From: Ken.Farrington@barclayscapital.com
Date: Tue Sep 30 2003 - 10:38:06 GMT-3
Great stuff. Cheers guys
-----Original Message-----
From: Reisner, Tim [mailto:TR126568@exchange.DAYTONOH.NCR.com]
Sent: 30 September 2003 14:09
To: Ken.Farrington@barclayscapital.com; ccielab@groupstudy.com
Subject: RE: BGP Experts Required here.
Ken,
Check this out in the Cisco doc:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr
_c/ipcprt2/1cfbgp.htm#1000898
The section on How BGP Selects Paths:
This note is in the list:
----------------------------------------------------------------------------
---- The most recent Internet Engineering Task Force (IETF) decision regarding BGP MED assigns a value of infinity to the missing MED, making the route lacking the MED variable the least preferred. The default behavior of BGP routers running Cisco IOS software is to treat routes without the MED attribute as having a MED of 0, making the route lacking the MED variable the most preferred. To configure the router to conform to the IETF standard, use the bgp bestpath med missing-as-worst router configuration command.---------------------------------------------------------------------------- ----
HTH
Tim Reisner CCIE #10418 NCR Corp. High Availability Networking Global Support Center Phone: (763)535-3274 Cell: (763)227-3179 Email: Tim.Reisner@NCR.com
-----Original Message----- From: Ken.Farrington@barclayscapital.com [mailto:Ken.Farrington@barclayscapital.com] Sent: Tuesday, September 30, 2003 7:21 AM To: ccielab@groupstudy.com Subject: BGP Experts Required here.
Hello all, I have a question re the setup below.
I have two routers peering eBGP with an external AS 350 and iBGP between the routers which (just for info) are separate locations.
Now if I receive a route (say 168.1.0.0) that has a metric inbound on both routers (router76 route for this network has metric of 33 ,and Router12 has metric for this route of 42) it looks as if the router that has the lowest metric from the route, does not accept the iBGP route from the other router with the higher metric, but the router with the higher metric accepts the iBGP route from the router with the lower metric.
If I receive a route where the metric is not set and all attributes are the same, it send each other the iBGP route (like 168.41.0.0) and uses it's local exit point via eBGP to that network.
Please can someone confirm that this is normal operation for iBGP.
Many thx indeed, Ken
\ 168.1.0.0 168.224.0.0 168.41.0.0 / \ External / \ AS 350 / \__________________________________/ |192.168.42.253 |192.168.41.254 | | |eBGP |eBGP | | | | | AS 300 | +--------+ +--------+ |Router76|---------------------|Router12| +--------+ iBGP +--------+ 192.168.42.253 192.168.41.253
on Router76
router bgp 300 neighbor 150.5.36.129 remote-as 300 neighbor 192.168.42.254 remote-as 350
Router76>sh ip int brief Interface IP-Address OK? Method Status Protocol Ethernet0 unassigned YES unset administratively down down Ethernet1 192.168.42.253 YES NVRAM up up Loopback0 150.5.32.65 YES NVRAM up up
Network Next Hop Metric LocPrf Weight Path ---------------------------------------------------------------------------- --------------------------- *> 168.1.0.0 192.168.42.254 33 0 350 i This entry only appears once ---------------------------------------------------------------------------- --------------------------- *>i168.224.0.0 192.168.41.254 31 100 0 350 i * 192.168.42.254 42 0 350 i ---------------------------------------------------------------------------- --------------------------- * i168.41.0.0 192.168.41.254 100 0 350 i This entry appears on both *> 192.168.42.254 0 350 i This entry appears on both ---------------------------------------------------------------------------- ---------------------------
On Router12
! router bgp 300 neighbor 150.5.32.65 remote-as 300 neighbor 192.254.41.254 remote-as 350 !
Router12>sh ip int brief Interface IP-Address OK? Method Status Protocol Ethernet0 unassigned YES unset administratively down down Ethernet1 192.168.41.253 YES NVRAM up up Loopback0 150.5.36.129 YES NVRAM up up
Network Next Hop Metric LocPrf Weight Path * 168.1.0.0 192.168.41.254 42 0 350 i *>i 192.168.42.254 33 100 0 350 i ---------------------------------------------------------------------------- --------------------------- *> 168.224.0.0 192.168.41.254 31 0 350 i This entry only appears once ---------------------------------------------------------------------------- --------------------------- *> 168.41.0.0 192.168.41.254 0 350 i This entry appears on both * i 192.168.42.254 100 0 350 i This entry appears on both ---------------------------------------------------------------------------- ---------------------------
________________________________________________________________ Ken Farrington Global Networks, Barclays Capital, 5 The North Colonnade, Canary Wharf, London, E14 4BB * Tel : 020 7773 3550 * Mob : 07768-866655 * ken.farrington@barcap.com
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