Key fields in the EIGRP header are as follows:
* The opcode field specifies the EIGRP packet type (update, query, reply, hello).
* The checksum applies to the entire EIGRP packet, excluding the IP header.
* The rightmost bit in the flags field is the initialization bit and is used in establishing a new neighbor relationship
* The sequence and ack fields are used to send messages reliably
* The AS number identifies the EIGRP process issuing the packet. The EIGRP process receiving the packet will process the packet only if the receiving EIGRP process has the same AS number; otherwise, the packet will be discarded.
EIGRP is very widely documented so googling for EIGRP header will robably give more details.
Other highlights:
- Its Hybrid and uses the DUAL (Diffused Update ALgorithm).
- Neighbor discovery and maintenance ensures only updates are sent when needed (hellos). The hello interval is 5 and 60 seconds, the default hold time is three times (15 and 180 seconds) the hello timer duration. Hold time is the amount of time a router will consider a neighbor alive without receiving a hello packet. The timers can be adjusted per interface with the ip hello-interval eigrp and ip hold-time eigrp.
We'll use the same topology we used for RIP to explore the EIGRP timers.
Loopbacks
R0: : 172.20.1.1/32
R1: : 172.20.2.1/32
Network Addresses:
R0: Serial1/0 : 192.168.10.1/30
R1: Serial1/0 : 192.168.10.2/30
ffr Addresses:
R0: Serial1/1.105 : 192.168.20.1/30
R1: Serial1/1.501 : 192.168.20.2/30
Ethernet interfaces:
R0: Fastethernet0/0: 20.20.20.1/24
r1: FastEthernet0/0: 10.10.10.1/24
Test reachability - before continuing - If all is well, add in EIGRP:
On R0
R0#sh run | section eigrp
router eigrp 1
network 192.168.0.0 0.0.255.255
no auto-summary
on R1
router eigrp 1
network 192.168.0.0 0.0.255.255
no auto-summary
What pops up immediately:
*Mar 1 00:02:32.223: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.10.1 (Serial1/0) is up: new adjacency
*Mar 1 00:02:32.383: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.20.1 (Serial1/1.501) is up: new adjacency
*Mar 1 00:02:35.447: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.20.1 (Serial1/1.501) is resync: peer graceful-restart
*Mar 1 00:02:35.455: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.10.1 (Serial1/0) is resync: peer graceful-restartR0#sh ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 192.168.20.2 Se1/1.105 12 00:10:52 894 5000 0 15
0 192.168.10.2 Se1/0 12 00:10:52 127 762 0 16
The value of the hold column shouldn't be more than the timer unless you're losing packets. Also the Q(queue) count column should always be '0' unless there's a problem.
For instance, lets block EIGRP on R0 and see what shows up on R1:
RO
R0(config)#access-list 100 deny eigrp any any
R0(config)#access-list 100 permit ip any any
int s1/0
ip access-group 100 in
This pops up on R0
*Mar 1 00:38:34.547: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.10.2 (Serial1/0) is down: holding time expiredand on R1 we now have the following:
R1#R1#sh ip eigrp neighbors
*Mar 1 00:38:34.899: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.10.1 (Serial1/0) is down: Interface Goodbye received
*Mar 1 00:38:39.623: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.10.1 (Serial1/0) is up: new adjacency
*Mar 1 00:39:59.147: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.10.1 (Serial1/0) is down: retry limit exceeded
*Mar 1 00:40:03.619: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.10.1 (Serial1/0) is up: new adjacency
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 192.168.10.1 Se1/0 11 00:00:36 1 5000 1 0
1 192.168.20.1 Se1/1.501 11 00:38:08 137 822 0 10
Note the queue count is 1 (one) , this implies an un-acknowledged hello.
On R0 the neighbor is removed:
R0#sh ip eigrp neighborsOther commands you can run to troubleshoot: I'll just BOLD key info from each command
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 192.168.20.2 Se1/1.105 13 00:38:54 894 5000 0 15
R1#show ip eigrp interfacesR1# show ip eigrp topology
IP-EIGRP interfaces for process 1
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se1/0 1 0/0 124 0/15 563 0
Se1/1.501 1 0/0 137 0/15 639 0
IP-EIGRP Topology Table for AS(1)/ID(172.20.2.1)R1# show ip eigrp accounting
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 192.168.10.0/30, 1 successors, FD is 2169856
via Connected, Serial1/0
P 192.168.20.0/30, 1 successors, FD is 2169856
via Connected, Serial1/1.501
R1# show ip eigrp traffic
IP-EIGRP Traffic Statistics for AS 1
Hellos sent/received: 1202/1179 <---remember we had blocked some hellos with access list 100
Updates sent/received: 109/9
Queries sent/received: 2/2
Replies sent/received: 4/2
Acks sent/received: 1/12
Input queue high water mark 4, 0 drops
SIA-Queries sent/received: 0/0
SIA-Replies sent/received: 0/0
Hello Process ID: 174
PDM Process ID: 169
IP-EIGRP accounting for AS(1)/ID(172.20.2.1) <<--note teh ID is our loopbackR1#show ip eigrp 100 ?
Total Prefix Count: 2 States: A-Adjacency, P-Pending, D-Down
State Address/Source Interface Prefix Restart Restart/
Count Count Reset(s)
A 192.168.10.1 Se1/0 1 0 0
A 192.168.20.1 Se1/1.501 1 0 0
accounting IP-EIGRP Accounting
interfaces IP-EIGRP interfaces
neighbors IP-EIGRP neighbors
topology IP-EIGRP Topology Table
traffic IP-EIGRP Traffic Statistics
!use this if running more than one AS on the same router
!Note since these routes are learnt via connected, our routing table currently wont
show ip eigrp neighbors
Already covered
show any eigrp routes. To get this we need to advertise our loopbacks via eigrp
R0#sh run | section eigrp
R1#sh ip route
!--------edited----------!
192.168.10.0/30 is subnetted, 1 subnets
C 192.168.10.0 is directly connected, Serial1/0
172.20.0.0/32 is subnetted, 1 subnets
C 172.20.2.1 is directly connected, Loopback0
192.168.20.0/30 is subnetted, 1 subnets
C 192.168.20.0 is directly connected, Serial1/1.501
10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0
See!! no EIGRP routes, now lets add the loopback addresses:
router eigrp 1
network 172.20.1.0 0.0.0.255
network 192.168.0.0 0.0.255.255
no auto-summary
and on R1:
R1#sh run | section eigrpour output:
router eigrp 1
network 172.20.2.0 0.0.0.255
network 192.168.0.0 0.0.255.255
no auto-summary
R0#sh ip route eigrpR1#sh ip route eigrp
172.20.0.0/32 is subnetted, 2 subnets
D 172.20.2.1 [90/2297856] via 192.168.20.2, 00:01:05, Serial1/1.105
[90/2297856] via 192.168.10.2, 00:01:05, Serial1/0
172.20.0.0/32 is subnetted, 2 subnetsNotes:
D 172.20.1.1 [90/2297856] via 192.168.20.1, 00:01:23, Serial1/1.501
[90/2297856] via 192.168.10.1, 00:01:23, Serial1/0
*EIGRP doesnt build neighbor relationships over secondary addresses.
*Please note changing the hello interval using ip hello-interval eigrp doesn't adjust the hold. time timer for you.
*Load balancing: routing protocols install routes to your routing table, the switching fabric within the router/switch does the load balancing based on various things. eg per packet load balancing, per destination etc etc...all this depends on your switching (cef, fast switching)
let me make a quick demonstration:
If we try to reach R1's loopback with CEF enabled and the routing table as it is now (by default we use per destination loadbalancing) -
R1#debug ip icmp
ICMP packet debugging is on
R1#
*Mar 1 01:04:19.239: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.20.1
*Mar 1 01:04:19.371: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.20.1
*Mar 1 01:04:19.459: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.20.1
*Mar 1 01:04:19.495: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.20.1
*Mar 1 01:04:19.507: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.20.1
Note the path taken is the same, all packets to the same destination will use
same path
now disable CEF (globally - no ip cef)on R0 and make the same 'ping'
R1#You can use sh ip interface to figure out what sort of switching is in use. I only did this to show how the routing protocol really only populates the routing database, how traffic is moved across networks is not its business.....I'll follow this up with EIGRP metrics,feasible distance, reported distance and feasible successors.....
*Mar 1 01:07:27.411: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.20.1
*Mar 1 01:07:27.495: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.10.1
*Mar 1 01:07:27.539: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.20.1
*Mar 1 01:07:27.583: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.10.1
*Mar 1 01:07:27.591: ICMP: echo reply sent, src 172.20.2.1, dst 192.168.20.1
Note the alternating paths, this is per Packet Load balancing
References:
Cisco
Oreilly's IP routing by By Ravi Malhotra - I found chapter4 available on oreilly:-) good for you
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