Do you still need that route?

The BGP advertisement is configured on the customer edge routers using the network command. Route advertisement is conditioned by the appearance of a corresponding network or subnet in the routing table of the edge router. If the network or subnet is manually entered into the routing table by configuring a static route to null 0, the condition is always true because the static route is always there, and the BGP advertisement is always performed.

If the customer edge router loses connectivity to the rest of the customer network but is still connected to the ISP network, BGP advertisement must cease. In this case, BGP advertisement can be stopped if BGP advertisements are bound to the reachability status of a specific subnet in the core of the customer network, according to the customer IGP. The problem with using a static route to null 0 is that it conditions the network statement in the BGP configuration so that BGP always advertises the route. If the customer edge router loses connectivity with the rest of the customer network, the router continues to advertise the entire customer address space. The ISP network receives a valid route from the customer edge router. Traffic is sent to this router, but because the router has lost connectivity with the rest of the network, the traffic is dropped (routed to the null 0 interface using the static route).

Rack1R1#show ip route eigrp
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/2809856] via 192.168.13.3, 00:00:39, Serial1/1
3.0.0.0/24 is subnetted, 1 subnets
D 3.3.3.0 [90/2297856] via 192.168.13.3, 00:00:41, Serial1/1
D 192.168.23.0/24 [90/2681856] via 192.168.13.3, 00:00:40, Serial1/1</pre>
 Rack1R1#conf te
 Enter configuration commands, one per line.  End with CNTL/Z.
 Rack1R1(config)#router bgp 12
 Rack1R1(config-router)#neigh 2.2.2.2 remote-as 12
 Rack1R1(config-router)#neigh 2.2.2.2 up lo0
 Rack1R1(config-router)#
 Rack1R1(config-router)#do show ip bgp summ
 BGP router identifier 1.1.1.1, local AS number 12
 BGP table version is 1, main routing table version 1
 Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
 2.2.2.2         4    12       0       0        0    0    0 never    Active
 Rack1R1(config-router)#exit

 %BGP-5-ADJCHANGE: neighbor 2.2.2.2; Up
 Rack1R1(config)#do show ip route 2.2.2.2
 Routing entry for 2.2.2.0/24
 Known via "eigrp 100", distance 90, metric 2809856, type internal
 Redistributing via eigrp 100
 Last update from 192.168.13.3 on Serial1/1, 00:01:44 ago
 Routing Descriptor Blocks:
 * 192.168.13.3, from 192.168.13.3, 00:01:44 ago, via Serial1/1
 Route metric is 2809856, traffic share count is 1
 Total delay is 45000 microseconds, minimum bandwidth is 1544 Kbit
 Reliability 255/255, minimum MTU 1500 bytes
 Loading 1/255, Hops 2

Rack1R1(config)#do show ip route 3.3.3.3
 Routing entry for 3.3.3.0/24
 Known via "eigrp 100", distance 90, metric 2297856, type internal
 Redistributing via eigrp 100
 Last update from 192.168.13.3 on Serial1/1, 00:02:09 ago
 Routing Descriptor Blocks:
 * 192.168.13.3, from 192.168.13.3, 00:02:09 ago, via Serial1/1
 Route metric is 2297856, traffic share count is 1
 Total delay is 25000 microseconds, minimum bandwidth is 1544 Kbit
 Reliability 255/255, minimum MTU 1500 bytes
 Loading 1/255, Hops 1

 Rack1R1(config)#ip route 100.10.10.1 255.255.255.255 3.3.3.3
 Rack1R1(config)#do show ip route 100.10.10.1
 Routing entry for 100.10.10.1/32
 Known via "static", distance 1, metric 0
 Routing Descriptor Blocks:
 * 3.3.3.3
 Route metric is 0, traffic share count is 1

Rack1R1(config)#ip route 100.20.10.1 255.255.255.255 null0
 Rack1R1(config)#do show ip route stat
 100.0.0.0/32 is subnetted, 2 subnets
 S       100.10.10.1 [1/0] via 3.3.3.3
 S       100.20.10.1 is directly connected, Null0
 Rack1R1(config)#do deb ip routing
 IP routing debugging is on
Rack1R1(config)#router bgp 12
 Rack1R1(config-router)#red static
 Rack1R1(config-router)#do show ip bgp
 BGP table version is 3, local router ID is 1.1.1.1
 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
 r RIB-failure, S Stale
 Origin codes: i - IGP, e - EGP, ? - incomplete

Network          Next Hop            Metric LocPrf Weight Path
 *100.10.10.1/32 0.0.0.0 0         32768 ?
 *100.20.10.1/32 0.0.0.0 0         32768 ?

Lets Shut down the lo0

 RT: delete route to 3.3.3.0 via 192.168.13.3, eigrp metric [90/2297856]
 RT: SET_LAST_RDB for 3.3.3.0/24
 OLD rdb: via 192.168.13.3, Serial1/1

 RT: no routes to 3.3.3.0
 RT: NET-RED 3.3.3.0/24
 RT: delete subnet route to 3.3.3.0/24
 RT: NET-RED 3.3.3.0/24
 RT: delete network route to 3.0.0.0
 RT: NET-RED 3.0.0.0/8
 Rack1R1(config-router)#do show ip bgp
 BGP table version is 3, local router ID is 1.1.1.1
 Status codes: s suppressed, d damped, h history, * valid, ; best, i - internal,
 r RIB-failure, S Stale
 Origin codes: i - IGP, e - EGP, ? - incomplete

 Network          Next Hop            Metric LocPrf Weight Path
 *100.10.10.1/32   0.0.0.0         0         32768 ?
 *100.20.10.1/32   0.0.0.0         0         32768 ?

 Rack1R1(config-router)#bgp scan-time 5
 Rack1R1(config-router)#do show ip bgp
 BGP table version is 3, local router ID is 1.1.1.1
 Status codes: s suppressed, d damped, h history, * valid, ; best, i - internal,
 r RIB-failure, S Stale
 Origin codes: i - IGP, e - EGP, ? - incomplete

 Network          Next Hop            Metric LocPrf Weight Path
 *100.10.10.1/32 0.0.0.0 0         32768 ?
 *100.20.10.1/32 0.0.0.0 0         32768 ?
 Rack1R1(config-router)#do show ip bgp
 BGP table version is 3, local router ID is 1.1.1.1
 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
 r RIB-failure, S Stale
 Origin codes: i - IGP, e - EGP, ? - incomplete

Network          Next Hop            Metric LocPrf Weight Path
 100.10.10.1/32  0.0.0.0 0         32768 ?
 100.20.10.1/32 0.0.0.0 0         32768 ?

Rack1R1(config-router)#do show ip route sta
 100.0.0.0/32 is subnetted, 1 subnets
 S       100.20.10.1 is directly connected, Null0
Rack1R1(config-router)#

 RT: del 100.10.10.1/32 via 3.3.3.3, static metric [1/0]
 RT: delete subnet route to 100.10.10.1/32
 RT: NET-RED 100.10.10.1/32
 Rack1R1(config-router)#do show ip bgp
 BGP table version is 4, local router ID is 1.1.1.1
 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
 r RIB-failure, S Stale
 Origin codes: i - IGP, e - EGP, ? - incomplete

Network          Next Hop            Metric LocPrf Weight Path
 * 100.20.10.1/32 0.0.0.0 0         32768 ?

So the static route to the null 0 will always be up and advertised via BGP

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2 thoughts on “Do you still need that route?

  1. nice tutor …
    [ask] can u explain what diffrent IBGP and EBGP ….????

  2. Thanks Giat,

    Border Gateway Protocol (BGP) is an interdomain (interautonomous system) routing protocol that is used to exchange routing information for the Internet. BGP, by design, is a very robust and scalable routing protocol, therefore, a very shot answer for your question, could be that eBGP is used towards other autonomous systems, iBGP is used within an AS.

    for more information besides the RFC check out this Cisco Link

    Have a good one
    Victor.-

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