Category Archives: 3.5.a Describe packet types

3.5.a Describe packet types

3.5.a [i] Packet types [hello, query, update, and such]

EIGRP uses five packet types:

● Hello/ Acks

● Updates

● Queries

● Replies

● Requests

Hellos are multicast for neighbor discovery/ recovery. They do not require acknowledgment. A hello with no data is also used as an acknowledgment (Ack). Acks are always sent using a unicast address and contain a non-zero acknowledgment number.

Updates are used to convey reachability of destinations . When a new neighbor is discovered, update packets are sent so the neighbor can build up its topology table. In this case, update packets are unicast. In other cases, such as a link cost change, updates are multicast. Updates are always transmitted reliably.

Queries and replies are sent when destinations go into Active state. Queries are always multicast unless they are sent in response to a received query. In this case, it is unicast back to the successor that originated the query. Replies are always sent in response to queries to indicate to the originator that it does not need to go into Active state because it has feasible successors. Replies are unicast to the originator of the query. Both queries and replies are transmitted reliably.

Request packets are used to get specific information from one or more neighbors. Request packets are used in route server applications. They can be multicast or unicast. Requests are transmitted unreliably.

The sequence TLV contains a list of the nodes that should not listen to multicast packets while the recovery takes place. While recovering, each reliable multicast packet transmitted has the CR (conditional receive) bit set to indicate that it should be processed only if the receiving node’s was not present in the preceding sequence TLV packet.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Location 3034).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13669-1.html#packet_formats

 

3.5.a Describe packet types

3.5.a [ii] Route types [internal, external]

EIGRP has the concept of internal and external routes. Internal routes are ones that have been originated within an EIGRP autonomous system (AS). Therefore, a directly attached network that is configured to run EIGRP is considered an internal route and is propagated with this information throughout the EIGRP AS. External routes are ones that have been learned by another routing protocol or reside in the routing table as static routes. These routes are tagged individually with the identity of their origination.

External routes are tagged with the following information:

● The router ID of the EIGRP router that redistributed the route.

● The AS number where the destination resides.

● A configurable administrator tag.

● Protocol ID of the external protocol.

● The metric from the external protocol.

● Bit flags for default routing.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3049-3051).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13681-eigrpfaq.html#rt

3.5.a Describe packet types

i didn’t catch this on my last reading… note also that doyle sets hello/ack as distinct packet types whereas cisco pairs them as below…

from doyle:

Requests were a type of packet originally intended for use in route servers. This
application was never implemented, and request packets are noted here only because
they are mentioned in some older EIGRP documentation.

from cisco: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml

Packet Formats

EIGRP uses five packet types:

  • Hello/Acks
  • Updates
  • Queries
  • Replies
  • Requests

Request packets are used to get specific information from one or more neighbors. Request packets are used in route server applications. They can be multicast or unicast. Requests are transmitted unreliably.

and wendell below from ccie r&s:

Hello Identifies neighbors, exchanges parameters, and is sent periodically as a
keepalive function
Update Informs neighbors about routing information
Ack Acknowledges Update, Query, and Response packets
Query Asks neighboring routers to verify their route to a particular subnet
Reply Sent by neighbors to reply to a Query
Goodbye Used by a router to notify its neighbors when the router is gracefully shutting down

note: wendell separates hello/ack, doesn’t mention request, sia-query, sia-reply and includes goodbye…

and wireshark chimes in:

eigrp_cap