1. EIGRP can be referred to as a hybrid protocol. It combines most of the characteristics of traditional distance vector protocols with some characteristics of link-state protocols. Specifically, EIGRP is "enhanced" by using four routing technologies:
-Neighbor discovery/recovery
-Reliable Transport Protocol (RTP)
-DUAL finite-state machine
-Protocol-dependent modules
2. It could be argued that EIGRP's weakest point is that it is a Cisco-proprietary protocol, but with the aid of redistribution, this point becomes moot.
3. EIGRP is a classless routing protocol. It directly interfaces to IP as protocol 88. EIGRP uses the multicast address of 224.0.0.10 for hellos and routing updates.
4. Only partial routing updates are sent when network topology changes occur.
5. EIGRP uses a composite metric much like IGRP, except that it is modified by a multiplier of 256.
-Bandwidth
-Delay
-Load
-Reliability
-MTU
6. When EIGRP calculates this metric for a route, it calls it the feasible distance to the route. EIGRP calculates a feasible distance to all routes in the network.
K1=K3=1,
BW=107/bandwidth-of-slowest-link
DELAY=Σ(delays-along-the-path)
7. EIGRP does not periodically advertise it routes. Because of this, it needs some way to locate and then exchange routing information with adjacent devices. EIGRP accomplishes this through the use of neighbors.
8. When EIGRP initializes, it sends out a multicast hello on address 224.0.0.10, on broadcast media.
9. EIGRP sends hellos every 5 seconds on the following interfaces:
LAN broadcast media, such as Ethernet, Token Ring, and FDDI
High-speed serial link greater than T1 speeds, such as Frame Relay HSSI links
Point-to-point serial links, such as PPP or HDLC
ATM and Frame Relay point-to-point subinterfaces
EIGRP sends hellos every 60 seconds on the following interfaces:
Low-speed serial links less than T1 speeds, including Frame Relay and
ATM and Frame Relay multipoint interfaces, and ATM SVCs
ISDN BRIs
Process:
1. Hellos are sent out each interface participating in EIGRP, except interfaces quieted by the passive interfaces. All EIGRP hellos and routing updates use the multicast address of 224.0.0.10.
2. Routers on the same IP subnet receive the multicast and respond with a full routing update. This is accomplished by setting the INITialization bit in the EIGRP header; the updates include all networks that EIGRP is aware of and the metric for those routes, except for those suppressed by split horizon. This update packet establishes a neighbor relationship (adjacency). The hello packet also includes a hold timer, which tells the router how long it should wait before receiving a hello and declaring the route unreachable and reporting it to the DUAL process. The hold timer is set to three times the value assigned for the hello timer. This usually is 15 or 180 seconds, depending on the media.
3. The other router responds to the initialization packet by sending a hello with the ACK bit set. EIGRP sets the ACK bit to acknowledge all messages that it receives that have data. This is one way that EIGRP has reliable transports.
4. The other router now inserts the new update into its route table. Because it has a new update, it sends an update to all its neighbors.
5. The neighbors that received the update respond with an acknowledgment packet.
6. The router holds the adjacency by the continuous exchange of hellos. If a hello is not received by the time the hold timer expires, the router marks the route as unreachable.
7. When the router forms an adjacency, it treats this as a virtual link to transport routing information.
Packets:
Hello— Used to discover and maintain neighbors. This packet type uses unreliable delivery.
Acknowledgments (ACKs)— Used to acknowledge updates. They are essentially hellos with no data in them. ACKs also use unreliable delivery.
Updates— Contain routing information. Updates can be either unicast or multicast, depending on how they are generated. Updates use reliable delivery. Uses RTP (Reliable Transport Protocol).
Queries— Used by the DUAL process to find feasible successor for routes. The query can be unicast or multicast. Queries always use reliable delivery.
Uses RTP.
Replies— Used by the DUAL process to aid in finding feasible successor for routes. Replies are always unicast and use reliable delivery. Use RTP.
DUAL: (Diffusing Update Algorithm)
With the help of DUAL, EIGRP keeps the following tables:
Neighbor table— EIGRP tracks every formed adjacency in the neighbor table. A neighbor will be held until an ACK is not received after 16 unicast retransmissions to that neighbor. At this time, the neighbor is dropped. Neighbors can be displayed with the show ip eigrp neighbors command.
Topology table— All learned routes reported by neighbors are kept in the topology table. The topology table also tracks the metrics and feasible distances associated with those routes. The topology table can be displayed with the show ip eigrp topology as_number command.
Route table/forwarding table— Only the routes with the lowest composite metric are entered into the final route or forwarding table. This is the route that the router will forward to.
The DUAL process is in control of determining feasible distances, feasible successors, and the successor of the routes in the EIGRP topology table. By having a backup path already defined in the topology table, the router can quickly converge to the new path in case the primary path fails.
The successor is the primary path for the route, or the path that the router will forward packets to. The feasible successor becomes the next-hop address only if the primary route to the destination becomes unreachable. The feasible successor is always downstream and, thereby, must have a distance or feasible distance that is less than that of the current preferred route.
We can not use passive-interface to stop a router sending routing updates by using "passive-interface..." command (in EIGRP or OSPF), because it will shut down the neighbor relationship of these two routers (no hello packets are exchanged).
1 comments:
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