Quick Notes: EIGRP Packets

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** From: Routing TCP/IP, Volume I, 2nd Edition **


EIGRP Packet Formats

  • The IP header of an EIGRP packet specifies protocol number 88, and the maximum length of the packet will be the IP maximum transmission unit (MTU) of the interface on which it is transmitted—usually 1500 bytes.
  • Following the IP header is an EIGRP header followed by various Type/Length/Value (TLV) triplets.
  • Version specifies the particular version of the originating EIGRP process. The version of the EIGRP process itself has not changed since its release.
  • Opcode specifies the EIGRP packet type.
    • 1 - Update
    • 3 - Query
    • 4 - Reply
    • 5 - Hello
    • 10 - SIA Query
    • 11 - SIA Reply
  • Checksum is a standard IP checksum. It is calculated for the entire EIGRP packet, excluding the IP header.
  • Flags currently include just two flags. The right-most bit is Init, which when set (0x00000001) indicates that the enclosed route entries are the first in a new neighbor relationship. The second bit (0x00000002) is the Conditional Receive bit, used in the proprietary Reliable Multicasting algorithm.
  • Sequence is the 32-bit sequence number used by the RTP.
  • ACK is the 32-bit sequence number last heard from the neighbor to which the packet is being sent. A Hello packet with a nonzero ACK field will be treated as an ACK packet rather than as a Hello. Note that an ACK field will only be nonzero if the packet itself is unicast because acknowledgments are never multicast.
  • Autonomous System Number is the identification number of the EIGRP domain.


General TLV Fields

  • Each Internal and External Routes TLV contains one route entry.
  • Every Update, Query, and Reply packet contains at least one Route TLV.
  • The Internal and External Routes TLVs include metric information for the route.
  • The metrics used by EIGRP are the same metrics used by IGRP, although scaled by 256.


IP Internal Route TLV

  • An internal route is a path to a destination within the EIGRP autonomous system.
  • Next Hop is the next-hop IP address. This address might or might not be the address of the originating router.
  • Delay is the sum of the configured delays expressed in units of 10 microseconds. A delay of 0xFFFFFFFF indicates an unreachable route.
  • Bandwidth is 256 × BW_EIGRP(min), or 2,560,000,000 divided by the lowest configured bandwidth of any interface along the route.
  • MTU is the smallest Maximum Transmission Unit of any link along the route to the destination. Although an included parameter, it has never been used in the calculation of metrics.
  • Hop Count is a number between 0x01 and 0xFF indicating the number of hops to the destination. A router will advertise a directly connected network with a hop count of 0; subsequent routers will record and advertise the route relative to the next-hop router.
  • Reliability is a number between 0x01 and 0xFF that reflects the total outgoing error rates of the interfaces along the route, calculated on a five-minute exponentially weighted average. 0xFF indicates a 100 percent reliable link.
  • Load is also a number between 0x01 and 0xFF, reflecting the total outgoing load of the interfaces along the route, calculated on a five-minute exponentially weighted average. 0x01 indicates a minimally loaded link.
  • Reserved is an unused field and is always 0x0000.
  • Prefix Length specifies the number of network bits of the address mask. Destination is the destination address of the route.



IP External Route TLV

  • An external route is a path that leads to a destination outside of the EIGRP autonomous system and that has been redistributed into the EIGRP domain.
  • Next Hop is the next-hop IP address.
  • Originating Router is the IP address or router ID of the router that redistributed the external route into the EIGRP autonomous system.
  • Originating Autonomous System Number is the autonomous system number of the router originating the route.
  • Arbitrary Tag may be used to carry a tag set by route maps.
  • External Protocol Metric is, as the name implies, the metric of the external protocol. This field is used, when distributing with IGRP, to track the IGRP metric.
  • Reserved is an unused field and is always 0x0000.
  • External Protocol ID specifies the protocol from which the external route was learned.
    • 0x01- IGRP
    • 0x02 - EIGRP
    • 0x03 - Static Route
    • 0x04 - RIP
    • 0x05 - Hello
    • 0x06 - OSPF
    • 0x07 - IS-IS
    • 0x08 - EGP
    • 0x09 - BGP
    • 0x0A - IDRP
    • 0x0B - Connected Link
  • Flags currently constitute just two flags. If the right-most bit of the eight-bit field is set (0x01), the route is an external route. If the second bit is set (0x02), the route is a candidate default route.
  • The remaining fields describe the metrics and the destination address. The descriptions of these fields are the same as those given in the discussion of the Internal Routes TLV.

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