From: Bob Woodson (woodso_b@xxxxxxxxx)
Date: Fri Aug 17 2001 - 14:07:24 GMT-3
Xuan,
Local_Pref is a well-known discretionary attribute
that is included in all update messages that a given
BGP speaker sends to the other BGP speakers located in
its own AS. A BGP speaker shall not include this
attribute in update messages that it sends to bgp
speakers located in a neighboring AS. If it is
contained in an update message that is received from a
BGP speaker which is not located in the same AS as the
receiving speaker, then this attribute shall be
ignored by the receiving speaker. Caslow Ch 14 p.456
--- 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 - 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 ?
> **Please
> read:http://www.groupstudy.com/list/posting.html
>
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