Information Technology Reference
In-Depth Information
This can have an interesting effect on traffic patterns in the network. An example is sending
traffic for prefix 10.4.0.0/16, which is originated by AS 65104 via R8 instead of R9. This
means that the traffic pattern is R10
R11. This is only
slightly suboptimal in this case; however, it is the concept that is of interest, as shown in
Example 5-20.
R8
R9
R11 instead of R10
R9
Example 5-20 Suboptimal Path Selection Favoring R8
R10#show ip bgp
BGP table version is 10, local router ID is 172.16.10.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 10.1.0.0/16 172.17.3.254 0 65100 65101 i
*> 172.17.3.253 0 65100 65101 i
* 10.2.0.0/16 172.17.3.254 0 65100 65102 i
*> 172.17.3.253 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 0 65100 65104 i
*> 172.17.3.253 0 65100 65104 i
172.17.3.253
172.17.3.253
172.17.3.253
Two issues are involved. The first issue is that all traffic is destined for a single core router,
R8. The second issue is that traffic might be suboptimal. The first issue can be resolved by
using eBGP multipath on R10. This solution does not address the concern of suboptimal
routing, however. The other solution is to provide R10 with the topological information
associated with each prefix. Setting an outbound route map on R8 and R9 to set the MED
to the IGP metric provides the topological information that R10 needs to make the optimal
decision. Example 5-21 shows the outbound MED configuration on R8 and R9. Example 5-22
shows the new BGP path information with MED.
Example 5-21 Outbound MED Configuration on R8 and R9
router bgp 65100
neighbor 172.17.3.1 route-map SET_MED out
!
route-map SET_MED permit 10
set metric-type internal
!
Example 5-22 New BGP Path Information with MED
R10#show ip bgp
BGP table version is 11, local router ID is 172.16.10.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
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