EIGRP FAQ

EIGRP FAQ

(12/03/2003)

Questions:

1) What does EIGRP stand for?
2) What is EIGRP?
3) Where can I find additional resources besides this crummy FAQ?
4) Can I see an example configuration of a gateway to IGRP?
5) How do I debug?
6) How do I redistribute static routes?
7) How do I redistribute RIP?

Answers:

1) What does EIGRP stand for?

Enhanced Interior Gateway Routing Protocol

EIGRP is a Cisco-proprietary routing protocol that routers use to share routing information. It provides both link state (calculated metrics using line quality, hops & more...) and distance vector (hop counting) protocol properties to minimize the negatives of either protocol. Hop counting is less resource intensive, but can pick sub-optimal routes over slow lines. Link-state can calculate better routes, but requires more calculations on the part of the router.
Cisco points out that there are 4 key features of EIGRP:

fast convergence (router stores neighbor's tables & can query for additional routes)
Supports variable length subnet masking (VLSM), which allows for masks other that class A, B, C
Supports partial updates to transmit routing changes only, not the entire table
Supports Appletalk (RTMP), IPX (RIP & SAP), and IP (RIP, OSPF, IS-IS, EGP, BGP)

EIGRP relies on several technologies:

Neighbor Discovery/Recovery: learn routes via directly connected interfaces. Uses periodic hello packets to discover failures.
Reliable Transport Protocol (RTP): delivers EIGRP packets. Can send either unicast or multicast packets (which do not require a response and can be sent to many peers at once for faster convergence)
Diffusion Update Algorithm (DUAL): a finite state machine that tracks routes from all neighbors. Selects feasible successor (least-cost neighbor) as the next hop. Tries to avoid recomputations, but if no sucessor is available, then recomputation occurs. DUAL makes routing decisions and stores in a routing table.
IP-EIGRP module: encapsulates EIGRP information in IP and transmits. Responsible for redistributing routes from other protocols. There are other modules as well for non-IP traffic.

Concepts:

Neighbor & Topology tables: each router keeps tables of its neighbors and networks behind them:

neighbor	protocol	metric	destination
1.1.1.1	IP	1	10.1.1.0
1.1.1.1	IPX	1	...
1.1.1.2	IP	2	10.1.1.0

route states:
- passive: no recomputation necessary. Passive as long as feasible sucessor is available
- active: recomputation going on.
route tagging
- Internal: learned via an EIGRP router using the same Autonomous System (AS) number as you.
- External: routes learned via other EIGRP AS, redistributed from other protocols like RIP, OSPF, or static routes

3) Additional Resources:

4) Can I see an example configuration of a gateway to IGRP?

router(s)	config	routing table
A-D	router igrp 68 network 10.1.0.0	n/a
E	router igrp network 10.1.0.0 router eigrp network 10.1.0.0	10.1.0.0/16 7 subnets I 10.1.1.0 E4 I 10.1.2.0 E4 I 10.1.3.0 E4 I 10.1.4.0 E4 C 10.1.5.0 E4 C 10.1.6.0 F0 D 10.1.7.0 F0
F	router eigrp network 10.1.0.0	D EX 10.1.1.0 F0 D EX 10.1.2.0 F0 D EX 10.1.3.0 F0 D EX 10.1.4.0 F0 D 10.1.5.0 F0 C 10.1.6.0 F0 C 10.1.7.0 E0

5) How do I debug?

sh ip eigrp topo

6) How do I redistribute static routes?

router(s)	config
A	router eigrp 68 network 10.0.0.0 network 11.0.0.0 default metric 1000 100 1 1 1500 redistribute static ip route 9.1.0.0 255.255.0.0 e1 ip route ... int s1 ip summary-address eigrp 68 9.0.0.0 255.0.0.0 int s0 ip summary-address eigrp 68 9.0.0.0 255.0.0.0 (summarize static routes)
B	router eigrp 68 network 11.0.0.0
C	router eigrp 68 network 11.0.0.0 network 12.0.0.0

If I wanted to filter what was injected from static routes, I could modify router A accordingly:

router eigrp 68
network 10.0.0.0
network 11.0.0.0
redistribute static
default metric 1000 100 1 1 1500
distribute-list 2 out static # filters what is imported from static routes to EIGRP
[ distribute-list 2 out s0 ] # would filter just static routes out s0 interface
no auto-summary

# this ACL filters what is imported from static routes to EIGRP
access-list 2 permit 1.1.1.0 0.0.0.255
access-list 2 deny 0.0.0.0
access-list 2 permit any

ip route 9.1.0.0 255.255.0.0 e1
ip route ...

7) How do I redistribute RIP?

Router A:

router rip
network 10.0.0.0
redistribute eigrp 68  # import EIGRP routes to RIP
default-metric 2   # default RIP metric for non-RIP routes
passive-interface s0  # don't broadcast RIP on s0

router eigrp 68
network 10.0.0.0
redistribute rip   # import RIP routes to EIGRP
default-metric 1544 100 255 1 1500
#  1544 = kb/s minimum bandwidth
#  100  = route delay in 10s of milliseconds
#  255  = (0-255) reliability of connection, 255=100%
#  1    = effective bandwidth
#  1500 = MTU in bytes
distribute-list 1 in   # filter what comes in from RIP
passive-interface e0

access-list 1 permit ip 10.0.1.0 0.0.0.255
....
access-list 1 permit ip 10.0.10.0 0.0.0.255
access-list 1 deny ip

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