Routing Loops with More than Two Routing Domains and Per-Route AD settings

• The problem with suboptimal path routing and routing loops can be resolved by changing the per-route AD on ASBRs.
• With the distance command, the AD is usually changed for all the internal and/or external routes; sometimes this may not resolve the issue for the above problem.
• To change the AD value for the selective routes, use the following command:
  • distance distance ip-adv-router wc-mask [acl-number/name]
• In this command, the required parameters match the neighboring router that advertises a route.
• Optionally, the distance command can also refer to an ACL. If included, that router compares the ACL to the prefix/length of each route learned from any matched neighbors and uses the listed AD only for routes permitted by the ACL.
• Example OSPF:

R19(config)# ip access-list 2 permit 1.18.18.0 0.0.0.255 R19(config)# router ospf 1 R19(config-router)# distance 98 0.0.0.18 0.0.0.0 2 R19# show ip route ospf ## Check if AD changed for specific route ##

• Example EIGRP

R20(config)# router eigrp 100
R20(config-router)# network 88.0.0.0
R19(config)# router eigrp 100
R19(config-router)# distance 88 20.19.20.20 0.0.0.0 3 # Only applies to iEIGRP routes from ACL 3 #
R19# show ip route eigrp # Look for the 88.88.88.0/24 and make sure it has an AD of 88 #

• This problem can be resolved by doing route filtering on ASBRs connecting IGP2 and IGP3.
• Filtering can be done in two ways: with per subnet filtering, or by using tags with routes.
• Performing filtering with per subnet requires long ACL/Prefix-list configurations.
• This method also has a higher chance of human error.

• A route tag is a unitless 32-bit integer that most routing protocols can assign to any given route.
• It can be assigned by a route-map referenced by a routing protocol distribute-list or redistribute command.
• The tag value follows the route advertisement, even through the redistribution process.
• The tag value is commonly used to identify routes, to do filtering, and to prevent routing loops.
• The concept is simple: One router sets the tag, the route (along with the tag) get advertised as per design, and then a certain router can match that route based on the tag value and either do filtering or change some attributes of the routes.
• The tag is set by using the command set tag in a route-map entry.

• Example

R20(config)# router rip
R20(config-router)# version 2
R20(config-router)# no auto-summary
R20(config-router)# network 18.0.0.0
R20(config-router)# redistribute ospf 1 metric 9
!
R18(config)# router rip
R18(config-router)# version 2
R18(config-router)# no auto-summary
R18(config-router)# network 18.0.0.0
R18(config-router)# redistribute ospf 1 metric 9
!
R18(config)# access-list 1 permit 2.2.2.0 0.0.0.255
R18(config)# route-map Set-Tag permit 10
R18(config-route-map)# match ip add 1
R18(config-route-map)# set tag 888
R18(config)# router rip
R18(config)# redistribute ospf 1 metric 9 route-map Set-Tag
!
R18# show ip route 2.2.2.0 # Look for the tag #

• When EIGRP is redistributed into OSPF, the tag from EIGRP is preserved.
• When EIGRP is redistributed into RIP, the tag from EIGRP is deleted and has to be added back on.