notes: EIGRP Packets

- EIGRP uses multiple packet types, they are all identified by protocol number 88 in the IP header.

1. Hellos: Are used by the neighbor discovery and recovery process. Hellos are unicast or multicast and use unreliable delivery.
- Establish neighbor relationships.

2. ACKs: Are Hello packets with no data in them. ACKs are always unicast and use unreliable delivery.
- Acknowledge reliable packets.

3. Updates: Convey route information. Updates could be unicast/multicast and always use reliable delivery.
- Send routing updates.

4. Queries/Replies: Used by DUAL for computations. Queries can be unicast or multicast, but replies are always unicast.
- Ask neighbors about routing information.
- Respond to queries.

- Any reliable multicast packets sent, that was not acknowledged by the neighbor it was sent too, will be followed by a retransmitted unicast packet to that neighbor.
- If an acknowledgement was not received after 16 of these unicast retransmissions, the neighbor will be declared dead.
- Retransmission timeout (RTO) is the time between the subsequent unicasts.
- Smooth round-trip time (SRTT) is the time, between a packet sent to the neighbor and the receipt of an acknowledgment.

EIGRP Reliability

Packets that require acknowledgment are as follows:
- Update
- Query
- Reply

Packet that do not are as follows:
- Hello
- Ack

Neighbor reset after retry limit (16) is reached. Slow neighbors are sent unicast packets instead.

The address used for hello packets is 224.0.0.10; AS numbers must match. Hellos are sent every 5 seconds on broadcast links and point-to-point serial links, point-to-point subinterface links, and multipoint circuits greater than T1. They are sent every 60 seconds on other link types. The hold time defaults to 3 times the hello time. Neighborships form even if the values do not match.