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Hi everybody.
How is everyone doing ?
Based on my book , the best path algorithm is:
1) weight
2) Local-pref
3) Locally injected routes
4) AS path
5)origin
6)Med
7) Neighbor type ( ibgp or ebgp)
9) oldest ebgp route
10) Lowest neighbor BGP RID
11) Lowest neighbor Ip address.
That is what my book says" If the BGP best path algorithm does not choose a best path by step 8( IGP metric to next hop)
,the routes which still tie for being best path will be allowed into ip routing table up to the number defined by the BGP maximum-paths router subcommand.
I created a set up where R1 will receive an bgp update about 10.0.0.0 from its ebgp peer R2 and R3 . Both are connected to R1 via serial interface as shown below:
Here R1 is receiving update 10.0.0.0 from R2 and R3 which tie at step 8( as per above list), R1 should install both routes to 10.0.0.0 in routing table however this is not what happened.
No attempt was made to manipulate path attributes; all path attributes are at default. With all default path attributes and both next hop 199.199.199.2 (R2) and 200.200.200.3 (R3) for 10.0.0.0 are located off R1 's s0/0 and s0/1 respectively, the best path algorithm should find the routes to prefix 10.0.0.0 at step 8 tie( the igp metric to reach next hop).
As per my book's excerpt above, if I configure " maximum-path 2" under bgp at R1, both these routes should be allowed into routing table. However , when I configured the above command, only one route was entered the routing table.
Below is R1's the configuration:
R1's config:
interface Serial0/0
ip address 199.199.199.1 255.255.255.0
clock rate 2000000
interface Serial0/1
ip address 200.200.200.1 255.255.255.0
clock rate 2000000
router bgp 3
no synchronization
bgp log-neighbor-changes
neighbor 199.199.199.2 remote-as 1
neighbor 200.200.200.3 remote-as 2
maximum-paths 2
no auto-summary
R1#show ip bgp
Network Next Hop Metric LocPrf Weight Path
* 10.0.0.0 199.199.199.2 0 0 1 i
*> 200.200.200.3 0 0 2 i
R1#show ip bgp
BGP table version is 3, local router ID is 200.200.200.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 10.0.0.0 199.199.199.2 0 0 1 i
*> 200.200.200.3 0 0 2 i
R1#show ip bgp 10.0.0.0
BGP routing table entry for 10.0.0.0/8, version 3
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Multipath: eBGP
Advertised to update-groups:
1
1
199.199.199.2 from 199.199.199.2 (199.199.199.2)
Origin IGP, metric 0, localpref 100, valid, external
2
200.200.200.3 from 200.200.200.3 (200.200.200.3)
Origin IGP, metric 0, localpref 100, valid, external, best
R1#show ip route
C 200.200.200.0/24 is directly connected, Serial0/1
C 199.199.199.0/24 is directly connected, Serial0/0
B 10.0.0.0/8 [20/0] via 200.200.200.3, 00:16:01
R1#
Here R1 has installed route 200.200.200.3 to reach 10.0.0.0. In order to confirm if the routes are really tied at step 8 and R1 is using the sep 9 ( oldest ebgp route) to break the tie, i will reset R1 and R3 neighbor relationship which will cause the R1 to install 199.199.199.2 (R2) to reach 10.0.0.0 because it will be older route than via R3.
R1#clear ip bgp 200.200.200.3
*Mar 1 00:34:50.575: %BGP-5-ADJCHANGE: neighbor 200.200.200.3 Down User reset
*Mar 1 00:34:52.031: %BGP-5-ADJCHANGE: neighbor 200.200.200.3 Up
R1#show ip route
C 200.200.200.0/24 is directly connected, Serial0/1
C 199.199.199.0/24 is directly connected, Serial0/0
B 10.0.0.0/8 [20/0] via 199.199.199.2, 00:00:18
R1#
It proves that indeed the routes tie at step 8 because R1 uses step 9 ( oldest ebgp route) to break the tie.
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The question is why R1 is not installing both routes i.e 199.199.199.2, 200.200.200.3 to reach 10.0.0.0 because they both tie at step 8 and according to my book all routes that tie at step 8 will be allowed into the routing table provided that " maximum-path" is configured accordingly.
Thanks and happy new year.
