you pretty much understand how it works. You might be muddying the waters a bit by bringing BGP into the picture
comment below: ""John Matney"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > > I've been reading the Cisco CCNP Cert Guide in partial preparation for > the BSCI exan and I've come across a bit in the Policy Routing section > that I just don't understand. > > The text states: > > "Policy routing does not allow traffic sent into another autonomous > system to take a different path from the one that would have been chosen > by that autonomous system." (pp. 551) CL: sure. makes sense. I'm not sure why the authors would take this tack, as policy routing applies only to inbound traffic. at best, it can set next hop, as you note. But nothing that the policy sets is untouchable by other routers, same autonomous ystem or not. > > ~From the reading, I understand that policy routing is configured on an > inbound interface and can filter on either source or both source and > destination addresses. PR, via a route map, can set properties such as > precedence, QoS and next-hop. All of these items only really have > relevance on the router in which policy routing is being done. In other > words, once the router policy routes the packet and specifies, for > instance, the next-hop interface. Now, if that next-hop router chooses > to drop, fragment or otherwise mangle the packet so be it, the first > router has no control over it anymore, its done its job. CL: yep > > So then, how does this quote apply? Perhaps, I'm completely missing the > point (wouldn't be the first time). A router can only do what its > configured to do. If I tell a packet to take path a to get to network b > but network b would perfer its incoming traffic to come in via path c, > the most network a can do to prevent this is to drop incoming traffic > via path a. Correct? CL: yep >Even if we were running a EGP such as BGP4 and the > distant router had a MED set to perfer path c, I could still push > packets via path a given that I knew it existed. CL: you can send a packet anyplace. that doesn't mean the destination router has to accept it. CL: but mixing policy routing and BGP in your mind is probably not a good idea. the BGP settings that are done via route-maps associated with neighbor statements apply to BGP routing information. Policy routing applies to data packets, not to routing protocol information. Does that make sense? CL: examples: router bgp 9902 neighbor 1.1.1.1 remote-as 9990 neighbor 1.1.1.1 route-map take_my_sttings out neighbor 1.1.1.1 route-map screw_your_settings in as opposed to interface s 0 ip policy route-map zzyzx > > Make sense? I'm a bit confused as to what the authors are getting to in > this passage. Could someone help? CL: HTH > > Thanks, > John > > > - -- > http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x88EE7695 > Key fingerprint = DBD7 6AE2 E7BE 1572 B245 BF54 4913 C85A 88EE 7695 > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.1.90-nr1 (Windows XP) > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org > > iD8DBQE9YZ1hSRPIWojudpURAoAQAKCMOZu+TQcZOSW39mqtZooDzRGoBwCgm+Ti > YMQGvYkbcXWMn/IhQZTmpnk= > =hAME > -----END PGP SIGNATURE----- Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=51705&t=51689 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
