9.4
Various Multihomed Connections to a Single Provider and Multiple Providers
9.4.2
Default only, one primary, and one backup link scenario
In the Figure, AS3 is multihomed to AS1. AS3 is not learning any BGP routes from AS1 and is sending its own routes via BGP. RTA will be running defaults toward AS1, with the New York link being the primary link and the San Francisco link being the secondary link. The following policies should apply:
  1. Outbound traffic from AS3 should always go on the New York link unless that link fails, in which case it should switch to the other link.

    This can be achieved by configuring two static routes in RTA pointing the defaults toward the two links. The default via the New York link will be set with a lower distance to be more preferred.

  2. Inbound traffic toward AS3 should always come on the New York link unless that link fails, in which case it should switch to the other link.

    This can be achieved by having RTA send different metrics toward AS1 on both links, with a lower metric on the New York link. This way, inbound traffic coming from AS1 will always come via the New York link.

  3. Prevent any BGP updates from coming into AS3.

This can be achieved by having AS3 configure a route map or distribute list that will block all incoming BGP routing updates. Usually, the provider (AS1, in this case) will not send you any updates per your request. Nevertheless, you should always protect the AS against the unknown. The provider could make a mistake and send you all his routes, and your AS would be vulnerable.

Note: Click on topology to view command outputs.

In RTA configuration, AS3 has used static routes to configure defaults toward AS1. The 0/0 toward RTD is given a distance of 40, lower than the distance of 50 toward RTC. The New York link will act as primary. Alternatively, AS3 could have accepted a single entry from AS1 and configured that entry as being the default.

Route maps SETMETRIC2 and SETMETRIC1 are used to set the outbound metric to 50 toward RTD and 100 toward RTC, respectively. Inbound traffic will prefer the NY link. Route map BLOCK is used to block all incoming BGP updates from AS1.

The RTA IP routing table shows how the default route is set. Note that distance 40 is being preferred over distance 50 for the 0/0 route, and the gateway of last resort is pointing to next-hop 192.68.9.2.

The RTC BGP table shows that AS3 is always accessed via the RTD-RTA link because of the lower metric 50. Prefix 172.16.220.0/24 can be reached via IBGP and EBGP. The IBGP route has been chosen as the best route. Note in this table that next hop for RTC to reach prefix 172.16.220.0/24 is 192.68.6.1, because RTD has configured its neighbor connection with RTC using the next-hop-self neighbor command.

Lab Activity
  In this lab, you will learn how to configure a multihomed connection to a single provider using default only routing, one primary, and one backup link.