TAGS FOR ROUTE FILTERING AND LOOP PREVENTION

Network admintrators and Service providers are required to be very careful when redistributing Routes between 2 or more routing protocols. The challenges can be faced on redistribution among any of the Routing protocols whether RIP, OSPF, EIGRP or even BGP.

tags-for-route-filtering-and-loop-prevention

Above shown is one such scenario where TAGs will help filter the routes and mitigating the loops from happening. Customer A has 2 Links from same Service provider as below –

tags-for-route-filtering-and-loop-prevention

Network 5.5.5.0/24 is advertised by Service provider from 2 entry-exit points

  1. From R6-R1 Link
  2. From R5-R4 Link

R1 injects the TAG 555 to while redistributing BGP network 5.5.5.0/24 into EIGRP 1 domain using the route-map tag555 as below

 

R1 Configuration –

Route-map tag555 permit 10

Match ip address net5

Set tag 555

Route-map tag555 permit 20

 

Router eigrp 1

Redistribute bgp 1 metric 1000 10 255 100 1500 route-map tag555

The same TAG of 555 is propagated into EIGRP domain and can be dropped while redistributing EIGRP to BGP on R4 which will stop the network Loop from happening.

 

R4 Configuration –

Route-map tag555 deny 10

Match tag 555

Route-map tag555 permit 20

 

Router bgp 3

Redistribute eigrp 1 metric 1000 route-map tag555

 

The same tag based blocking needs to be implemented for Network (5.5.5.0/24) learned by R4 via its eBGP neighbor R5.Below is the approach to be followed –

tags-for-route-filtering-and-loop-prevention

 

R4 Configuration –

Route-map tag666 permit 10

Match ip address net5

Set tag 666

Route-map tag666 permit 20

 

Router eigrp 1

Redistribute bgp 1 metric 1000 10 255 100 1500 route-map tag666

 

R1 Configuration –

Route-map tag666 deny 10

Match tag 666

Route-map tag666 permit 20

 

Router bgp 3

Redistribute eigrp 1 metric 1000 route-map tag666

Now to check whether the tag555 and tag666 have been implemented and able to block Network 5.5.5.0/24, we issue the following commands on

 

R4 Configuration –

Show ip bgp neighbor 192.168.45.5 advertised-routes

(The above command is issued to verify that R4 is not advertising network 5.5.5.0/24 to R5. R4 denies routes with tag 555 advertised to R5 using Route Map)

R4#sh ip bgp neighbor 192.168.45.5 advertised-routes

   Network                 Next Hop              Metric     LocPrf     Weight            Path

*> 2.2.2.0/24               192.168.34.3          1000                                32768              ?

*> 3.3.3.0/24               192.168.34.3          1000                                32768              ?

*> 192.168.12.0         192.168.34.3            1000                                32768               ?

*> 192.168.23.0          192.168.34.3           1000                                32768               ?

*> 192.168.34.0          0.0.0.0                     0                                       32768              ?

*> 192.168.45.0          0.0.0.0                     0                                       32768              i

Total number of prefixes 6

 

R1 Configuration –

Show ip bgp neighbor 192.168.15.5 advertised-routes

(The above command is issued to verify that R1 is not advertising network 5.5.5.0/24 to R6. R1 denies routes with tag 666 advertised to R6 using Route Map)

R1#show ip bgp neighbor 192.168.16.6 advertised-routes

Network                 Next Hop                Metric    LocPrf Weight            Path

*> 2.2.2.0/24              192.168.12.2              1000                                32768              ?

*> 3.3.3.0/24              192.168.12.2              1000                                32768              ?

*> 192.168.12.0          0.0.0.0                       0                                       32768              ?

*> 192.168.15.0           0.0.0.0                      0                                       32768              i

*> 192.168.23.0         192.168.12.2              1000                                32768              ?

*> 192.168.34.0           192.168.12.2            1000                                32768              ?

*> 192.168.45.0           192.168.12.2            1000                                32768              ?

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