Unlike OSPF which uses the Router ID (RID) to identify the OSPF neighbor, the primary use of the EIGRP RID is to prevent routing loops. The RID is used to identify the originating router for external routes. If an external route is received with the same RID as the local router, the route is discarded. This feature is designed to reduce the possibility of routing loops in networks were route redistribution is being performed on more than one ASBR.
NOTE: Route redistribution is a core ROUTE topic that is described in detail later in this guide.
When determining the Router ID, EIGRP will select the highest IP address that is configured on the router. If Loopback interfaces are also configured on the router, those interfaces are preferred, since a Loopback interface is the most stable interface that can exist on a router. The RID will never change unless the EIGRP process is removed, if the router ID is manually configured. The RID will always be listed in the EIGRP Topology Table as illustrated below:
| R1#show ip eigrp topology IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply, r – reply Status, s – sia Status P 10.2.2.0/24, 1 successors, FD is 128256 |
NOTE: It is important to understand that the RID and the neighbor ID will typically be different, although this may be the case in routers with a single interface, for example.
The EIGRP RID is configured using the eigrp router-id [address] router configuration command. When this command is entered, the RID is automatically updated with the new address in the EIGRP Topology Table. To demonstrate this point, let us begin by looking at the current RID on the router as stated in the Topology Table:
| R1#show ip eigrp topology IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply, r – reply Status, s – sia Status … [Truncated Output] … |
A RID of 1.1.1.1 is now configured on the router as follows:
| R1(config)#router eigrp 150 R1(config-router)#eigrp router-id 1.1.1.1 R1(config-router)# *Mar 1 05:50:13.642: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 150: Neighbor 150.1.1.2 (Serial0/0) is down: route configuration changed *Mar 1 05:50:16.014: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 150: Neighbor 150.1.1.2 (Serial0/0) is up: new adjacency |
Following the change, the EIGRP neighbor relationship is reset and the new RID is immediately reflected in the EIGRP Topology Table:
| R1#show ip eigrp topology IP-EIGRP Topology Table for AS(150)/ID(1.1.1.1)Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply, r – reply Status, s – sia Status … [Truncated Output] … |
When configuring the EIGRP Router ID, the following should be remembered:
- You cannot configure the RID as 0.0.0.0
- You cannot configure the RID as 255.255.255.255
All external routes that are originated by the router now contain the EIGRP RID. This can be verified in the following output of neighbor router R2:
| R2#show ip eigrp topology 192.168.254.0/24 IP-EIGRP (AS 150): Topology entry for 192.168.254.0/24 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 7289856 Routing Descriptor Blocks: 150.1.1.1 (Serial0/0), from 150.1.1.1, Send flag is 0x0 Composite metric is (7289856/6777856), Route is External Vector metric: Minimum bandwidth is 1544 Kbit Total delay is 220000 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 External data: Originating router is 1.1.1.1 AS number of route is 0 External protocol is Connected, external metric is 0 Administrator tag is 0 (0x00000000) |
The RID is not included for internal EIGRP routes as illustrated in the following output:
| R2#show ip eigrp topology 10.3.3.0/24 IP-EIGRP (AS 150): Topology entry for 10.3.3.0/24 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2297856 Routing Descriptor Blocks: 150.1.1.1 (Serial0/0), from 150.1.1.1, Send flag is 0x0 Composite metric is (2297856/128256), Route is Internal Vector metric: Minimum bandwidth is 1544 Kbit Total delay is 25000 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 |