From: Xuan.Sun@xxxxxxxxxxx
Date: Fri Aug 17 2001 - 13:58:31 GMT-3
When BGP selects the route, it follows the rules
1) Weight
2) Local_Pref
3) AS_Path
4) ORIGIN
5) MED
6) EBGP> confed. EBGP > IBGP
7) IGP metric
8) router ID
When you look at the Page 189, Fig 3-11 in Doyle's Routing TCP/IP II. The
Diamond learns 192.168.193.0 from Mammoth, a EBGP peer and Sugarbush, a
IBGP peer. From the "sh ip bgp" in Diamond, it takes the Mammoth's path, a
route from EBGP peer. Why ?
The Local_Pref for IBGP peer has a default 100. But the Local_Pref for EBGP
is empty. What does the empty mean ? Will Local_Pref (Step 2) ignore if
there is a router from EBGP peer. So it reaches Step 6. The route from EBGP
peer is selected.
I tried to align the result of "sh ip bgp". You will see any route from
EBGP peer is empty. You will see BGP take the route from EBGP peer.
Diamond#sh ip bgp
BGP table version is 29, local router ID is 192.168.1.246
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
* i192.168.100.0 192.168.1.238 0 100 0 300 i
*> 192.168.1.242 0 0 300 i
*> 192.168.192.0 192.168.1.245 0 0 100 ?
* i 192.168.1.238 100 0 100 ?
*> 192.168.192.0/21 192.168.1.245 0 100 i
* i 192.168.1.238 100 0 100 i
*> 192.168.193.0 192.168.1.245 0 0 100 ?
* i 192.168.1.238 100 0 100 ?
*> 192.168.194.0 192.168.1.245 0 0 100 ?
* i 192.168.1.238 100 0 100 ?
*> 192.168.195.0 192.168.1.245 0 0 100 ?
* i 192.168.1.238 100 0 100 ?
*> 192.168.196.0 192.168.1.245 0 0 100 ?
* i 192.168.1.238 100 0 100 ?
*> 192.168.197.0 192.168.1.245 0 0 100 ?
* i 192.168.1.238 100 0 100 ?
*> 192.168.198.0 192.168.1.245 0 0 100 ?
* i 192.168.1.238 100 0 100 ?
*> 192.168.199.0 192.168.1.245 0 0 100 ?
* i 192.168.1.238 100 0 100 ?
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