IT Certification and Training Blog

EIGRP Timer Talk

Posted by Kelson Lawrence on Feb 8, 2011 10:25:00 AM

By Andrew Messier

describe the image The topology for this study consists of two routers connected on a LAN. FastEthernet 0/0 on Router1 connects to FastEthernet 0/0 on Router2. Full connectivity exists between both routers, and Enhanced Interior Gateway Routing Protocol (EIGRP) using Autonomous System (AS) 100 is configured to run on both FastEthernet ports. Below is sample configuration for this setup:

Router1#show running-config
hostname Router1
interface FastEthernet0/0  
 ip address 10.0.0.1 255.255.255.0
router eigrp 100  
 network 10.0.0.0

Router2#show running-config
hostname Router2
interface FastEthernet0/0  
 ip address 10.0.0.2 255.255.255.0
router eigrp 100  
 network 10.0.0.0

Device’s configured to run EIGRP require adjacencies, or neighbor relationships, to exchange EIGRP topology information. Each device sends EIGRP hello packets out every interface configured to run the routing protocol. Hello packets are sent as multicast traffic with the destination address equal to 224.0.0.10. When a device receives a hello packet on an interface configured to run EIGRP, it will form an adjacency with the sending device. A router will by default display a message to the console when it forms a new adjacency. Sample output from Router2 is shown below:

*Mar  1 00:14:25.487: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.0.0.1 (FastEthernet0/0) is up: new adjacency

A device will continue to share topology information and maintain the neighbor relationship as long as it continues to receive hello packets within a configured time interval. Cisco calls this time interval the hold time. The hello-interval is the time that each device waits before sending out the next hello packet. Open Shortest Path First (OSPF) requires that two devices be configured with the same hello-interval value to form an adjacency; however, this is not true for EIGRP. Unlike OSPF, EIGRP does not share its configured hello-time with its neighbor. A device advertises its own hold-time in each hello packet. Sample capture for a hello packet from Router1 is shown below:

Frame 318 (74 bytes on wire, 74 bytes captured)
Ethernet II, Src: cc:00:0e:78:00:00 (cc:00:0e:78:00:00), Dst: IPv4mcast_00:00:0a (01:00:5e:00:00:0a)
Internet Protocol, Src: 10.0.0.1 (10.0.0.1), Dst: 224.0.0.10 (224.0.0.10)
Cisco EIGRP     
    Version = 2     
    Opcode = 5 (Hello)     
    Checksum = 0xee69     
    Flags = 0x00000000     
    Sequence  = 0     
    Acknowledge = 0         
    Autonomous System : 100     
    EIGRP Parameters         
        Type = 0x0001 (EIGRP Parameters)         
        Size = 12 bytes         
        K1 = 1         
        K2 = 0         
        K3 = 1         
        K4 = 0         
        K5 = 0         
        Reserved         
        Hold Time = 15     
    Software Version: IOS = 12.3, EIGRP = 1.2         
        Type = 0x0004 (Software Version)         
        Size = 8 bytes          
        IOS  release version = 12.3         
        EIGRP release version = 1.2

It is a common misnomer that the hold-time configured on a device is how long it will wait before ending the relationship with its neighbor. The hold-time is actually how long the peer will wait before it ends the relationship. Device’s track the hold-time of each neighbor upon receipt of a hello packet, and the Hold Timer will wait the value of the hold-time before expiring. Any packet received from the peer will reset the Hold Timer. One can view the time remaining before a neighbor is removed by issuing the show ip eigrp neighbor command. In the sample output shown below, Router2 will wait another 11 seconds before ending its relationship with 10.0.0.1 (Router1). If it receives another hello packet from 10.0.0.1 before then, the timer will start over.

Router2#show ip eigrp neighbors
IP-EIGRP neighbors for process 100
H   Address           Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                      (sec)         (ms)       Cnt Num
0   10.0.0.1          Fa0/0             11 00:21:59  830  4980  0  2

The default hello-interval on slow links, which is defined by Cisco to be multi-point links T1 or lower, is 60 seconds. The default hello-interval on links faster than T1 is 5 seconds. A point-to-point link T1 or lower with a clock and bandwidth configured to be 64 Kb still sends hello packets every 5 seconds. The hold-time is by default three times the hello-interval; so, slow links have a 180 second value and fast links have a 15 second value. The hold-time and hello-interval can be configured using the ip hold-time eigrp as-number seconds and ip hello-interval eigrp as-number seconds commands. Both commands must be entered in interface-configuration mode on the interface belonging to the adjacency desired to be configured. The following commands were issued on Router1 to configure FastEthernet0/0 to advertise a hold-time of five minutes:

Router1#configure terminal
Router1(config)#interface fastethernet0/0
Router1(config-if)#ip hold-time eigrp 100 300

Sample capture from Router1 and output on Router2 after issuing the show ip eigrp neighbors command is shown below:

Frame 1298 (74 bytes on wire, 74 bytes captured)
Ethernet II, Src: cc:00:0e:78:00:00 (cc:00:0e:78:00:00), Dst: IPv4mcast_00:00:0a (01:00:5e:00:00:0a)
Internet Protocol, Src: 10.0.0.1 (10.0.0.1), Dst: 224.0.0.10 (224.0.0.10)
Cisco EIGRP     
    Version = 2     
    Opcode = 5 (Hello)   
    Checksum = 0xed4c     
    Flags = 0x00000000     
    Sequence = 0     
    Acknowledge = 0     
    Autonomous System : 100     
    EIGRP Parameters         
        Type = 0x0001 (EIGRP Parameters)         
        Size = 12 bytes         
        K1 = 1         
        K2 = 0         
        K3 = 1         
        K4 = 0         
        K5 = 0         
        Reserved         
        Hold Time = 300     
    Software Version: IOS = 12.3, EIGRP = 1.2         
        Type = 0x0004 (Software Version)         
        Size = 8 bytes          
        IOS  release version = 12.3         
        EIGRP release version = 1.2

Router2#show ip eigrp neighbors
IP-EIGRP neighbors for process 100
H   Address           Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                      (sec)         (ms)       Cnt Num
0   10.0.0.1          Fa0/0            298 00:32:25  830  4980  0  2
 

 

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Router1#configure terminal
Router1(config)#interface fastethernet0/0
Router1(config-if)#ip hold-time eigrp 100 300

Tags: Andrew Messier, EIGRP, timer, router configuration