summary route 10.0.0.0/8 is advertised from R4 on both interfaces (Gi1/0/1 and Gi1/0/2). Let's take a look at the configuration:
R4#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#router eigrp LAB
R4(config-router)#address-family ipv4 unicast autonomous-system 100
R4(config-router-af)#af-interface GigabitEthernet1/0/1
R4(config-router-af-interface)#summary-address 10.0.0.0 255.0.0.0
R4(config-router-af-interface)#exit
R4(config-router-af)#af-interface GigabitEthernet1/0/2
R4(config-router-af-interface)#summary-address 10.0.0.0 255.0.0.0
R4(config-router-af-interface)#endThe previous configuration is reflected in R1 routing table as shown next, this however, is still load balancing the traffic across the two paths from R1:
R1#show ip route eigrp
Codes: L - local, 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, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
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
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 7 subnets, 3 masks
D 10.0.0.0/8 [90/16000] via 192.168.3.2, 00:04:16, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 00:04:16, GigabitEthernet1/0/4
172.16.0.0/30 is subnetted, 2 subnets
D 172.16.2.0 [90/15360] via 192.168.1.2, 03:50:08, GigabitEthernet1/0/4
D 172.16.4.0 [90/15360] via 192.168.3.2, 03:50:08, GigabitEthernet1/0/3However, traffic from R1 to subnet 10.20.60.0/24 and 10.30.70.0/24 needs to be preferred over GigabitEthernet1/0/4 (connected to R2). To accomplish this result, a leak-map can be configured on R4 to leak the more specific prefixes but keeping the summarization in place.
R4#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#ip prefix-list LEAKED-PREFIXES permit 10.20.60.0/24
R4(config)#ip prefix-list LEAKED-PREFIXES permit 10.30.70.0/24
R4(config)#route-map LEAKED-PREFIXES
R4(config-route-map)#match ip address prefix-list LEAKED-PREFIXES
R4(config-route-map)#exit
R4(config)#router eigrp LAB
R4(config-router)#address-family ipv4 unicast autonomous-system 100
R4(config-router-af)#af-interface GigabitEthernet1/0/1
R4(config-router-af-interface)#summary-address 10.0.0.0 255.0.0.0 leak-map LEAKED-PREFIXES
R4(config-router-af-interface)#endAfter applying the previous configuration, R1 starts seeing a more specific entry for 10.20.60.0/24 and 10.30.70.0/24 that are now being learned through interface GigabitEthernet1/0/4, as shown next:
R1#show ip route eigrp
Codes: L - local, 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, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
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
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 9 subnets, 3 masks
D 10.0.0.0/8 [90/16000] via 192.168.3.2, 01:26:41, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 01:26:41, GigabitEthernet1/0/4
D EX 10.20.60.0/24
[170/66560] via 192.168.1.2, 00:01:29, GigabitEthernet1/0/4
D 10.30.70.0/24
[90/16000] via 192.168.1.2, 00:01:29, GigabitEthernet1/0/4
172.16.0.0/30 is subnetted, 2 subnets
D 172.16.2.0 [90/15360] via 192.168.1.2, 05:12:33, GigabitEthernet1/0/4
D 172.16.4.0 [90/15360] via 192.168.3.2, 05:12:33, GigabitEthernet1/0/3
R1#show ip cef exact-route 10.10.10.1 10.20.60.1
10.10.10.1 -> 10.20.60.1 =>IP adj out of GigabitEthernet1/0/4, addr 192.168.1.2
R1#show ip cef exact-route 10.10.10.1 10.30.70.1
10.10.10.1 -> 10.30.70.1 =>IP adj out of GigabitEthernet1/0/4, addr 192.168.1.2Scenario 5: Influence Path Selection by modifying the Administrative Distance (AD) of a prefix
The idea of this example is to modify the AD for the prefix 10.30.90.0/24, therefore, the traffic destined to it can be routed via R3.
Note: This approach is another resource to influence EIGRP, however, this is less preferred than the use of an Offset-List. Be careful if you are using multiple routing protocols on the same device as this method can affect them as well.
Note: This method only affects Internal EIGRP routes, the configuration does not modify the AD of External EIGRP routes.
Notice that R1 is learning the route 10.30.90.0/24 through R2 (192.168.1.2) and R3 (192.168.3.2) with the same metric:
R1#show ip route eigrp
Codes: L - local, 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, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
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
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 12 subnets, 2 masks
D EX 10.20.40.0/24
[170/66560] via 192.168.3.2, 00:00:26, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 00:00:26, GigabitEthernet1/0/4
D EX 10.20.50.0/24
[170/66560] via 192.168.3.2, 00:00:26, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 00:00:26, GigabitEthernet1/0/4
D EX 10.20.60.0/24
[170/66560] via 192.168.3.2, 00:00:26, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 00:00:26, GigabitEthernet1/0/4
D 10.30.70.0/24
[90/16000] via 192.168.3.2, 00:00:26, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 00:00:26, GigabitEthernet1/0/4
D 10.30.80.0/24
[90/16000] via 192.168.3.2, 00:00:26, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 00:00:26, GigabitEthernet1/0/4
D 10.30.90.0/24
[90/16000] via 192.168.3.2, 00:00:26, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 00:00:26, GigabitEthernet1/0/4
172.16.0.0/30 is subnetted, 2 subnets
D 172.16.2.0 [90/15360] via 192.168.1.2, 00:00:26, GigabitEthernet1/0/4
D 172.16.4.0 [90/15360] via 192.168.3.2, 00:00:26, GigabitEthernet1/0/3To accomplish the change, an ACL needs to be configured that is used to match the desired subnet, afterwards the AD of the prefix can be modified by also specifying the advertising neighbor with the use of the command distance <route AD> <IP Source address> <Wildcard bits> <ACL>.
In this example, in order to prefer the advertisement from R3, a lower AD value is used (85), the IP address of R3 EIGRP neighbor (192.168.3.2) is added with a wild card of 0.0.0.0 and then the ACL to match the prefix is added:
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#access-list 30 permit 10.30.90.0 0.0.0.255
R1(config)#router eigrp LAB
R1(config-router)#address-family ipv4 unicast autonomous-system 100
R1(config-router-af)#topology base
R1(config-router-af-topology)#distance 85 192.168.3.2 0.0.0.0 30
R1(config-router-af-topology)#endThe result can be seen in the RIB and FIB output from R1, where the routing entry for 10.30.90.0/24 has its AD changed to 85 and the preferred EIGRP neighbor is R3 (192.168.3.2):
R1#show ip route eigrp
Codes: L - local, 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, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
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
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 12 subnets, 2 masks
D EX 10.20.40.0/24
[170/66560] via 192.168.3.2, 00:00:14, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 00:00:14, GigabitEthernet1/0/4
D EX 10.20.50.0/24
[170/66560] via 192.168.3.2, 00:00:14, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 00:00:14, GigabitEthernet1/0/4
D EX 10.20.60.0/24
[170/66560] via 192.168.3.2, 00:00:14, GigabitEthernet1/0/3
[170/66560] via 192.168.1.2, 00:00:14, GigabitEthernet1/0/4
D 10.30.70.0/24
[90/16000] via 192.168.3.2, 00:00:14, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 00:00:14, GigabitEthernet1/0/4
D 10.30.80.0/24
[90/16000] via 192.168.3.2, 00:00:14, GigabitEthernet1/0/3
[90/16000] via 192.168.1.2, 00:00:14, GigabitEthernet1/0/4
D 10.30.90.0/24
[85/16000] via 192.168.3.2, 00:00:14, GigabitEthernet1/0/3
172.16.0.0/30 is subnetted, 2 subnets
D 172.16.2.0 [90/15360] via 192.168.1.2, 00:00:14, GigabitEthernet1/0/4
D 172.16.4.0 [90/15360] via 192.168.3.2, 00:00:14, GigabitEthernet1/0/3
R1#show ip route 10.30.90.0
Routing entry for 10.30.90.0/24
Known via "eigrp 100", distance 85, metric 16000, precedence routine (0), type internal
Redistributing via eigrp 100
Last update from 192.168.3.2 on GigabitEthernet1/0/3, 00:00:31 ago
Routing Descriptor Blocks:
* 192.168.3.2, from 192.168.3.2, 00:00:31 ago, via GigabitEthernet1/0/3
Route metric is 16000, traffic share count is 1
Total delay is 21 microseconds, minimum bandwidth is 1000000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
R1#show ip cef 10.30.90.0
10.30.90.0/24
nexthop 192.168.3.2 GigabitEthernet1/0/3Scenario 6: Influence Path Selection with Route Filtering
In this example, the idea is to selectively influence the path selection by filtering some routes or prefixes coming into R1.
R1 must prefer R2 path when the destination is any of the next subnets 10.30.70.0/24, 10.30.80.0/24 and 10.20.40.0/24. When the destination is subnet 10.30.90.0/24, 10.20.50.0/24 and 10.20.60.0/24 R1 must prefer R3 path.
To accomplish this, a prefix-list is used to match the desired routes and a distribute-list is configured under the EIGRP process to apply the route filter in an inbound direction, as shown next:
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#ip prefix-list R2-Preferred permit 10.30.70.0/24
R1(config)#ip prefix-list R2-Preferred permit 10.30.80.0/24
R1(config)#ip prefix-list R2-Preferred permit 10.20.40.0/24
R1(config)#
R1(config)#ip prefix-list R3-Preferred permit 10.30.90.0/24
R1(config)#ip prefix-list R3-Preferred permit 10.20.50.0/24
R1(config)#ip prefix-list R3-Preferred permit 10.20.60.0/24
R1(config)#router eigrp LAB
R1(config-router)#address-family ipv4 unicast autonomous-system 100
R1(config-router-af)#topology base
R1(config-router-af-topology)#distribute-list prefix R2-Preferred in GigabitEthernet1/0/4
R1(config-router-af-topology)#distribute-list prefix R3-Preferred in GigabitEthernet1/0/3
R1(config-router-af-topology)#endNote: Notice that the "prefix" option is needed when applying the distribute-list as an ip prefix-list is being used to match the desired routes
Note: One of the main differences between methods such as the use of an offset-list, is that the distribute-list prevents the not permitted prefixes from being inserted into the RIB and the EIGRP Topology table.
The result is that R1 routing table shows the desired path selection:
R1#show ip route eigrp
Codes: L - local, 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, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
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
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 12 subnets, 2 masks
D EX 10.20.40.0/24
[170/66560] via 192.168.1.2, 00:00:12, GigabitEthernet1/0/4 <--- R2
D EX 10.20.50.0/24
[170/66560] via 192.168.3.2, 00:00:24, GigabitEthernet1/0/3 <--- R3
D EX 10.20.60.0/24
[170/66560] via 192.168.3.2, 00:00:24, GigabitEthernet1/0/3
D 10.30.70.0/24
[90/16000] via 192.168.1.2, 00:00:12, GigabitEthernet1/0/4
D 10.30.80.0/24
[90/16000] via 192.168.1.2, 00:00:12, GigabitEthernet1/0/4
D 10.30.90.0/24
[90/16000] via 192.168.3.2, 00:00:24, GigabitEthernet1/0/3Related Information
Revision History
| Revision | Publish Date | Comments |
|---|---|---|
1.0 |
12-Jan-2024 |
Initial Release |
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