Eigrp Prefix List

Question 1

1. Eigrp Filter Prefix List
2. Eigrp Prefix List Alpha
3. Cisco Eigrp Distribute List Prefix

Configuration Example: Inbound and Outbound Distribute List Route Filters. Figure 4-5 shows the network topology for the configuration that follows, which demonstrates how to configure inbound and outbound route filters to control routing updates using the commands covered in this chapter. Assume that all basic configurations and EIGRP routing have been configured correctly. However, when the following distribute-list with prefix-list 'EIGRP-ROUTES' is applied only the subnet 3.3.3.3/32 is learned through the subinterface f0/0.3 and the physical interface f0/0. You can test with many forms, changing the permit/deny of prefix-lists and the application to the distribute-list command.

Question 2

Explanation

Use the external keyword along with the redistribute command to redistribute OSPF external routes.

In order to use an prefix-list in a 'match' statement, we have to use the command 'match ip address prefix-list …'. The syntax of a 'match' statement is as follows:

match ip address {access-list-number [access-list-number… | access-list-name…] | access-list-name [access-list-number…| access-list-name] | prefix-list prefix-list-name [prefix-list-name…]}

Reference: https://www.cisco.com/c/en/us/td/docs/ios/iproute_pi/command/reference/iri_book/iri_pi1.html

Question 3

Explanation

The first statement of the above route-map (route-map BGP-TO-OSPF deny 10) will prevent network 172.16.1.0/24 from being redistributed into OSPF.

Question 4

Question 5

Explanation

It seems there is a loop because of mutual redistributions among RIP, OSPF and EIGRP domains. So we should filter out the prefix 10.1.1.0/24 when redistributed from OSPF to EIGRP (the second redistribution point) to prevent routing loop.

Question 6

Explanation

Only the subnet that matches prefix-list OSPF-TAG-PRF-1 will be redistributed into OSPF (as indicated by 'route-map OSPF-TAG-1 permit 10'). This subnet must match the prefix-list OSPF-TAG-PRF-1 so it must be 10.2.0.0/18 to 10.2.0.0/24. Only the subnet 10.2.2.0/24 matches this requirement.

Note: The prefix-list 'OSPF-TAG-PRF' denies a route (deny 10.1.0.0/16 le 24) is used in a route-map deny clause (route-map OSPF-TAG-1 deny 5) so according to Cisco:

'If you use an ACL in a route-map permit or deny clause, and the ACL denies a route, then the route-map clause match is not found and the next route-map clause is evaluated.'

Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/49111-route-map-bestp.html

Therefore in this case the first route-map clause will be ignored so we don't need to care about it.

Question 7
Unclouded cloud manager app.

Explanation

If you configure the redistribution of OSPF into BGP without keywords, only OSPF intra-area and inter-area routes are redistributed into BGP, by default.

You can redistribute both internal and external (type-1 & type-2) OSPF routes via this command: 'Router(config-router)#redistribute ospf 1 match internal external 1 external 2'

Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5242-bgp-ospf-redis.html

Question 8

Explanation

In this topology, we are doing mutual redistribution at multiple points (between OSPF and EIGRP on R3 & R4), which is a very common cause of network problems, especially routing loops so you should use route-map to prevent redistributed routes from redistributing again into the original domain.

In this question, route-map is also used for this purpose. In particular, the route-map 'SET-TAG' is used to prevent any routes that have been redistributed into EIGRP from redistributed again into OSPF domain by tagging these routes with tag 1:

These routes are prevented from redistributed again by route-map FILTER_TAG by denying any routes with tag 1 set:

ip prefix-list list-name [seq seq-value] {deny | permit prefix/prefixlength}[ge ge-value] [le le-value]
ip default-network network-number

Eigrp Filter Prefix List

router eigrp
network network-id [wildcard-mask]
passive-interface iface
neighbor ip-address outgoing-iface
metric weights 0 k1 k2 k3 k4 k5
redistribute route-method
eigrp router-id 32-bit-rid
eigrp stub [option]
offset-list acl {in|out} offset iface
timers active-time minutes
maximum-paths number
variance multiplier
distribute-list sacl {in|out} iface
distribute-list prefix name {in|out} iface
distribute-list route-map name {in|out} iface

interface
bandwidth kbps
delay usec
ip hello-interval eigrp asn seconds
ip hold-time eigrp asn seconds
ip authentication mode eigrp asn md5
ip authentication key-chain eigrp asn name
no ip split-horizon eigrp asn
ip bandwidth-percent eigrp asn percent ip summary-address eigrp asn prefix subnet-mask

The first statement of the above route-map (route-map BGP-TO-OSPF deny 10) will prevent network 172.16.1.0/24 from being redistributed into OSPF.

Question 4

Question 5

Explanation

It seems there is a loop because of mutual redistributions among RIP, OSPF and EIGRP domains. So we should filter out the prefix 10.1.1.0/24 when redistributed from OSPF to EIGRP (the second redistribution point) to prevent routing loop.

Question 6

Explanation

Only the subnet that matches prefix-list OSPF-TAG-PRF-1 will be redistributed into OSPF (as indicated by 'route-map OSPF-TAG-1 permit 10'). This subnet must match the prefix-list OSPF-TAG-PRF-1 so it must be 10.2.0.0/18 to 10.2.0.0/24. Only the subnet 10.2.2.0/24 matches this requirement.

Note: The prefix-list 'OSPF-TAG-PRF' denies a route (deny 10.1.0.0/16 le 24) is used in a route-map deny clause (route-map OSPF-TAG-1 deny 5) so according to Cisco:

'If you use an ACL in a route-map permit or deny clause, and the ACL denies a route, then the route-map clause match is not found and the next route-map clause is evaluated.'

Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/49111-route-map-bestp.html

Therefore in this case the first route-map clause will be ignored so we don't need to care about it.

Question 7
Unclouded cloud manager app.

Explanation

If you configure the redistribution of OSPF into BGP without keywords, only OSPF intra-area and inter-area routes are redistributed into BGP, by default.

You can redistribute both internal and external (type-1 & type-2) OSPF routes via this command: 'Router(config-router)#redistribute ospf 1 match internal external 1 external 2'

Reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5242-bgp-ospf-redis.html

Question 8

Explanation

In this topology, we are doing mutual redistribution at multiple points (between OSPF and EIGRP on R3 & R4), which is a very common cause of network problems, especially routing loops so you should use route-map to prevent redistributed routes from redistributing again into the original domain.

In this question, route-map is also used for this purpose. In particular, the route-map 'SET-TAG' is used to prevent any routes that have been redistributed into EIGRP from redistributed again into OSPF domain by tagging these routes with tag 1:

These routes are prevented from redistributed again by route-map FILTER_TAG by denying any routes with tag 1 set:

ip prefix-list list-name [seq seq-value] {deny | permit prefix/prefixlength}[ge ge-value] [le le-value]
ip default-network network-number

Eigrp Filter Prefix List

router eigrp
network network-id [wildcard-mask]
passive-interface iface
neighbor ip-address outgoing-iface
metric weights 0 k1 k2 k3 k4 k5
redistribute route-method
eigrp router-id 32-bit-rid
eigrp stub [option]
offset-list acl {in|out} offset iface
timers active-time minutes
maximum-paths number
variance multiplier
distribute-list sacl {in|out} iface
distribute-list prefix name {in|out} iface
distribute-list route-map name {in|out} iface

interface
bandwidth kbps
delay usec
ip hello-interval eigrp asn seconds
ip hold-time eigrp asn seconds
ip authentication mode eigrp asn md5
ip authentication key-chain eigrp asn name
no ip split-horizon eigrp asn
ip bandwidth-percent eigrp asn percent ip summary-address eigrp asn prefix subnet-mask

Eigrp Prefix List Alpha

show ip protocols

show ip route
show ip route longer-prefixes
show ip eigrp neighbors [detail]
show ip eigrp topology [all-links]
show ip eigrp topology [prefix/[length]
show ip eigrp traffic
show ip eigrp accounting
show ip eigrp interfaces [detail] [iface]
show key chain
show clock (for authentication)
show route-map name
debug eigrp packet
clear ip eigrp [asn] neighbors

Cisco Eigrp Distribute List Prefix

R2# show ip protocols
Routing Protocol is 'eigrp 1'
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 1
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
7.0.0.0
192.168.1.0
Routing Information Sources:
Gateway Distance Last Update
192.168.1.1 90 00:01:11
Distance: internal 90 external 170

R2#show interfaces
FastEthernet0/0 is up, line protocol is up
Hardware is DEC21140, address is ca01.1360.0000 (bia ca01.1360.0000)
Internet address is 192.168.1.2/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Half-duplex, 100Mb/s, 100BaseTX/FX
..

R2# show ip eigrp interfaces
IP-EIGRP interfaces for process 1
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Fa0/0 1 0/0 71 0/1 280 0
Lo7777 0 0/0 0 0/1 0 0

R2# show ip eigrp traffic
EIGRP-IPv4 Traffic Statistics for AS(15)
Hellos sent/received: 2564306/7677183
Updates sent/received: 201/519
Queries sent/received: 71/171
Replies sent/received: 174/221
Acks sent/received: 845/693
SIA-Queries sent/received: 0/0
SIA-Replies sent/received: 0/0
Hello Process ID: 180
PDM Process ID: 179
Socket Queue: 0/10000/6/0 (current/max/highest/drops)
Input Queue: 0/2000/6/0 (current/max/highest/drops)