Cisco OSPF Command and Configuration Handbook (paperback)

2017-07-07 02:10:07

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7-10 distribute-list access-list- name out routing-process

Syntax Description:

access-list-name: ” Standard IP access list name that determines which routes redistributed into OSPF will be prevented from being installed in the OSPF database. This has the effect of preventing the blocked redistributed routes from being advertised to OSPF neighbors.

routing-process ” The routing process that has been redistributed into OSPF (RIP, IGRP, EIGRP, OSPF, BGP, EGP, static, or connected).

Purpose: To prevent routes, redistributed into OSPF from another routing rocess, from being installed in the OSPF database.

Initial Cisco IOS Software Release: 11.2

Configuration Example: Preventing Routes Redistributed into OSPF from Being Installed in the OSPF Database

In Figure 7-6, Router B is receiving the EIGRP routes 10.1.1.4/30, 3.3.3.0/24, and 4.4.4.0/24 from Router C. Router B is redistributing EIGRP into OSPF and these routes will be advertised to Router A as OSPF external type 2 routes. Start by configuring Routers A, B, and C as shown in the listing that follows the figure.

Figure 7-6. A Distribute List/Named Access List Prevents Redistributed Routes from Being Installed in the OSPF Database

Router A interface Loopback0 ip address 1.1.1.1 255.255.255.255 ! interface Ethernet0/0 ip address 172.16.1.1 255.255.255.0 ! router ospf 1 network 172.16.1.0 0.0.0.255 area 0 ________________________________________________________________________________ Router B interface Loopback0 ip address 2.2.2.2 255.255.255.255 ! interface Ethernet0 ip address 172.16.1.2 255.255.255.0 ! interface Serial1 bandwidth 64 ip address 10.1.1.5 255.255.255.252 clockrate 64000 ! router eigrp 1 network 10.0.0.0 ! router ospf 1 redistribute eigrp 1 subnets network 2.2.2.2 0.0.0.0 area 1 network 172.16.1.0 0.0.0.255 area 0 ________________________________________________________________________________ Router C interface Loopback0 ip address 3.3.3.3 255.255.255.0 ! interface Loopback1 ip address 4.4.4.4 255.255.255.0 ! interface Serial0 ip address 10.1.1.6 255.255.255.252 ! router eigrp 1 network 3.0.0.0 network 4.0.0.0 network 10.0.0.0 no auto-summary

Verify that Routers A and B have established a FULL OSPF neighbor relationship.

rtrA# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/BDR 00:00:30 172.16.1.2 Ethernet0/0 rtrB# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/DR 00:00:38 172.16.1.1 Ethernet0

Verify that Router B is receiving routes 3.3.3.0/24 and 4.4.4.0/24 from Router C via EIGRP.

rtrB# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR Gateway of last resort is not set 2.0.0.0/32 is subnetted, 1 subnets C 2.2.2.2 is directly connected, Loopback0 3.0.0.0/24 is subnetted, 1 subnets D 3.3.3.0 [90/40640000] via 10.1.1.6, 00:38:45, Serial1 4.0.0.0/24 is subnetted, 1 subnets D 4.4.4.0 [90/40640000] via 10.1.1.6, 00:38:45, Serial1 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, Ethernet0 10.0.0.0/30 is subnetted, 1 subnets C 10.1.1.4 is directly connected, Serial1

Verify that Router A is receiving routes 10.1.1.4/30, 3.3.3.0/24, and 4.4.4.0/24 from Router B as OSPF external type 2 routes.

rtrA# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 , E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets C 1.1.1.1 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets O IA 2.2.2.2 [110/11] via 172.16.1.2, 00:27:48, Ethernet0/0 3.0.0.0/24 is subnetted, 1 subnets O E2 3.3.3.0 [110/20] via 172.16.1.2, 00:05:41, Ethernet0/0 4.0.0.0/24 is subnetted, 1 subnets O E2 4.4.4.0 [110/20] via 172.16.1.2, 00:27:48, Ethernet0/0 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, Ethernet0/0 10.0.0.0/30 is subnetted, 1 subnets O E2 10.1.1.4 [110/20] via 172.16.1.2, 00:05:42, Ethernet0/0

For illustrative purposes, verify that these three routes are in the OSPF database on Router B.

rtrB# show ip ospf database external OSPF Router with ID (2.2.2.2) (Process ID 1) Type-5 AS External Link States LS age: 441 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 3.3.3.0 (External Network Number ) Advertising Router: 2.2.2.2 LS Seq Number: 80000001 Checksum: 0x3F50 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0 LS age: 1788 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 4.4.4.0 (External Network Number ) Advertising Router: 2.2.2.2 LS Seq Number: 80000003 Checksum: 0x1773 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0 LS age: 459 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 10.1.1.4 (External Network Number ) Advertising Router: 2.2.2.2 LS Seq Number: 80000001 Checksum: 0xD7B3 Length: 36 Network Mask: /30 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0

Modify the configuration on Router B to allow only the installation of the EIGRP route 4.4.4.0/24 into the OSPF database using a named access list.

Router B router ospf 1 redistribute eigrp 1 subnets network 2.2.2.2 0.0.0.0 area 1 network 172.16.1.0 0.0.0.255 area 0 distribute-list filter-eigrp out eigrp 1 ! ip access-list standard filter-eigrp permit 4.4.4.0 0.0.0.255

Verification

Verify that the EIGRP routes 10.1.1.4/30 and 3.3.3.0/24 have been blocked from entering the OSPF database on Router B.

rtrB# show ip ospf database external OSPF Router with ID (2.2.2.2) (Process ID 1) Type-5 AS External Link States LS age: 419 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 4.4.4.0 (External Network Number ) Advertising Router: 2.2.2.2 LS Seq Number: 80000004 Checksum: 0x1574 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0

Verify that Router A is receiving only one external type 2 route from Router B.

rtrA# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets C 1.1.1.1 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets O IA 2.2.2.2 [110/11] via 172.16.1.2, 00:38:45, Ethernet0/0 4.0.0.0/24 is subnetted, 1 subnets O E2 4.4.4.0 [110/20] via 172.16.1.2, 00:38:45, Ethernet0/0 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, Ethernet0/0

Troubleshooting

Step 1. Verify that there is a neighbor relationship between the OSPF routers using the show ip ospf neighbor command.

Step 2. Verify that the distribute-list command is referencing the correct access list name and routing process.

Step 3. Verify the syntax of the named access list.

Step 4. Verify that the intended routes are in the OSPF database using the command show ip ospf database external.