Posts Tagged ‘routing’

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Configuring IPv6 OSPF routing in Cisco IOS

Wednesday, September 10th, 2008

Hi guys! it’s time for me to write about configuration of OSPF IPv6 routing in Cisco IOS.
I will use example addresses from my workplace for this tutorial.
This is really simple, first configure the IPv6 addresses on the interfaces.

Router1
Router1(config)# interface fastethernet 0/0
Router1(config-if)#ipv6 address 2001:1ad8::1/126

Router2
Router2(config)#int fa 0/0
Router2(config-if)#ipv6 address 2001:1ad8::2/126

Verify the IPv6 connectivity with ping:
Router2#ping ipv6 2001:1ad8::1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:1AD8::1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/10/20 ms

Set a nullroute to redistribute to IPv6 OSPF, and configure IPv6 OSPF routing on Router1:
Router1(config)#ipv6 route 2001:1ad8:500::/64 null 0
Router1(config)# interface fa 0/0
Router1(config-if)#ipv6 ospf 1 area 0
Router1(config-if)#ipv6 router ospf 1
Router1(config-rtr)#redistribute static

Do the same thing on Router2, except for the static route and redistribution.
Router2(config)#int fastethernet 0/0
Router2(config-if)#ipv6 ospf 1 area 0

Now verify the IPv6 OSPF router neighborship
Router2#show ipv6 ospf neighbor
Neighbor ID     Pri   State           Dead Time   Interface ID    Interface
172.16.1.1        1   FULL/BDR        00:00:31    4               FastEthernet0/0

Check for the route
Router2#sh ipv6 route ospf  | include ^O
OE2  2001:1AD8:500::/64 [110/20]

As always, seems like you are afraid of the comment box, I know you’re there, now speak! ;)

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Posted in Cisco configuration | 2 Comments »

Understanding Cisco BGP Best Path Selection Algoritm

Sunday, September 7th, 2008

Do you completely understand the BGP Best Path Selection?

I know I have to look it up from time to time…

In BGP running on a Cisco router, this is the process:

  • Use paths with the highest Weight
  • Use paths with the highest Local Preference
  • Use paths sourced with the network or redistribute command over paths sourced from the aggregate-address command.
  • Choose the route with the shortest AS-Path
  • Use paths origined from (in this order) IGP, EGP and Unknown. (IGP paths are prefered over EGP, EGP over unknown)
  • Choose the path with the lowest MED (‘MED is cost’, so the path with the lowest MED is prefered)
  • Choose eBGP paths over iBGP
    If there are multiple prefered iBGP paths, use the one with the lowest IGP metric.
  • Now see if there are multiple paths , and if the router is running with the bgp-multipath command. (then they will be installed)
  • If there are multiple eBGP paths for the destination, choose the oldest one (the one first received).
    If there are no current best path, or you run the bgp best path compare-routerid command.
  • Choose the route originating from the router with the lowest router-id.
  • (Route Reflectors) If paths originate from the same router, choose the path with the lowest cluster list length.
  • At last, choose the route that originates from the lowest neighbor address

Why isn’t the prefix received and installed to the routing table?
This happens from time to time, this is often because there are no IGP route to the NEXT_HOP in the BGP UPDATE.  It can also occur if the local-as is present in the AS_PATH attribute.

How can I see which prefixes are filtered, and which are received?
You can use the command neighbor 10.20.30.40 soft-reconfiguration inbound to make the router store rejected, filtered and other routing information in memory for you. You can then use the show ip bgp neighbor 10.20.30.40 received-routes and show ip bgp neighbor 10.20.30.40 advertised-routes to see you received and advertised routes.

Please use the comment box if you have any additions or find any mistakes, or simply just want to say hi! :)

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Posted in Cisco configuration | No Comments »

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