BGP/ISIS traffic loop

Here is the following topology:

 

 

R1 is a core router for SP1 and R4 is a core router for SP2, both R2 and R3 are routers which are part of SP1 and SP2 networks respectively and act as edge routers.

-          R1 and R2 are ISIS neighbors, all directly connected and loopback interfaces are advertised using this protocol.

-          R1 and R2 are iBGP peers [AS65001]

-          R1 and R4 are eBGP peers [AS65001 and AS65004] using loopback IP’s

-          R1 learns default route and networks 192.168.41-43.0/24 from R4

-          R2 learns default route through BGP from R1

-          R2 has static route for 4.4.4.4/32 with next-hop to 10.1.23.3 which advertised to R1 through ISIS.

Now let’s simulate situation where the link between R2 and R3 is down due to physical or layer-2 failure:

1.       R1 BGP peer with R4 is still up due to tolerance behavior of the BGP protocol

2.       R2 has static route to 4.4.4.4 to 10.1.23.3 but this directly connected interface is down so a recursive lookup will occur and R2 will use the default route to send traffic

 

3.       R2 learns the default route from R1 through BGP so next-hop is R1

This situation in large SP can cause huge amount of traffic going back and forth between R1 and R2 and will cause a network outage in the network.

In order to overcome this issue I have configured a track object to Fa1 on R2 and bond it to the static route of R4:

track 1 interface FastEthernet1 line-protocol ! ip route 4.4.4.4 255.255.255.255 10.1.23.3 track 1

 

This will cause R1 to discard from the routing table all routes learned from R4 even if the BGP peer is still up.

R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2        ia - IS-IS inter area, * - candidate default, U - per-user static route        o - ODR, P - periodic downloaded static route   Gateway of last resort is 4.4.4.4 to network 0.0.0.0   C    192.168.12.0/24 is directly connected, Loopback2      1.0.0.0/32 is subnetted, 1 subnets C       1.1.1.1 is directly connected, Loopback0 C    192.168.13.0/24 is directly connected, Loopback3      2.0.0.0/32 is subnetted, 1 subnets i L2    2.2.2.2 [115/10] via 10.1.12.2, FastEthernet0 B    192.168.42.0/24 [20/0] via 4.4.4.4, 00:01:02 B    192.168.43.0/24 [20/0] via 4.4.4.4, 00:01:02 C    192.168.11.0/24 is directly connected, Loopback1 B    192.168.41.0/24 [20/0] via 4.4.4.4, 00:01:03      10.0.0.0/24 is subnetted, 1 subnets C       10.1.12.0 is directly connected, FastEthernet0 B*   0.0.0.0/0 [20/0] via 4.4.4.4, 00:01:03   R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2        ia - IS-IS inter area, * - candidate default, U - per-user static route        o - ODR, P - periodic downloaded static route   Gateway of last resort is 4.4.4.4 to network 0.0.0.0   C    192.168.12.0/24 is directly connected, Loopback2      1.0.0.0/32 is subnetted, 1 subnets C       1.1.1.1 is directly connected, Loopback0 C    192.168.13.0/24 is directly connected, Loopback3      2.0.0.0/32 is subnetted, 1 subnets i L2    2.2.2.2 [115/10] via 10.1.12.2, FastEthernet0 B    192.168.42.0/24 [20/0] via 4.4.4.4, 00:01:05 B    192.168.43.0/24 [20/0] via 4.4.4.4, 00:01:05 C    192.168.11.0/24 is directly connected, Loopback1 B    192.168.41.0/24      10.0.0.0/24 is subnetted, 1 subnets C       10.1.12.0 is directly connected, FastEthernet0   R1#  show ip bgp summary BGP router identifier 192.168.13.1, local AS number 65001 BGP table version is 23, main routing table version 23 4 network entries using 528 bytes of memory 4 path entries using 208 bytes of memory 2/1 BGP path/bestpath attribute entries using 336 bytes of memory 1 BGP AS-PATH entries using 24 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory Bitfield cache entries: current 1 (at peak 2) using 28 bytes of memory BGP using 1124 total bytes of memory BGP activity 8/4 prefixes, 8/4 paths, scan interval 60 secs   Neighbor        V          AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd 2.2.2.2         4      65001     937     942       23    0    0 15:42:11        0 4.4.4.4         4      65004     902     893       19    0    0 00:01:25        4

 In the first show command the link is up, in the second show command the link is down  altough the BGP peer is still up.