Transcript 20088-1 CCNA3 3.1-03 EIGRP -jp

Module 3
Cisco 3- EIGRP
EIGRP
Perrine and Brierley
4/9/2016
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Module 3
Cisco 3- EIGRP
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EIGRP
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco
proprietary routing protocol based on IGRP.
EIGRP supports CIDR, and hence VLSM.
Compared to IGRP, EIGRP boasts faster convergence times, improved
scalability and superior handling of routing loops.
Technically, EIGRP is an advanced distance-vector routing protocol that
relies on features commonly associated with link-state protocols. Some of
OSPF’s best traits, such as partial updates & neighbor discovery, are
similarly put to use by EIGRP.
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EIGRP
Recall that OSPF’s strengths include scalability and multivendor support. So if your core routers are a mixed bag of
products from several different vendors, OSPF and RIP
may be your only options.
But OSPF’s benefits, especially it’s hierarchical design, come
at a price: administrative complexity.
EIGRP is an ideal choice for large, multiprotocol networks built
primarily on Cisco routers (says Cisco).
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Comparison of OSPF and EIGRP
OSPF
EIGRP
Supports CIDR & VLSM, rapid
convergence, partial updates,
neighbor discovery.
Enables the admin to define
route summarization.
Is an open standard; has
multi-vendor support
Supports CIDR & VLSM, rapid
convergence, partial updates,
neighbor discovery
Uses automatic route
summarization & user-defined
route summaries.
Is proprietary; can only be used
with Cisco routers.
Is scalable; admin defined
‘areas’ provide manageable
hierarchy.
Is scalable; no hierarchical
domains exist
Is difficult to implement
Is easy to implement.
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EIGRP and IGRP Compatibility
EIGRP offers multiprotocol support and IGRP does not.
EIGRP scales IGRP’s metric by a factor of 256. That’s
because EIGRP uses a metric that is 32 bits long, & IGRP
uses a 24-bit metric. By dividing or multiplying by 256,
EIGRP can easily exchange information with IGRP.
EIGRP imposes a max hop limit of 224, where IGRP has
255.
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EIGRP and IGRP Compatibility
Sharing or redistribution, is automatic between IGRP
& EIGRP as long as both processes use the same
AS number.
metric = [K1 x bandwidth + K2 x bandwidth) /
(256ms – load) + (K3 x delay)] x
[K5 / (reliability+K4)]
Where by default:
= 0 K5 = 0
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K2 = 0
K3 = 1
K4
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EIGRP vs. IGRP
EIGRP tags routes learned from IGRP as external
because they did not originate from EIGRP routers.
External EIGRP routes are denoted by EX in the
routing table.
But IGRP cannot differentiate between internal &
external routes
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EIGRP
EIGRP is an advanced distance vector routing protocol, but has
advantages over simple distance vector protocols:
• rapid convergence
By using an routing algorithm, Diffusing Update Algorithm (DUAL)
which guarantees loop-free operation & allows all routers involved
in a topology change to synchronize at the same time.
• partial bounded updates
EIGRP routers make partial, incremental updates, & unlike OSPF, the
routers send these partial updates only to the routers that need the
information. This is called bounded updates.
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EIGRP cont.
• minimal consumption of bandwidth when the network
is stable
•No timed routing updates - instead small hello packets.
•Exchanged a regular intervals
•don’t use a significant amount of bandwidth.
• support for VLSM and CIDR
• multiple network-layer support
EIGRP supports IP, IPX and AppleTalk via protocoldependent modules
(PDM).
• complete independence from routed protocols
PDM protect EIGRP from painstaking revision.
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EIGRP Terminology & Tables
EIGRP routers keep route and topology information in RAM for quick
access.
EIGRP terms and tables:
neighbor table
Maintains a neighbor table that list adjacent routers. There is a
neighbor table for each protocol that EIGRP supports.
topology table
Topology table for each configured network protocol. All learned
routes to a destination are maintained in the topology table.
routing table
EIGRP chooses the best (successor) routes to a destination from
the topology table & places these routes in the routing table.
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EIGRP Terminology
successor
a route selected as the primary route used to reach a destination.
Successors are kept in the routing table.
feasible successor
Is a backup route. They are kept in the topology table.
EIGRP routers establish adjacencies with neighbor routers by sending
hello packets, sent every 5 seconds (default).
By forming adjacencies, EIGRP routers do the following:
• dynamically learn of new routes that join their network
• identify routers that become either unreachable or inoperable
• rediscover routers that had previously been unreachable.
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EIGRP Technologies cont.
Reliable Transport Protocol (RTP) is a transport layer (layer 4)
protocol that can guarantee ordered delivery of EIGRP packets to all
neighbors. To stay independent of IP, EIGRP uses its own proprietary
transport-layer protocol to guarantee delivery of routing information.
EIGRP uses RTP to provide reliable or unreliable service as the
situation warrants. Hello packets are not required to be reliable delivery.
RTP supports both unicasting & multicasting , and it can multicast and
unicast to different peers simultaneously.
The centerpiece of EIGRP is DUAL, the EIGRP route calculation
engine. It uses a finite state machine.
DUAL tracks all the routes advertised by neighbors and uses the
composite metric of each route to compare them. DUAL also
guarantees that each path is loop-free.
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EIGRP Technologies cont.
A successor is a neighboring router that is
currently used for packet forwarding; it provides the
least cost route to the destination and is not part of
the routing loop
A feasible successor provides the next lowest
cost path without introducing routing loops.
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EIGRP Data Structure
Neighbor Table
Routing Table
Topology Table
Reported distance (RD)
Feasible distance (FD)
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EIGRP Data Structure
Neighbor Table
the most important table in EIGRP. The neighbor
relationships in the neighbor table are basis for all EIGRP
routing updates and convergence activity. supports reliable,
sequenced delivery of packets.
Routing Table
Contain the routes installed by DUAL as the best loop free
paths to a given destination. It canmaintain up to 4 routes
per destination.
Topology Table
stores all the information it needs to calculate a set of
distances and vectors to all reachable destinations.
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EIGRP Data Structure
Reported distance (RD)
The distance reported by an adjacent neighbor to a
specific destination.
Feasible distance (FD)
The lowest calculated metric to each destination.
Table is sorted with the successor routes at the top,
followed by feasible successors.
At the bottom are what DUAL believes are routing
loops.
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To see the topology table, use the command:
Router# show ip eigrp topology [all]
Example of output:
P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33
via 10.2.0.2 (2681856/2169856), Serial1
Viewing (2681856/2169856), 2681856 is the FD, and 2169856 is the RD.
If no feasible successors to the destination exist, DUAL places the route
in the active state.
Entries in the topology table can be in one of 2 states
• passive route – route that is stable and available for use
• active route – route in the process of being recomputed by DUAL.
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Internal routes originate from within the EIGRP AS.
External routes originate from outside the system.
Routes learned (redistributed) from other routing protocols
such as RIP, OSPF and IGRP are external.
Static routes originating from outside the EIGRP AS and
redistributed inside are also external routes.
NOTE:
The internal administrative distance of EIGRP is 90.
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EIGRP Packet Types
EIGRP relies on 5 packet types to maintain its various
tables and establish complex relationships with
neighbor routers.
The 5 packet types:
1. Hello
2. Acknowledgment
3. Update
4. Query
5. Reply
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Hello Packets
EIGRP relies on hello packets to discover, verify & rediscover neighbor
routers. The default hello interval depends on the bandwidth of the
interface:
Bandwidth
Less than 1.544Mbps
Example Link
Multipoint Frame Relay
Greater than 1.544Mbps T1, Ethernet
Default Hello
Default Hold
Interval
Time
60 sec
180 sec
5 sec
15 sec
Keep the hold timer 3 times the hello interval. EIGRP hello packets are
multicast. On IP networks,
EIGRP routers send hellos to the multicasts IP address 224.0.0.10.
Recall that OSPF requires neighbor routers to have the same hello & dead
intervals to communicate. EIGRP has no such restriction.
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Acknowledgement Packets
An EIGRP router uses acknowledgement packets to indicate receipt of
any EIGRP packet during a reliable exchange.
To be reliable, a sender’s message must be acknowledged by the
recipient.
Hello packets are always sent unreliable, & require no acknowledgement.
Update Packets
Update packets are used when a router discovers a
new neighbor. They are also used when a router
detects a topology change.
All update packets are sent reliably.
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Query and Reply Packets
EIGRP routers use query packets whenever they need
specific information from one or all of its neighbors. A reply
packet is used to respond to a query.
Query can be multicast or unicast
Replies are always unicast.
Both packet types are sent reliably.
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EIGRP Convergence (DUAL)
DUAL’s sophisticated algorithm results in EIGRP’s
exceptional fast convergence, says Cisco
A router’s topology table includes a list of all routes
advertised by the neighbors. For each network, the
router keeps the real (computed) cost of getting to
that network & also keeps the advertised cost
(reported distance) from its neighbor.
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EIGRP Convergence (DUAL)
The best path is the path with the lowest
metric route that is calculated by adding the
metric between the next-hop router and the
destination (the reported distance) to the
metric between the local router and the nexthop router.
This computed cost, or distance is the FD.
The next-hop router(s) are selected as the
best path is the successor. If several routes
have the same FD, then there can be several
successors to a destination.
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10
20
RTX
Network
24
COST
10
RTY
FDDI
RTA
Serial
1
10
RTZ
100
100
RTA reaches Network 24 via:
Neighbor
RTX
RTY
RTZ
Computed Cost to 24
40
31
230
Report Distance to 24
30
21
220
For RTA, RTY is the successor to Network 24, because it
has the lowest computed cost (31). Hence RTA’s FD=31.
If RTY goes down, then is there a feasible successor – or
not? If so, what is it?
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10
20
RTX
Network
24
COST
10
RTY
FDDI
RTA
Serial
1
10
RTZ
100
Neighbor
RTX
RTY
RTZ
100
Computed Cost to 24
40
31
230
Report Distance to 24
30
21
220
Remember: RTA’s FD=31
Feasible successor =
1. It’s RD < RTA’s FD AND
2. Has the lowest computed cost of all other routes
DUAL calculates the feasible successor to be through RTX
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10
20
RTX
Network
24
COST
10
RTY
FDDI
RTA
Serial
1
10
RTZ
100
Neighbor
RTX
RTY
RTZ
100
Computed Cost to 24
40
31
230
Reported Distance to 24
30
21
220
Suppose RTX also goes down. Is there a feasible successor for RTA to Network 24?
Feasible successor =
1.
It’s RD < RTA’s FD AND
2.
Has the lowest computed cost of all other routes
NO! DUAL will set the route to Network 24 from passive to active state, and RTA
will query its neighbor’s about Network 24 again.
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Consider the output from the command ‘show ip eigrp
topology all’:
P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33
via 10.2.0.2 (2681856/2169856), Serial0
via 10.2.1.2 (2681856/2169856), Serial1
via 10.2.2.2 (2891856/2769856), Ethernet 0
Remember:
Successor is the route that has the best lowest cost or FD.
Feasible successor =
It’s RD < link’s FD AND has the lowest computed cost
of all other routes
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Consider the output from ‘show ip eigrp topology all’:
P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33
via 10.2.0.2 (2681856/2169856), Serial0
via 10.2.1.2 (2681856/2169856), Serial1
via 10.2.2.2 (2891856/2769856), Ethernet 0
What are the successor(s)?
10.2.0.2 AND 10.2.1.2
What are the feasible successor(s)?
NONE
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EIGRP Operation
The 4 key steps to EIGRP operation are:
1.
building the neighbor table
2.
discovering routes
3.
choosing routes
4.
maintaining routes
Choose the routes based on these 5 factors:
1.
bandwidth
2.
delay
3.
reliability
4.
load
5.
MTU (maximum transmission unit)
So, unless otherwise configured by an administrator, bandwidth & delay
are the only 2 factors that determine EIGRP’s metric value.
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Configuring EIGRP
Configuring EIGRP for IP Networks
Router(config)# router eigrp autonomoussystem-number
Router(config-router)# network networknumber
network-number is the NETWORK address of
the interface of the router
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Configuring EIGRP
The ip bandwidth-percent command configures the
percentage of bandwidth that EIGRP can use on an
interface.
By default, EIGRP is set to use up to 50 percent of the
bandwidth of an interface to exchange routing
information.
The command relies on the bandwidth of an interface.
Some cases the engineer sets the bandwidth to a lower
number than the actual bandwidth of the link (in order to
manipulate the routing metric).
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Summarizing EIGRP routes for IP
EIGRP automatically summarizes routes at a
classful boundary.
For discontiguous subnetworks, you do NOT
want summarization.
The command that turns summarization off on
EIGRP is:
Router(config-router)# no autosummary
Hence, with summarization turned off, EIGRP
routers will advertise subnets.
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Summarizing EIGRP routes for IP
Manual summary routes are configured on a
per-interface basis by:
Router(config-if)#ip summary-address eigrp
autonomous-system-number ip-address
mask administrative-distance
By default, EIGRP summary routes have an
administrative distance of 5. The value can
range between 1 and 255.
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END
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