Configure Aggregate Addresses - Cisco NCS 5500 Series Configuration Manual
Bgp configuration ios xr
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Implementing BGP
Configure Aggregate Addresses
Back Door: Example
Here, 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.
Configure Aggregate Addresses
Perform this task to create aggregate entries in a BGP routing table.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
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