Figure 2: Back Door Example
In Figure 2: Back Door Example , on page 30, Routers A and C and Routers B and C are running eBGP.
Routers A and B are running an IGP (such as Routing Information Protocol [RIP], Interior Gateway Routing
Protocol [IGRP], Enhanced IGRP, or Open Shortest Path First [OSPF]). The default distances for RIP, IGRP,
Enhanced IGRP, and OSPF are 120, 100, 90, and 110, respectively. All these distances are higher than the
default distance of eBGP, which is 20. Usually, the route with the lowest distance is preferred.
Router A receives updates about 160.10.0.0 from two routing protocols: eBGP and IGP. Because the default
distance for eBGP is lower than the default distance of the IGP, Router A chooses the eBGP-learned route
from Router C. If you want Router A to learn about 160.10.0.0 from Router B (IGP), establish a BGP back
door. See .
In the following example, a network back-door is configured:
RP/0/RP0/CPU0:router(config)# router bgp 100
RP/0/RP0/CPU0:router(config-bgp)# address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-bgp-af)# network 160.10.0.0/16 backdoor
Router A treats the eBGP-learned route as local and installs it in the IP routing table with a distance of 200.
The network is also learned through Enhanced IGRP (with a distance of 90), so the Enhanced IGRP route is
successfully installed in the IP routing table and is used to forward traffic. If the Enhanced IGRP-learned
route goes down, the eBGP-learned route is installed in the IP routing table and is used to forward traffic.
Although BGP treats network 160.10.0.0 as a local entry, it does not advertise network 160.10.0.0 as it normally
would advertise a local entry.
Route Dampening
Route dampening is a BGP feature that minimizes the propagation of flapping routes across an internetwork.
A route is considered to be flapping when it is repeatedly available, then unavailable, then available, then
unavailable, and so on.
For example, consider a network with three BGP autonomous systems: autonomous system 1, autonomous
system 2, and autonomous system 3. Suppose the route to network A in autonomous system 1 flaps (it becomes
unavailable). Under circumstances without route dampening, the eBGP neighbor of autonomous system 1 to
autonomous system 2 sends a withdraw message to autonomous system 2. The border router in autonomous
system 2, in turn, propagates the withdrawal message to autonomous system 3. When the route to network A
reappears, autonomous system 1 sends an advertisement message to autonomous system 2, which sends it to
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
30
Implementing BGP
Route Dampening
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