1.3.c EIGRP Operations

1.3.c i General operations

Here’s a quick overview of EIGRP:

Distance-vector routing protocol.
Transport is IP protocol 88 (does not use TCP or UDP).
- Also implements it’s own Reliable Transport Protocol (RTP).
Hello Timer – Interval at which Hello messages are sent out an interface.
Hold Timer – Used to determine when a neighbor has failed. This is based on not receiving any EIGRP messages in the timer period.
Multicast address – 224.0.0.10 or FF02::A
Full updates are sent when a neighbor is discovered, then partial updates are used.
Arbitrary hierarchy, meaning summarization is supported at any point in the network.
Supports IPv4 & IPv6.
Administrative distances:
- 90 for Internal prefixes
- 170 for External prefixes

Configure/Modify hello-/hold-timer

! Named mode
router eigrp TEST
 address-family ipv4 unicast autonomous-system 100
  af-interface GigabitEthernetX/X
   hello-interval 30
   hold-time 90

! Classic mode
interface GigabitEthernetX/X
 ip hello-interval eigrp 100 30
 ip hold-time eigrp 100 90

1.3.c ii Topology table

To view the EIGRP topology table, you can use the following:

R1#sh ip eigrp topology
EIGRP-IPv4 VR(TEST) Topology Table for AS(100)/ID(150.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status 

P 155.1.79.0/24, 2 successors, FD is 2621440
        via 155.1.13.3 (2621440/1966080), GigabitEthernet0/1
        via 155.1.146.6 (2621440/1966080), GigabitEthernet0/2
P 155.1.5.0/24, 1 successors, FD is 2621440
        via 155.1.146.4 (2621440/1966080), GigabitEthernet0/2
        via 155.1.0.5 (9831055360/1310720), Tunnel0
...
...

If you need more information about a specific network:

R1#sh ip eigrp topology 155.1.79.0/24 
EIGRP-IPv4 VR(TEST) Topology Entry for AS(100)/ID(150.1.1.1) for 155.1.79.0/24
  State is Passive, Query origin flag is 1, 2 Successor(s), FD is 2621440, RIB is 20480
  Descriptor Blocks:
  155.1.13.3 (GigabitEthernet0/1), from 155.1.13.3, Send flag is 0x0
      Composite metric is (2621440/1966080), route is Internal
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 30000000 picoseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2
        Originating router is 150.1.7.7
  155.1.146.6 (GigabitEthernet0/2), from 155.1.146.6, Send flag is 0x0
      Composite metric is (2621440/1966080), route is Internal
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 30000000 picoseconds
..
..

There are also the following additional options:

R1#sh ip eigrp topology ?
  A.B.C.D          Network to display information about
  A.B.C.D/nn       Prefix <network>/<length>, e.g., 192.168.0.0/16
  active           Show only active entries
  all-links        Show all links in topology table
  detail-links     Show all links in topology table
  pending          Show only entries pending transmission
  secondary-paths  Show secondary paths in topology.
  summary          Show a summary of the topology table
  zero-successors  Show only zero successor entries
  |                Output modifiers
  <cr>             <cr>

This table is separate from the routing table or RIB. EIGRP will use the topology table to choose the best path, then send it as a candidate to the RIB. If there is another route with a better administrative distance, then the route will still exist in the topology table but will be in the RIB with the other method.

1.3.c iii Packet types

The EIGRP packet header (20 bytes) has a 4 bit Opcode field that specifies the packet type.

These values and packet types are below:

Update (1)
- During neighbor initialization, sent unicast to exchange topology information.
- During normal operation, sent multicast.
- During normal operation, only sent when something changes and only includes info about what changed.
- Sent reliably with RTP.
Query (3)
- Sent to advertise that a route is in Active state and the sender is requesting alternate path info from it’s neighbors.
- Sent reliably with RTP.
- Only reach one-hop away.
Reply (4)
- If a router receives a Query and has a loop-free route it will send a Reply.
- Sent reliably with RTP.
Hello (5)
- Used to form neighbor relationships and as a keepalive to maintain them.
- Sent to 224.0.0.10 by default.
Ack (5)
- Used to acknowledge updates.
- Sent reliably with RTP.
SIA Query (10)/SIA Reply (11)
- The Stuck-In-Active process is described in the next section.

1.3.c iv Stuck In Active

If EIGRP is stable, all routes will be passive. A route is put into the Active state if it needs to run DUAL for that prefix. In this case, the router will send out Queries to it’s neighbors and must wait for all of it’s neighbors to send back a Reply in order to run the DUAL computation and select a best path.

If a router sends out a Query and it doesn’t get a Reply from it’s neighbor, the router would not being to finish converging. Possible reasons for a router to not respond to a Query are:

Neighbor’s CPU is overloaded and cannot process all incoming packets
Packet loss on the link
Congestion on low-bandwidth links
The neighbor’s topology table is too large, either requiring the Query to propagate to a large number of routers or causing a very large number of prefixes to impacted by the single link failure.

To combat this issue, EIGRP will start the Active timer when a Query is first sent out. The default value is 3 minutes and can be set to 1-65535 minutes (you can also set it to infinity).

If a neighbor does not Reply to a Query within half of the Active timer time, the router sends a Stuck-In-Active Query (SIA-Query). This SIA Query is essentially asking the neighbor to confirm if it’s working on the original Query.

If the neighbor responds to the SIA-Query with an SIA-Reply, the Active timer is reset. A maximum of three SIA-Queries can be sent to a neighbor. If the route is still Active after three SIA-Queries, three SIA-Replies, and the half of the Active timer expires again, then the neighbor is dropped.

The neighbor is also dropped if the SIA-Query is sent and no SIA-Reply is received before the Active timer expires.

In order to view Active routes, use the show ip eigrp topology active command.This output should be empty.

R1#sh ip eigrp topology active  
EIGRP-IPv4 VR(NAMED) Topology Table for AS(100)/ID(150.1.1.1)

R1#

1.3.c v Graceful shutdown

EIGRP graceful shutdown you and your neighbors to quickly converge when you stop running EIGRP globally or on an interface. This is achieved by sending a hello packet with all K-Values set to 255.

This is a simple lab with two routers connected to each other running EIGRP. First, I’ll go to R1 and shut down EIGRP on the connected interface.

R1(config)#router eigrp NAMED
R1(config-router)#address-fam ipv4 as 100
R1(config-router-af)#af-interface g0/0
R1(config-router-af-interface)#shutdown
R1(config-router-af-interface)#
*Sep  6 23:59:20.374: %DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 10.12.0.2
 (GigabitEthernet0/0) is down: interface down

You can see that the neighbor immediately goes down on my end. On the other side we also receive a console message:

R2#
*Sep  6 23:59:21.212: %DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 10.12.0.1
 (GigabitEthernet0/0) is down: Interface PEER-TERMINATION received
R2#

This allows R2 to converge quickly without having to wait for timers to expire. The reason this happens is because R1 sends the following packet:

Notice that the packet is sent to the multicast address and that the K-Values are all set to 255. This K-Value mismatch will cause the neighbors to go down. However, due to them all being set to 255, EIGRP knows it’s a graceful shutdown and not simply a mismatch.

This same behavior would occur if I shut down EIGRP globally, or shut down the address family.

Digital Amnesia