From: Peter Rybaczyk (psrsam@xxxxxxxxxxxxx)
Date: Fri Aug 17 2001 - 22:15:22 GMT-3
Xuan,
Take a look at this. I've effectively duplicated the scenario from Doyle II, p.
189 but with different addresses and host names. When you view output with show
ip
bgp, the routes received from external neighbors do not show a local preference
,
but when you specify a route after the show ip bgp (like in the 2nd trace below
),
the local preference shows up for both routes, the one received from external a
nd
the one received from internal neighbor. So if the local preference is really
associated with the route from the external neighbor but just not displayed, th
en
the integrity of the route selection process is there, and the tie breaker is #
6
in your comment, (EBGP> confed. EBGP > IBGP.
If anyone has insight on this, please share.
Thanks.
Peter
R5#show ip bgp
BGP table version is 14, local router ID is 172.16.90.1
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
*> 172.16.70.0/24 192.68.5.1 0 3 i
* i 192.168.100.1 0 100 0 3 i
*> 172.16.220.0/24 192.68.5.1 0 3 i
* i 192.168.100.1 0 100 0 3 i
*> 192.68.6.0 0.0.0.0 0 32768 i
*> 199.1.0.0/22 192.68.5.1 0 3 i
* i 192.168.100.1 100 0 3 i
*> 199.1.1.0 192.68.5.1 0 3 i
* i 192.168.100.1 0 100 0 3 i
*> 199.1.2.0 192.68.5.1 0 3 i
* i 192.168.100.1 0 100 0 3 i
*> 199.1.3.0 192.68.5.1 0 3 i
* i 192.168.100.1 0 100 0 3 i
R5#sh ip bgp 199.1.3.0
BGP routing table entry for 199.1.3.0/24, version 14
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Flag: 0x210
Advertised to non peer-group peers:
192.168.100.1
3
192.68.5.1 from 192.68.5.1 (172.16.2.254)
Origin IGP, localpref 100, valid, external, best
3
192.168.100.1 from 192.168.100.1 (192.168.100.1)
Origin IGP, metric 0, localpref 100, valid, internal
R5#
Xuan.Sun@Seagate.com wrote:
> 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 - interna
l
> 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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