IGP Cost

BGP prefers the route with the smallest IGP cost during BGP route selection.

This rule helps BGP to choose the route with the smallest cost to its recursive next hop address quickly. If the bestroute igp-metric-ignore command is configured, BGP does not compare the IGP cost. In Figure 1, OSPF runs in AS 65001, an EBGP peer relationship is established between Device E and Device A and between Device E and Device B, and an IBGP peer relationship is established between Device A and Device C, between Device A and Device D, between Device B and Device C, and between Device B and Device D; Device E is configured to import routes (1.1.1.9/32 for example) from AS 100 to BGP.

Figure 1 Networking diagram with IGP cost configurations

Run the display bgp routing-table [ ip-address ] command on Device C and Device D to check the configurations. Device C is used as an example.

# Display the routing table of Device C.

[~DeviceC] display bgp routing-table
 BGP Local router ID is 10.1.1.1
 Status codes: * - valid, > - best, d - damped, x - best external, a - add path,
               h - history,  i - internal, s - suppressed, S - Stale
               Origin : i - IGP, e - EGP, ? - incomplete
 RPKI validation codes: V - valid, I - invalid, N - not-found

 Total Number of Routes: 4
      Network            NextHop        MED        LocPrf    PrefVal Path/Ogn

 *>i  1.1.1.9/32         10.1.5.2        0          100        0      100i
 * i                     10.1.6.2        0          100        0      100i
 *>i  10.1.5.0/30        10.1.3.2        0          100        0      i
 *>i  10.1.6.0/30        10.1.2.2        0          100        0      i

The preceding command output shows that two routes 1.1.1.9/32 are available in the routing table of Device C and that Device C selects the route learned from Device A.

[~DeviceC] display bgp routing-table 1.1.1.9
 BGP local router ID : 10.1.1.1
 Local AS number : 65001
 Paths:   2 available, 1 best, 1 select
 BGP routing table entry information of 1.1.1.9/32:
 From: 10.1.3.2 (2.2.2.9)
 Route Duration: 00h00m44s
 Relay IP Nexthop: 10.1.3.2
 Relay IP Out-Interface: GigabitEthernet0/1/8
 Original nexthop: 10.1.5.2
 Qos information : 0x0
 AS-path 100, origin igp, MED 0, localpref 100, pref-val 0, valid, internal, best, select, active, pre 255
 Not advertised to any peer yet

 BGP routing table entry information of 1.1.1.9/32:
 From: 10.1.2.2 (10.1.2.2)
 Route Duration: 00h00m39s
 Relay IP Nexthop: 10.1.1.2
 Relay IP Out-Interface: GigabitEthernet0/1/00/1/0
 Original nexthop: 10.1.6.2
 Qos information : 0x0
 AS-path 100, origin igp, MED 0, localpref 100, pref-val 0, valid, internal, pre 255, IGP cost 2, not preferred for IGP cost
 Not advertised to any peer yet

The preceding command output shows that the route with next hop address 10.1.6.2 is ignored because its IGP cost is larger than that of the other route. Table 1 describes the attribute comparison of the routes learned from Device A and Device B.

**Table 1** Attribute comparison of the routes learned from Device A and Device B.
Route Attribute	Route Learned from Device A	Route Learned from Device B	Comparison
PrefVal	0	0	The same.
Local_Pref	100	100	The same.
Route type	Learned from a peer	Learned from a peer	The same.
AIGP	-	-	The same.
AS_Path	100	100	The same length.
Origin	IGP	IGP	The same.
MED	0	0	The same.
Peer type	IBGP	IBGP	The same.
IGP cost	-	2	The route learned from Device A is optimal. NOTE: If a BGP route carries no IGP cost value, BGP considers its IGP cost to be 0. If no IGP routes are used during BGP peer relationship establishment or the costs of used IGP routes are 0, the IGP cost is not displayed in the display bgp routing-table ip-address command output.