Content

| 9.9 |  | Redundancy, Symmetry, and Load Balancing |
| 9.9.3 |  | Default routing in BGP networks |

It has been seen that default routes minimize the size of a routing table. They can also provide networks with redundancy in the event of failures and connectivity interruptions. BGP, like almost all IGPs, can distribute a default route. To provide redundancy, default information could be received from multiple BGP sources. In a BGP system, the Local Preference attribute can be manipulated on the various default routes. This is so that one default route is identified as a primary, the highest Local Preference, and others are kept as backups. If the primary fails, a backup route with the next highest preference can take its place.

In Figure , RTA is connected to RTB and is learning about two default routes, one by way of 1.1.1.1 and a second by way of 2.2.2.2. By using the Local Preference attribute, 1.1.1.1 can be given preference and make it the primary default route. RTA will use 2.2.2.2 as a default only after 1.1.1.1 fails.

In Figure , the same behavior can be achieved as long as IBGP is running inside the AS. The Local Preference attribute, which is exchanged between IBGP peers, will determine the primary and backup links.

It is important to control default information in BGP because improper configuration can cause serious Internet routing problems. For example, a misconfigured BGP speaker could end up flooding a default route to all of its neighbors and quickly find itself consumed with default routed traffic from surrounding autonomous systems. To protect against misadvertisements, the Cisco IOS provides a way to target default information at a specific neighbor by using the

default-originate option with the neighbor command:

> RTC(config)#router bgp 3
> RTC(config-router)#neighbor 172.16.20.1 remote-as 1
> RTC(config-router)#neighbor 172.16.20.1 default-originate

If RTC is configured as shown in this configuration, it will send default information only to the specified neighbor. If a BGP router is to be configured to advertise a default to all of its peers, use the

network command shown as follows:

Note: Both neighbors, 172.16.20.1 and 172.17.1.1, will receive a default route from RTC, if it has one to send.

> RTC(config)#router bgp 3
> RTC(config-router)#neighbor 172.16.20.1 remote-as 1
> RTC(config-router)#neighbor 172.17.1.1 remote-as 2
> RTC(config-router)#network 0.0.0.0

Many network administrators choose to filter dynamically learned default routes to avoid situations in which traffic ends up where it is not supposed to be. Without dynamically learned default routes, a router must be statically configured with default information. Statically configured default routes typically provide more control over routing within an AS.

| Lab Activity e-Lab Activity: Default Routing in BGP Networks This lab configures a router to target default information to a specific neighbor so that it doesn't end up flooding a default route to all its neighbors.