From: Peter van Oene (pvo@xxxxxxxxxxxx)
Date: Wed Jul 24 2002 - 12:43:35 GMT-3
>To: Hunt Lee <ciscoforme3@yahoo.com.au>
>From: Peter van Oene <pvo@usermail.com>
>Subject: Re: BGP Next Hop?
>
>This is normal behavior. When multiple EBGP paths exist, Cisco enables a
>next-hop self functionality toward the IBGP mesh. See the following link
>for more info (step nine in the best path algorithm)
>
>Pete
>
>http://www.cisco.com/warp/public/459/25.shtml
>
>
>
>
>At 12:48 AM 7/25/2002 +1000, you wrote:
>>Can someone please clarify this for me?
>>
>> RTB
>> /
>>RTD -- RTA
>> \
>> RTC
>>
>>RTB & RTC are both in AS1
>>RTA & RTD are both in AS2
>>
>>RTD's interface (connecting to RTA) :- 25.25.25.1
>>RTA's interface (back to RTD) :- 25.25.25.2
>>RTA's interface (to RTB) :- 10.1.1.1
>>RTB's interface (back to RTA) :- 10.1.1.2
>>RTA's interface (to RTC) :- 10.1.2.1
>>RTC's interface (back to RTA) :- 10.1.2.2
>>
>>RTA is connected with RTB & C with Serial connections, while
>>connecting with RTD with Ethernet
>>
>>Both RTB & RTC are advertising a network 172.17.1.0/24 (directly
>>connected via Eth from both routers). Also, in order to do BGP load
>>balancing to that network, I have use the "maximum-paths 6" command
>>at RTA.
>>
>>RouterA#sh ip route
>>Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B -
>>BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
>>N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
>>E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
>>i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter
>>area
>> * - candidate default, U - per-user static route, o - ODR
>> P - periodic downloaded static route
>>
>>Gateway of last resort is not set
>>
>> 172.17.0.0/24 is subnetted, 1 subnets
>>B 172.17.1.0 [20/0] via 10.1.2.2, 00:36:24
>> [20/0] via 10.1.1.2, 00:36:18
>> 25.0.0.0/24 is subnetted, 1 subnets
>>C 25.25.25.0 is directly connected, Ethernet0
>> 10.0.0.0/30 is subnetted, 2 subnets
>>C 10.1.2.0 is directly connected, Serial1
>>C 10.1.1.0 is directly connected, Serial0
>>RouterA#
>>
>>Ok, here's what I am very confused. When I lookup RTD's BGP table
>>(shown below), for the BGP route 172.17.1.0/24, instead of the
>>Next-hop being 10.1.1.2 or 10.1.2.2, it is 25.25.25.2. Since I
>>thought that in EBGP peering, the next hop is the IP address of the
>>neighbor that announced the route, while IBGP peering preserve the
>>Next-hop attribute learned from EBGP peers. So in the case, if I
>>haven't use next-hop-self on RTA, I would think the Next-Hop in RTD's
>>BGP table would be either 10.1.1.2 or 10.1.2.2....
>>
>>RouterD#sh ip bgp
>>BGP table version is 9, local router ID is 25.25.25.1
>>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
>>*>i10.1.1.0/30 25.25.25.2 100 0 1 i
>>*>i10.1.2.0/30 25.25.25.2 100 0 1 i
>>*>i25.25.25.0/24 25.25.25.2 0 100 0 i
>>*>i172.17.1.0/24 25.25.25.2 0 100 0 1 i
>>RouterD#
>>
>>
>>RouterA#sh ip bgp
>>BGP table version is 12, local router ID is 25.25.25.2
>>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
>>*> 10.1.1.0/30 10.1.2.2 0 1 i
>>* 10.1.1.2 0 0 1 i
>>*> 10.1.2.0/30 10.1.1.2 0 1 i
>>* 10.1.2.2 0 0 1 i
>>*> 25.25.25.0/24 0.0.0.0 0 32768 i
>>*> 172.17.1.0/24 10.1.1.2 0 0 1 i
>>* 10.1.2.2 0 0 1 i
>>RouterA#
>>
>>
>>RouterB#sh ip bgp
>>BGP table version is 7, local router ID is 172.17.1.1
>>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
>>*> 10.1.1.0/30 0.0.0.0 0 32768 i
>>*>i10.1.2.0/30 172.17.1.2 0 100 0 i
>>* i25.25.25.0/24 10.1.2.1 0 100 0 2 i
>>*> 10.1.1.1 0 0 2 i
>>* i172.17.1.0/24 172.17.1.2 0 100 0 i
>>*> 0.0.0.0 0 32768 i
>>RouterB#
>>
>>
>>RouterC#sh ip bgp
>>BGP table version is 7, local router ID is 172.17.1.2
>>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
>>*>i10.1.1.0/30 172.17.1.1 0 100 0 i
>>*> 10.1.2.0/30 0.0.0.0 0 32768 i
>>* i25.25.25.0/24 10.1.1.1 0 100 0 2 i
>>*> 10.1.2.1 0 0 2 i
>>* i172.17.1.0/24 172.17.1.1 0 100 0 i
>>*> 0.0.0.0 0 32768 i
>>RouterC#
>>
>>
>>Thanks
>>Hunt Lee
>>
>>http://www.sold.com.au - SOLD.com.au
>>- Find yourself a bargain!
This archive was generated by hypermail 2.1.4 : Sat Sep 07 2002 - 19:36:42 GMT-3