Most people have learned that the EIGRP hold timer is renewed when it receives a hello packet from a neighbor. This is correct however the hello packet is not the only packet that renewes it…all EIGRP packets do. In order to demonstrate this we’ll take a look at the following two routers:
First I will configure EIGRP on both routers, nothing special I just want to make sure we have a neighbor adjacency:
R1(config)#router eigrp 12 R1(config-router)#network 192.168.12.0
R2(config)#router eigrp 12 R2(config-router)#network 192.168.12.0
Now I will increase the hold time so it doesn’t drop the neighbor adjacency so quickly. I’ll set it to 1 hour:
R2(config-if)#ip hold-time eigrp 12 3600
When we take a look at R1 you’ll see that it uses 3600 seconds as the hold time for R2:
R1#show ip eigrp neighbors IP-EIGRP neighbors for process 12 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 192.168.12.2 Fa0/0 3597 00:00:51 3 200 0 21
We have 3597 seconds and counting…now I will set the hello timer to a low value so that we can find out if other EIGRP packets will renew the hold time. I’ll set it to 5 minutes:
R2(config)#interface fastEthernet 0/0 R2(config-if)#ip hello-interval eigrp 12 300
R1 will now only receive a hello packet from R2 every 5 minutes. To show that different EIGRP packets will renew the hold time I will enable a debug on R1 so that we can see what kind of packets it receives from R2:
R1#debug eigrp packets EIGRP Packets debugging is on (UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB, SIAQUERY, SIAREPLY)
Now I will create a new loopback interface on R2 and advertise it in EIGRP. This will cause some traffic between R1 and R2. Before I do this, let’s take a quick look at the current state of the hold time again:
look again at table:
R1#show ip eigrp neighbors IP-EIGRP neighbors for process 12 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 192.168.12.2 Fa0/0 3504 00:04:23 3 200 0 21
Right now we are down to 3504 seconds…let’s advertise that loopback interface in EIGRP:
Yesterday I did the same lab, hehe, same results.... Continue reading in our forum
what is the default hello time ?
It’s 5 seconds for LAN interfaces and 60 seconds for slow (WAN) interfaces. The holddown timer is 3x the hello timer.
In case of hybrid FR hub and spoke configuration, all with serial interfaces ( R1 is the hub router - with 2 logical sub-interfaces, one point-to-point, and another one multipoint, R2 is a spoke router for the point-to-point connection, R3,R4,R5 are the spokes for the multipoint connection). In this case, R1 point-to-point subif has different timer values (Hello: 5s) than the multipoint subif (Hello: 60s) as the latter is considered to be slower link. However all this hub-and-spoke topology is configured in one EIGRP AS (AS:100). My question is do we n... Continue reading in our forum
This is an excellent question and it reveals some of the nuances of EIGRP.
When using Ethernet as the layer 2 protocol, holdown and hello timers for EIGRP don’t have to be consistent over the whole AS. Each EIGRP neighbour relationship must have its timers configured correctly, but those can be different from relationship to relationship. However there is one case where timers must be configured the same, and this is the case you describe above.
This case is when you have multiple routers on a network that form adjacencies. All those routers mus... Continue reading in our forum