Re: BGP behavior for tracing routes

From: Fahad Khan (fahad.khan@gmail.com)
Date: Sun Aug 17 2008 - 07:39:29 ART

Dear Hammad,

-When working with BGP , "sh ip bgp" can be considered to be more important
than "sh ip route".
-Ping is the bidirectional technology; source and destination must have
routes to eachother for a successfull ping.
-BGP is said be an Application layer protocol needed for policy routing ,
normally not for layer 3 reachibility.
-You must know about the importance of "next-hop reachibility" and how you
can make next-hop reachable with different techniques.
-learn the modification of next-hop ip in cases of EBGP and IBGP peerings.

HTH

On 8/17/08, Zealist Hamamd <zealist@gmail.com> wrote:
>
> Hi experts,
>
> It might be silly ques but i m askin :)
> I was practicing "BGP Bestpath Selection - Weight*"* Lab in IE Tech
> workbook for BGP.
> While doing that i noticed that If u ping or trace routes learned via BGP
> and when no other protocol is running u must have to do that with source
> add
> of an interface also advertised in BGP other wise it wont work. while in
> other protocols if u have a route in ur routing table u can ping it anyway.
> So why is this behavior is in BGP?? or it is commone for for all
> protocols??
>
> Also
>
> topology is this R1,R3 r in AS1 and R4,R5 in AS2
>
> R1-->R3 ibgp ------ R3-->R5 ebgp --- R5-->R4 ibgp and R1-->R4 ebgp
>
> now ques is that at R4 there isnt a route for link bw (R1 and R3) and
> similarly on R5 and even then when we ping from R3 we get response(if we
> give source interface of R3 which is
> in BGP). So isn't it be v difficult while we r going to troubleshoot BGP in
> real world where there r countless networks?? shld we always consider
> next-hop only in routing table??
>
>
> i m giving output of sh cmds
> FOR R1
> ------------
>
> R1#sh ip rout
> Gateway of last resort is not set
>
> 155.1.0.0/24 is subnetted, 4 subnets
> C 155.1.146.0 is directly connected, FastEthernet1/0
> C 155.1.13.0 is directly connected, Serial0/0
> B 155.1.5.0 [20/0] via 155.1.146.4, 00:13:45
> B 155.1.37.0 [200/0] via 155.1.13.3, 00:11:34
> R1#
> FOR R3:
> --------------
>
> R3#sh ip rou
> Gateway of last resort is not set
>
> 155.1.0.0/24 is subnetted, 4 subnets
> C 155.1.13.0 is directly connected, Serial0/0
> C 155.1.0.0 is directly connected, Serial0/1
> B 155.1.5.0 [200/0] via 155.1.13.1, 00:20:14
> C 155.1.37.0 is directly connected, Loopback0
> R3#
>
>
> For R4:
> ---------
> R4#sh ip rou
> Gateway of last resort is not set
>
> 155.1.0.0/24 is subnetted, 4 subnets
> C 155.1.146.0 is directly connected, FastEthernet0/0
> B 155.1.5.0 [200/0] via 155.1.45.5, 00:14:44
> C 155.1.45.0 is directly connected, Serial1/0
> B 155.1.37.0 [20/0] via 155.1.146.1, 00:09:21
> R4#
> For R5:
> ---------
>
> R5#sh ip route
> Gateway of last resort is not set
>
> 155.1.0.0/24 is subnetted, 4 subnets
> C 155.1.0.0 is directly connected, Serial0/0
> C 155.1.5.0 is directly connected, Loopback1
> C 155.1.45.0 is directly connected, Serial0/1
> B 155.1.37.0 [20/0] via 155.1.0.3, 00:08:47
> R5#
>
>
> Regards,
>
> --
> zealist@gmail.com
>
>
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-- 
*FAHAD KHAN

BE Computer Systems NED,

CCNA,CCDA,CCNP,FOUNDFE,CLSE,QOS,JNCIA,JNCIS,MCP,CCIE  (Written)

Systems Support Engineer, Premier Systems (Pvt) limited,

Karachi, Pakistan

92-321-2370510.*

Blogs and organic groups at http://www.ccie.net

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