BGP routing principle
BGP (Border Gateway Protocol) is a dynamic route discovery protocol between autonomous systems. It is currently widely used in the Internet. The BGP protocol has very rich attributes and the routing process is relatively complicated. In a network using CISCO equipment, the BGP routing protocol performs routing according to the following rules.
1. The path with the highest WEIGHT value takes precedence. WEIGHT is a CISCO proprietary parameter, and it is only valid for the local router configured with this parameter, and cannot be passed. By default, locally originated paths have the same WEIGHT value (32768), and all other paths have a WEIGHT value of 0.
2. LOCAL-PREF value (local priority). In CISCO IOS, the default value of LOCAL-PREF is 100. This attribute can only be passed within this AS.
3. Based on the origin (originaTIon) to evaluate the route, the local origin route of the router has priority. The lower priority order is: default-originate (configured for each neighbor), default-informaTIon-originate (configured for each address cluster), network, redistribute, aggregate-address
4. Assess the length of the AS-PATH, the shortest path in the AS-PATH list takes precedence. But you can ignore this step by configuring bgp bestpath as-path ignore (hide command).
5. Evaluate the origin attribute of the route, and the route with the lowest origin attribute value takes precedence. IGP: origin value = 0; EGP: origin value = 1; INCOMPLETE: origin value = 3
6. Evaluate MED. The path with the smallest MED value wins. This attribute can be passed out of AS. By default, the comparison will only be performed if the first AS of the two paths is the same. If configured: bgp always-compare-med, then MED will be compared for all paths, regardless of whether they come from the same AS.
7. The EBGP path takes precedence over the IBGP path.
8. BGP prefers the path with the lowest IGP metric to the next hop of BGP.
9. If maximum-paths [ibgp] n is configured, n is between 2-6 and there are multiple equal cost paths, then BGP will insert up to n received paths in the IP routing table. This activates the BGP multipath load sharing feature. If the optional keyword ibgp is not used, then the multipath feature applies only to the ebgp path. When this option is not activated, its default value is 1.
10. When both paths are external paths, BGP will preferentially select the path received first (the oldest path).
11. BGP preferentially selects the route from the BGP router with the lowest router ID.
12. If the originating router ID or router ID of multiple paths are the same, BGP will prefer the path with the shortest CLUSTER-LIST length.
13. BGP prefers the path from the lowest neighbor address.
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