3.5.d General operations

  • 3.5.d [ii] Topology table, update, query, active, passive
  • 3.5.d [iii] Stuck in active

Simulataneous Router Discovery/Route Exchange

 

Router comes up and sends hellos

Reply from neighbor includes update

Acks are sent

Update process occurs in the opposite direction

 

eigrp

 

EIGRP DUAL

 

The lowest cost route is calculated by adding the cost between the next hop router and the destination (Reported Distance, RD), and the cost between the local router and the next hop. This sum is the Feasible Distance (FD).

feasibledistance

 

The successor is the next hop router to the destination that the local router has chosen. Multiple successors may exist if they provide equal cost paths.

 

If a backup path to the destination exists it may become a feasible successor. To qualify, this next hop router must report an RD that is less than the FD of the successor.

feasiblesuccessor2

In the above diagram, the successor is the route through the NextHop because the feasible distance is 1 + 1, the least cost path to the destination. The route through the Backup becomes a feasible successor because its RD of 1 is less than the FD of the successor, thus meeting the Feasibility Condition, RD<FD. In this case, the least cost path (LocalRouter to NextHop to Destination) will be the only route represented in the IP Routing Table. The Backup path however, will be represented as a Feasible Successor in the EIGRP Topology table.

 

Note: in the above diagram the cost values are artificial representations of a dual metric for simplicity. The actual calculated metric is much more complex. For instance, all things being equal the topology below creates 2 equal cost paths to the destination router’s loopback, 4.4.4.4/24

 

LocalRouter#sh ip route eigrp

4.0.0.0/24 is subnetted, 1 subnets

D 4.4.4.0 [90/154112] via 192.168.13.3, 00:28:28, FastEthernet1/0

[90/154112] via 192.168.12.2, 00:28:28, FastEthernet0/0

D 192.168.24.0/24 [90/26112] via 192.168.12.2, 00:28:28, FastEthernet0/0

D 192.168.34.0/24 [90/26112] via 192.168.13.3, 00:28:28, FastEthernet1/0

90 is the AD of Eigrp, and 154112 is the calculated metric (equal cost paths in this case, therefore both reside in the IP Routing table and will be load balanced)

eigrp_net

LocalRouter#sh ip eigrp topo

(truncated)

 

P 4.4.4.0/24, 2 successors, FD is 154112

via 192.168.13.3 (154112/153856), FastEthernet1/0

via 192.168.12.2 (154112/153856), FastEthernet0/0

 

To create the scenario in the earlier topology use the delay command in interface configuration mode (delay is the recommended method for manipulation).

 

LocalRouter(config-if)#int f1/0 (connection to Backup in the diagram)

LocalRouter(config-if)#delay 3

 

LocalRouter#sh ip route eigrp

4.0.0.0/24 is subnetted, 1 subnets

D 4.4.4.0 [90/154112] via 192.168.12.2, 00:02:04, FastEthernet0/0

D 192.168.24.0/24 [90/26112] via 192.168.12.2, 00:02:04, FastEthernet0/0

D 192.168.34.0/24 [90/26368] via 192.168.12.2, 00:02:04, FastEthernet0/0

 

 

LocalRouter#sh ip eigrp topo

IP-EIGRP Topology Table for AS(1)/ID(192.168.12.1)

 

Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,

r – reply Status, s – sia Status

 

P 4.4.4.0/24, 1 successors, FD is 154112

via 192.168.12.2 (154112/153856), FastEthernet0/0

via 192.168.13.3 (154624/153856), FastEthernet1/0

 

Note: the feasibility condition has been met RD<FD.

 

All of this means that if the successor route goes down, the feasible successor will assume responsibilty for the destination without a query, it simply gets promoted to the IP Routing Table.

 

LocalRouter(config-if)#shut

LocalRouter(config-if)#

*Mar 1 03:11:24.311: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.12.2 (FastEthernet0/0) is down: interface down

LocalRouter(config-if)#

*Mar 1 03:11:26.295: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down

*Mar 1 03:11:27.295: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down

LocalRouter(config-if)#do sh ip route eigrp

D 192.168.12.0/24 [90/27136] via 192.168.13.3, 00:00:15, FastEthernet1/0

4.0.0.0/24 is subnetted, 1 subnets

D 4.4.4.0 [90/154624] via 192.168.13.3, 00:00:15, FastEthernet1/0

D 192.168.24.0/24 [90/26880] via 192.168.13.3, 00:00:15, FastEthernet1/0

D 192.168.34.0/24 [90/26624] via 192.168.13.3, 00:00:15, FastEthernet1/0

 

If the interface recovers, that route with its better metric returns to the IP Routing table as successor.

 

LocalRouter(config-if)#no shut

LocalRouter(config-if)#

*Mar 1 03:13:07.139: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

LocalRouter(config-if)#

*Mar 1 03:13:07.727: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.12.2 (FastEthernet0/0) is up: new adjacency

*Mar 1 03:13:08.139: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

LocalRouter(config-if)#do sh ip route eigrp

4.0.0.0/24 is subnetted, 1 subnets

D 4.4.4.0 [90/154112] via 192.168.12.2, 00:00:04, FastEthernet0/0

D 192.168.24.0/24 [90/26112] via 192.168.12.2, 00:00:04, FastEthernet0/0

D 192.168.34.0/24 [90/26368] via 192.168.12.2, 00:00:04, FastEthernet0/0

 

EIGRP Queries

 

Passive is the stable condition for a route, as below.

 

LocalRouter#sh ip eigrp topo

IP-EIGRP Topology Table for AS(1)/ID(192.168.12.1)

 

Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,

r – reply Status, s – sia Status

 

P 4.4.4.0/24, 1 successors, FD is 154112

 

If there is a change in the topology as was demonstrated above, EIGRP will check for the presence of a feasible successor. If one is not found, a query will be initiated. This is known as going active. When a route becomes active the router multicasts queries to its neighbors for a valid route to the subnet. A neighbor receiving such a query

 

A note on auto-summary:

Command History

 

Release

Modification

10.0

This command was introduced.

12.2(8)T

The command default behavior changed to disabled.

 

Remember to check the version of EIGRP in use. Prior to 12.2(8)T the no auto-summary command was necessary for EIGRP to be used classlessly (VLSM).