Information Technology Reference
In-Depth Information
Example 5-22
New BGP Path Information with MED (Continued)
Network Next Hop Metric LocPrf Weight Path
*> 10.1.0.0/16 172.17.3.254 2297856 0 65100 65101 i
* 172.17.3.253 2323456 0 65100 65101 i
* 10.2.0.0/16 172.17.3.254 2323456 0 65100 65102 i
*> 172.17.3.253 2297856 0 65100 65102 i
*> 10.3.0.0/16 0.0.0.0 0 32768 i
*> 10.4.0.0/16 172.17.3.254 2297856 0 65100 65104 i
* 172.17.3.253 2323456 0 65100 65104 i
172.17.3.254
2297856
172.17.3.253
2297856
172.17.3.254
2297856
This allows router R10 to intelligently choose what traffic to send to each core router. Traffic
is sent to the correct core router to provide optimal routing, and traffic is distributed across
the core routers.
A similar scenario is seen in AS 65102, which has two core routers, each with differing
costs to reach remote locations. This is another example in which advertising IGP metrics
via BGP MEDs can optimize traffic flow. In general, because AS 65100 is under the same
administrative control, the MED settings can be trusted. Setting the MED values to the IGP
metric for all advertisements to the regional BGP speakers helps ensure optimal routing, as
a general design guideline.
Failure and Recovery Scenarios
A couple of interesting failure scenarios should be examined with this architecture. Of
primary interest is the failure of the regional border routers, core routers, and core links.
The regional IGP process handles failures that occur within the region.
There are multiple aspects to network reconvergence following the failure of a regional
border router. The first aspect is the default route that is originated. The regional IGP
process handles the removal of the now-defunct default route. Figure 5-16 shows a failure
in AS 65102 of the regional border router R7.
The other aspect is tearing down the eBGP session between the core router and the failed
regional border router and withdrawing the prefixes from the core. This affects the inbound
traffic flow. Until these prefixes are removed from the BGP RIB on the core router and are
withdrawn from the rest of the core, traffic is black-holed on the core router that is peering
with the regional border router. If the interface on the core router transitions down, the BGP
process tears down the BGP session with R7 and removes the path information for that peer.
