BGP uses a hierarchical process to evaluate multiple sources for the same network prefix.This process determines the best source path to use in reaching a given network prefix. BGP 4 can support multiple parallel paths that have the same path parameters. If the BGP speaker is configured to support multiple paths, the router will load balance connections across all paths.The selection process evalu- ates the neighbor advertising a route and the AS path attributes for a particular network prefix to determine the best path to use in the local routing table and the forwarding database.
This process is outlined in Table 10.6 with a brief explanation of the decision criteria used at each step, as well as the analysis performed by the BGP system.
The variables involved in this process, and their use, was explained in the previous section.The selection process flows from one step to the next to determine the best value for a given parameter.The selection process can exit at any point to determine the best path.
Table 10.6BGP Path Selection Process
Path Selection Step Path Selection Criteria
1. Next Hop Comparison Next hop destination must be reachable from the local routing table.
2. Synchronization If synchronization is enabled, the route must Comparison be in the IGP. This applies only to IBGP
derived routes.
3. Weight Comparison Highest value is better.
4. Local Preference Highest value is better.
Comparison
5. Local Origination Prefer routes that are locally originated with Comparison the network n.n.n.nor aggregaten.n.n.n.
BGP commands to IGP redistribution.
6. AS Path Comparison Prefer the fewest number of AS hops (AS_PATH).
7. Origin Comparison Prefer the route with the best origin type.
Order of preference:
1 – IGP 2 – EGP
3 – Incomplete
Continued
8. Multiexit Discriminator Select the path with the lowest MED value.
Comparison This applies only when the neighboring AS is the same, that is, two connections between different ASs.
9. BGP Source Comparison Select an EBGP path versus an IBGP path.
Select confederation EBGP versus confedera- tion IBGP.
10. IGP Metric to Next_Hop Select the shortest IGP path to the BGP Comparison NEXT_HOP (lowest IGP metric).
11. BGP Multipath support “N” paths will be installed for load balancing if this feature is enabled. The paths are external and originate from the same AS.
12. ROUTER_ID Comparison Select the path from the BGP speaker with the highest ROUTER_ID value.
BGP Path Selection Example
The best way to understand the BGP path selection process is to use an actual BGP network prefix and follow the decision process used to determine the best path. For example, we can look at a screen capture from the BGP speaker “route- server.cerf.net“ and follow the path selection process for the network prefix 208.188.42.0 (see Figure 10.15).We will need to refer back to Figure 10.11 to see the parameters for this BGP speaker and the BGP neighbors. Figure 10.16 depicts the routing table to the NEXT_HOP sources for this network prefix.
Table 10.7 lists all path selection values from the four sources of this network prefix, and identifies the best path selected.
Figure 10.15 gives the AS_PATH, origin, IGP metric value, local preference value, NEXT_HOP identifier, ROUTER_ID, and the path that this router believes is the best source.The BGP speaker sees the network 208.188.42.0 as a supernet of 208.188.0.0. As discussed in the CIDR sidebar, a supernet is several consecutively numbered networks that are advertised as a CIDR block using a network and mask. In this case, the network is 208.188.0.0 and the mask is 255.255.192.0, or, in CIDR terminology, this is a /18 block of addresses.
Table 10.6Continued
Path Selection Step Path Selection Criteria
Table 10.7BGP Path Selection Process for 208.188.0.0 Path Source Source SourceSource Evaluation Selection 134.24.88.55198.32.176.25192.41.177.69192.157.69.5Result Step 1. Next_Hop ReachableReachableReachableReachableAll reachable via static Comparisonroutes (Fig. 10.16) 2. Synchro-N/AN/AN/AN/AN/A with EBGP nization Comparison 3. Weight N/AN/AN/AN/ANone (Fig. 10.11) Comparison 4. Local 100100100100All equal (Fig. 10.15) Preference Comparison 5. Local IGPIGPIGPIGPAll equal (Fig. 10.15) Origination Comparison 6. AS Path 1740 2548 1740 2548 1740 2548 1740 2548 All have 3 AS hops Comparison7132713271327132(Fig. 10.15) 7. Origin IGPIGPIGPIGPAll equal (Fig. 10.15) Comparison 8. Multiexit N/AN/AN/AN/ANone (Fig. 10.15) Discriminator Comparison 9. BGP EBGPEBGPEBGPEBGPAll equal (Fig. 10.15) Source Comparison Continued
Table 10.7Continued Path Source Source SourceSource Evaluation Selection 134.24.88.55198.32.176.25192.41.177.69192.157.69.5Result Step 10. IGP 0000All equal (Fig. 10.16) Metric to Next_Hop Comparison 11. BGP N/AN/AN/AN/ANone (Fig. 10.11) Multipath Support 12. ROUTER_134.24.88.55198.32.176.25192.41.177.69192.157.69.5Largest ID ID Com-198.32.176.25 parison(Fig. 10.15) Best Path*
Figure 10.16 depicts the routing information used to reach the NEXT_HOP sources for the network 208.188.0.0. In this case, we can use router 134.24.88.25, 198.32.176.25, 192.41.177.69, or 192.157.69.5 to get to the network
208.188.0.0.The route to all sources is known via static IP routing, and all met- rics are identical.
Figure 10.15BGP Route Information for 208.188.42.0
Figure 10.16Routing Table Entry for NEXT_HOP Sources for 208.188.42.0
Following the path selection process for the network prefix 208.188.0.0, we find that all selection parameters are identical among the four BGP speakers except for the ROUTER_ID. In this case, the ROUTER_ID 198.32.176.25 is the highest value and thus the best path to use for forwarding packets. Figure 10.17 confirms the next-hop forwarding destination for packets destined for the network 208.188.0.0 /18.
Figure 10.17 depicts the information as it appears in the IP routing table, which will be used to develop the forwarding information to advertise to other BGP speakers. It also depicts the router 198.32.176.25 as the next hop along the IP path to reach 208.188.0.0/18 IP addresses.
Figure 10.17Routing Table Entry for 208.188.42.0
There are several relevant sources of information that can be accessed by anyone connected to the Internet.
www.rsng.net This is the location of the Route Server Next Generation Project (RSng), which has a wealth of information
Internet Information Sources for External BGP and the Internet