Discussion

As an ISP, you might use static routes to represent your customer links and networks,
especially for small-size customers that you connect to with just a single link
or a low-speed link. In these cases, you want to redistribute these static links into
your OSPF network. Create a simple routing policy that accepts all static routes and
then apply it as an export policy to OSPF. This adds Type 7 (AS External) routes to
the link-state database:
aviva@RouterG> show ospf database
OSPF link state database, area 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router 192.168.17.1 192.168.17.1 0x800001c6 1633 0x2 0x13b3 60
Router 192.168.18.1 192.168.18.1 0x8000015f 384 0x2 0xf833 60
Router *192.168.19.1 192.168.19.1 0x800000f3 11 0x2 0x180b 48
Network 10.0.0.2 192.168.17.1 0x80000004 1633 0x2 0xbf17 32
Network 10.0.1.1 192.168.18.1 0x80000004 84 0x2 0xc014 32
Network 10.0.2.1 192.168.18.1 0x800000b4 684 0x2 0x3aea 32
OSPF AS SCOPE link state database
Type ID Adv Rtr Seq Age Opt Cksum Len
Extern *0.0.0.0 192.168.19.1 0x80000001 11 0x2 0x4b0a 36
Extern *192.168.42.1 192.168.19.1 0x80000001 11 0x2 0xbf01 36
The only drawback is that if the link between the customer and the ISP goes down,
the network still appears to be reachable to the world, and you would need to stop
advertising it if this occurred. However, short links between a customer and an ISP
should rarely or never fail.
12.11 Adjusting OSPF Link Costs
Problem
You want to direct traffic within an OSPF area toward a particular interface.
Solution
Increase the cost on one interface to have OSPF use a different interface:
[edit protocols ospf area 0.0.0.0]
aviva@RouterJ# set interface fe-1/0/0.0 metric 3
Discussion
When choosing paths to a destination, OSPF uses the one with the lowest metric. By
default, all links faster than 100 Mbps have a metric of 1. This recipe shows how to
adjust the metric in our three-router backbone area so that traffic from RouterH
always goes through RouterG instead of directly to RouterJ. You might want to do
this if the interface between RouterH and RouterJ is congested with other traffic (see
Figure 12-1).
406 | Chapter 12: OSPF
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.