Preventing Sub-optimal routing in mutual redistribution - Part 1

Following from my previous redistribution post. Let's explore a few ways we can prevent sub-optimal routing from redistributing routes at multiple routers.

Consider the topology from my previous redistribution post and let's continue.

Basically there are 3 ways to prevent sub-optimal routing.

1. Manually setting Administrative distance
2. Route tags
3. Using metric and metric types

We will explore all 3. But first, let's get a baseline of what is happening. For starting configs consider my previous redistribution post.

Router R1

R1#show ip route 10.0.0.0/27 is subnetted, 2 subnets C 10.0.0.0 is directly connected, FastEthernet1/0 O 10.0.0.32 [110/2] via 10.0.0.3, 00:34:18, FastEthernet1/0 192.168.1.0/27 is subnetted, 2 subnets O E2 192.168.1.32 [110/20] via 10.0.0.3, 00:34:18, FastEthernet1/0 C 192.168.1.0 is directly connected, FastEthernet1/1 R1#traceroute 192.168.1.34 Translating "192.168.1.34" Type escape sequence to abort. Tracing the route to 192.168.1.34 1 10.0.0.3 56 msec 52 msec 20 msec 2 10.0.0.34 44 msec * 28 msec 3 192.168.1.34 64 msec * 60 msec R1# !--R1 is taking a suboptimal path to get to R4 fast1/1.

First let's manipulate the Administrative distance to fix this.

Router R1

R1#show run | sec ospf ! router ospf 1 log-adjacency-changes redistribute rip subnets network 10.0.0.0 0.0.0.255 area 0 distance ospf external 125 !--Any external route in OSPF is now assigned a AD of 125 which is worse than the RIP AD (120) So we are now using the RIP learned route from R4 to get to R4 fast1/1. Note that you can add an access list at the end of the distance command to fine tune the external routes which will be assigned this new AD. ! R1#show ip route 10.0.0.0/27 is subnetted, 2 subnets C 10.0.0.0 is directly connected, FastEthernet1/0 O 10.0.0.32 [110/2] via 10.0.0.3, 00:02:01, FastEthernet1/0 192.168.1.0/27 is subnetted, 2 subnets R 192.168.1.32 [120/1] via 192.168.1.4, 00:00:07, FastEthernet1/1 C 192.168.1.0 is directly connected, FastEthernet1/1 R1#traceroute 192.168.1.34 Translating "192.168.1.34" Type escape sequence to abort. Tracing the route to 192.168.1.34 1 192.168.1.4 68 msec * 44 msec R1# !

Another way to manipulate the AD with a certain router ID.

Router R1

R1#show ip ospf database Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 192.168.1.0 10.0.0.1 1083 0x80000002 0x000E3B 0 192.168.1.32 192.168.1.33 1642 0x80000002 0x005355 0 Notice that the adv router id is 192.168.1.33. R1#show run | sec ospf ! router ospf 1 log-adjacency-changes redistribute rip subnets network 10.0.0.0 0.0.0.255 area 0 distance 125 192.168.1.33 0.0.0.0 !--Any external route originated by router id 192.168.1.33 (R2) is assigned an AD for 125. You can also apply access-list to fine tune the external routes from 192.168.1.33 (R2) which will get the AD of 125. ! R1#show ip route 10.0.0.0/27 is subnetted, 2 subnets C 10.0.0.0 is directly connected, FastEthernet1/0 O 10.0.0.32 [110/2] via 10.0.0.3, 00:02:01, FastEthernet1/0 192.168.1.0/27 is subnetted, 2 subnets R 192.168.1.32 [120/1] via 192.168.1.4, 00:00:07, FastEthernet1/1 C 192.168.1.0 is directly connected, FastEthernet1/1 R1#traceroute 192.168.1.34 Translating "192.168.1.34" Type escape sequence to abort. Tracing the route to 192.168.1.34 1 192.168.1.4 68 msec * 44 msec R1# !

Part 2 shows how we can prevent sub-optimal routing by using route tags.