Re: BGP question.

  • To: "'Scott Huddle'" <huddle at mci dot net>, "'Jumpot Phuritatkul'" <howdy at nectec dot or dot th>
  • Subject: Re: BGP question.
  • From: Barry Raveendran Greene <bgreene at cisco dot com>
  • Date: Thu, 19 Jun 1997 17:32:59 +0800
  • Cc: "'apops at apnic dot net'" <apops at apnic dot net>
  • Encoding: 111 TEXT
  • Sender:
    • Hello Jumpot,
      This one is a little difficult. Your objective is the following:
      	1) A to go to C via path (1) A-B-C
      	2) B to go to C via path (2) B-D-C
      Here's the problem. You have one forward table (i.e. sh ip route on a Cisco 
      router). This will hold the route selected from the routing protocols. You 
      can manipulate the route selection process (i.e. like using the local 
      preference in BGP), but you will still only get the "best" entry in the 
      forward table selected for use. When you have two "equal" cost entries in 
      the forward table, Cisco "round robins" packets evenly over the two equal 
      entries (this does not mean load balance since packets are not of equal 
      So all packets coming into a router will use the forward table to determine 
      which interface to "forward" the packets.
      Now for the possible "hack" that may work (not guaranteed) if you have a 
      7500 w/VIP cards. With Cisco's VIP architecture and distributed switching, 
      forward tables are pushed out to the VIP processors. If you place the 
      serial connection for "A" and "C" on the same VIP card. Put "B" on another 
      VIP card with the LAN interfaces from B's site on that same VIP card the 
      serial link from "D" (preferable on a different bus). Packets coming in 
      from A will prefer the interface on the same VIP card - C. Packet from B's 
      LAN interfaces will prefer the D interface.
      I'm not sure that will work as described. Plus, this only takes care of 
      traffic flows coming from A (downstream). For Internet traffic in Asia, 
      that's 60%-80% for the traffic.
      If you have a 7500 w/VIP card and you wish to pursue this hack, let me 
      Jumpot Phuritatkul wrote:
      >But from my intention, B will announce C and D networks by originating
      >from B because I don't want A to see AS C and D. So  that A will see C
      >and D networks by only AS-path B .
      >>From your solution, at B will get C's routes with 2 AS-path (C and D-C)
      >and B will choose D-C to be the best path because of setting localprefs.
      >Consequently, B will announce the best path of C to A with originating 
      from B
      >then A will see C'routes from B which was only one AS-path B and it is the 
      >best  path that B send to A that is path (2) but I want A to go to B via
      >path (1). How can I do ?
      >				(1)
      >	     [A] --------[B]----------------[C]
      >        	          |    (2)           |
      >                	  +------[D]---------+
      >On Wed, 18 Jun 1997, Scott Huddle wrote:
      >> If B localprefs D higher it will prefer that route to C
      >> (and yet still have it if D goes down)  B can announce
      >> C and D to A and it (A) will select BC over
      >> BDC by virtue of path length.
      >> Did I miss something?
      >> -scott
      >> At 06:00 PM 6/18/97 +0700, Jumpot Phuritatkul wrote:
      >> >Hi,
      >> >	Would someone give me a solution for this problem?
      >> >
      >> >				(1)		
      >> > 	[A] --------[B]----------------[C]
      >> >		     |	  (2)		|
      >> >		     +------[D]---------+
      >> >
      >> >	There are 4 networks of A, B, C and D.
      >> >	A,B,C and D have their own Autonomous System Numbers.
      >> >A is upstream provider of B. B want to announce B,C and D's networks 
      >> >AS origin of B to A . I want A to go to C via path (1) A-B-C , and B to 
      >> >to C via path (2) B-D-C . How can I set BGP command at B,C and D?
      >> >Please help me solve this problem.
      >> >
      >> >Regards,
      >> >Jumpot Phuritatkul
      >> >Network Technology Lab.
      >> >NECTEC, Thailand .
      >> >
      >> >
      >> >To unsubscribe: send "unsubscribe" to apops-request at apnic dot net
      >> >
      >> >
      Barry Raveendran Greene                    |       ||        ||        |
      Senior Consultant                          |       ||        ||        |
      Corporate Consulting Engineering           |      ||||      ||||       |
      tel: +65 738-5535 ext 235                  |  ..:||||||:..:||||||:..   |
      e-mail: bgreene at cisco dot com                  |  c i s c o S y s t e m s  |
      To unsubscribe: send "unsubscribe" to apops-request at apnic dot net