EIGRP summary

1, EIGRP uses IP protocol with protocol number 88 while ospf is using protocol number 89.’

2, K-values in all eigrp routers in an autonomous system must match because it si critical that all routers using the same value to calculate metrics, the router will not be able to establish an adjacency if K-value does not match
K-values can be viewed by using command:
#show ip protocols
Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
By default K2,K4 and K5 are 0, that means only bandwidth and delay are used to calculate metrics.

3, Eigrp packets types:
EIGRP query

4,Eigrp Hello packets are sent to the multicast group in ipv4 and FF02::A in ipv6. If a static neighbor is configured, yellow packets are sent as unicast to the explicitly configured neighbor address. The default interval is 5 secs.

5,RD (Recorded Distance) is the neighbor’s distance to the destination. CD (Computed Distance) is the metric of RD plus the cost of the router towards the specific neighbor. FD (Feasible Distance ) is a record of the lowest known distance since last transition from the active to passive state, therefor FD is a historical record.

6,Eigrp route can be put in “active” state when router detected a topology change and the new shortest path provided by a neighbor that is not a Feasible Successor,the router can not use such a neighbor right away because ti could cause a routing loop. When route is in “active” state, the entry in the routing table is locked until that the route get all reply packets from all the neighbor s to confirm that there is no routing loop. Then the router will put the route entry into passive state again.

7,SIA (Stuck in Active)
When a query is first sent out by a router, a timer (active timer) is started, (by default 3 mins), If all expected replied are not received before the active timer expires, the route in question will be designated fas SIA, and the neighbor that failed to reply will be removed from the neighbor table and their adjacencies torn down.
To solve this problem, SIA-query and SIA-Reply are used during Active timer period.at most 3 SIA-query can be sent, each after half of the active timer, when the router got SIA-reply the active timer will be reset.

8, Unequal-cost Loadbalance

9,Stub Router
A stub router advertises only a subset of its own EIGRP-enabled networks to neighbors, this subset can be defined in the eigrp stub command using the summary.connected, state, redistribute, receive-only and leak-map.
Neighbors of a stub router will never sent a query packet to a stub router.

10, passive interface
passive interface does not process any EIGRP packet, but the network configured on the interface is still advertised.

11, If the default route happens to be configured as as static route out an interface, not the next-hop IP address, IOS treats this route as being directly connected.

12, EIGRP neighborship requirements
Active Hello packets
AS Number

13, EIGRP OTP (Over The Top)
EIGRP OTP (Over the Top) allows you to run EIGRP between routers that are not directly connected. It is a great feature to use when you want to run EIGRP between routers that are connected to a service provider network and you don’t want the hassle of other solutions like MPLS VPN and you don’t want the service provider’s involvement with your routing. With EIGRP OTP, you can configure everything on your own routers as long as you are able to reach the remote router(s).

EIGRP OTP uses an overlay VPN and thus uses tunneling, it is a bit similar to other solutions like DMVPN / multipoint GRE but it uses LISP (Locator/Identifier Separation Protocol) instead.
There is a control plane and data plane for LISP.On the data plane, LISP uses UDP to encapsulate data and tunnel traffic. EIGRP OTP only uses LISP for the data plane, EIGRP is used for the control plane.

14, TCP default receive window size is 64kb, and can be scaled up to 16Mb in windows system.

15, EIGRP passive interface


How to influence EIGRP metrics to affect route selection

EIGRP updates contain five metrics: minimum bandwidth, delay, load, reliability, and maximum transmission unit (MTU). Of these five metrics, by default, only minimum bandwidth and delay are used to compute best path. Unlike most metrics, minimum bandwidth is set to the minimum bandwidth of the entire path, and it does not reflect how many hops or low bandwidth links are in the path. Delay is a cumulative value which increases by the delay value of each segment in the path.

Therefore we can change delay on interface to affect route selection, but this method can only be used when need to influence route selection learned via EIGRP neighbor on that interface.

Another more sophisticated way is to use offset-list, the metric of the route on the router can be modified using an offset-list on the neighbor router. Offset-list will insert the value to affect RD and FD advertised towards the peer router.